Molecular regulatory mechanisms of staminate strobilus development and dehiscence in Torreya grandis

Gymnosperms are mostly dioecious, and their staminate strobili undergo a longer developmental period than those of angiosperms. However, the underlying molecular mechanisms remain unclear. This study aimed to identify key genes and pathways involved in staminate strobilus development and dehiscence in Torreya grandis. Through weighted gene co-expression network analysis (WGCNA), we identified fast elongation-related genes enriched in carbon metabolism and auxin signal transduction, whereas dehiscence-related genes were abundant in alpha-linolenic acid metabolism and the phenylpropanoid pathway. Based on WGCNA, we also identified PHYTOCHROME-INTERACTING FACTOR4 (TgPIF4) as a potential regulator for fast elongation of staminate strobilus and 2 WRKY proteins (TgWRKY3 and TgWRKY31) as potential regulators for staminate strobilus dehiscence. Multiple protein-DNA interaction analyses showed that TgPIF4 directly activates the expression of TRANSPORT INHIBITOR RESPONSE2 (TgTIR2) and NADP-MALIC ENZYME (TgNADP-ME). Overexpression of TgPIF4 significantly promoted staminate strobilus elongation by elevating auxin signal transduction and pyruvate content. TgWRKY3 and TgWRKY31 bind to the promoters of the lignin biosynthesis gene PHENYLALANINE AMMONIA-LYASE (TgPAL) and jasmonic acid metabolism gene JASMONATE O-METHYLTRANSFERASE (TgJMT), respectively, and directly activate their transcription. Overexpression of TgWRKY3 and TgWRKY31 in the staminate strobilus led to early dehiscence, accompanied by increased lignin and methyl jasmonate levels, respectively. Collectively, our findings offer a perspective for understanding the growth of staminate strobili in gymnosperms.

Large-Scale Identification and Characterization Analysis of VQ Family Genes in Plants, Especially Gymnosperms

VQ motif-containing (VQ) proteins are a class of transcription regulatory cofactors widely present in plants, playing crucial roles in growth and development, stress response, and defense. Although there have been some reports on the member identification and functional research of VQ genes in some plants, there is still a lack of large-scale identification and clear graphical presentation of their basic characterization information to help us to better understand this family. Especially in gymnosperms, the VQ family genes and their evolutionary relationships have not yet been reported. In this study, we systematically identified 2469 VQ genes from 56 plant species, including bryophytes, gymnosperms, and angiosperms, and analyzed their molecular and evolutionary features. We found that amino acids are only highly conserved in the VQ domain, while other positions are relatively variable; most VQ genes encode relatively small proteins and do not have introns. The GC content in Poaceae plants is the highest (up to 70%); these VQ proteins can be divided into nine subgroups. In particular, we analyzed the molecular characteristics, chromosome distribution, duplication events, and expression levels of VQ genes in three gymnosperms: Ginkgo biloba, Taxus chinensis, and Pinus tabuliformis. In gymnosperms, VQ genes are classified into 11 groups, with highly similar motifs in each group; most VQ proteins have less than 300 amino acids and are predicted to be located in nucleus. Tandem duplication is an important driving force for the expansion of the VQ gene family, and the evolutionary processes of most VQ genes and duplication events are relatively independent; some candidate VQ genes are preliminarily screened, and they are likely to be involved in plant growth and stress and defense responses. These results provide detailed information and powerful references for further understanding and utilizing the VQ family genes in various plants.

The D14 and KAI2 Orthologs of Gymnosperms Sense Strigolactones and KL Mimics, Respectively, and the Signals Are Transduced to Control Downstream Genes

Strigolactones (SLs), lactone-containing carotenoid derivatives, function as signaling molecules in the rhizosphere, inducing symbiosis with arbuscular mycorrhizal. In addition, as a class of plant hormones, SLs control plant growth and development in flowering plants (angiosperms). Recent studies show that the ancestral function of SLs, which precede terrestrialization of plants, is as rhizosphere signaling molecules. SLs were then recruited as a class of plant hormones through the step-by-step acquisition of signaling components. The D14 gene encoding the SL receptor arose by gene duplication of KARRIKIN INSENSITIVE2 (KAI2), the receptor of karrikins and KAI2 ligand (KL), an unknown ligand, in the common ancestor of seed plants. KL signaling targets SMAX1, a repressor protein. On the other hand, the SL signaling targets SMXL78 subclade repressors, which arose by duplication of SMAX1 in angiosperms. Thus, gymnosperms contain the SL receptor D14 but not SMXL78, the SL signaling-specific repressor proteins. We studied two gymnosperm species, ginkgo (Ginkgo biloba) and Japanese umbrella pine (Sciadopitys verticillata), to clarify whether SLs are perceived and the signals are transduced in gymnosperms. We show that D14 and KAI2 of ginkgo and Japanese umbrella pine specifically perceive an SL analog and KL mimic, respectively. Furthermore, our results suggest that both SL signaling and KL signaling target SMAX1, and the specific localization of the receptor may result in the specificity of the signaling in gymnosperms.

The Dynamic Changes of Alternative Electron Flows upon Transition from Low to High Light in the Fern Cyrtomium fortune and the Gymnosperm Nageia nagi

In photosynthetic organisms except angiosperms, an alternative electron sink that is mediated by flavodiiron proteins (FLVs) plays the major role in preventing PSI photoinhibition while cyclic electron flow (CEF) is also essential for normal growth under fluctuating light. However, the dynamic changes of FLVs and CEF has not yet been well clarified. In this study, we measured the P700 signal, chlorophyll fluorescence, and electrochromic shift spectra in the fern Cyrtomium fortune and the gymnosperm Nageia nagi. We found that both species could not build up a sufficient proton gradient (∆pH) within the first 30 s after light abruptly increased. During this period, FLVs-dependent alternative electron flow was functional to avoid PSI over-reduction. This functional time of FLVs was much longer than previously thought. By comparison, CEF was highly activated within the first 10 s after transition from low to high light, which favored energy balancing rather than the regulation of a PSI redox state. When FLVs were inactivated during steady-state photosynthesis, CEF was re-activated to favor photoprotection and to sustain photosynthesis. These results provide new insight into how FLVs and CEF interact to regulate photosynthesis in non-angiosperms.

Variation of photosynthesis during plant evolution and domestication: implications for improving crop photosynthesis

Studies investigating the mechanisms underlying the variation of photosynthesis along plant phylogeny and especially during domestication are of great importance, and may provide new insights to further improve crop photosynthesis. In the present study, we compiled a database including 542 sets of data of leaf gas exchange parameters and leaf structural and chemical traits in ferns and fern allies, gymnosperms, non-crop angiosperms, and crops. We found that photosynthesis was dramatically improved from ferns and fern allies to non-crop angiosperms, and further increased in crops. The improvement of photosynthesis during phylogeny and domestication was related to increases in carbon dioxide diffusional capacities and, to a lesser extent, biochemical capacity. Cell wall thickness rather than chloroplast surface area facing intercellular airspaces drives the variation of mesophyll conductance. The variation of the maximum carboxylation rate was not related to leaf nitrogen content. The slope of the relationship between mass-based photosynthesis and nitrogen was lower in crops than in non-crop angiosperms. These findings suggest that the manipulation of cell wall thickness is the most promising approach to further improve crop photosynthesis, and that an increase of leaf nitrogen will be less efficient in improving photosynthesis in crops than in non-crop angiosperms.

A revised view on the evolution of glutamine synthetase isoenzymes in plants

Glutamine synthetase (GS) is a key enzyme responsible for the incorporation of inorganic nitrogen in the form of ammonium into the amino acid glutamine. In plants, two groups of functional GS enzymes are found: eubacterial GSIIb (GLN2) and eukaryotic GSIIe (GLN1/GS). Only GLN1/GS genes are found in vascular plants, which suggests that they are involved in the final adaptation of plants to terrestrial life. The present phylogenetic study reclassifies the different GS genes of seed plants into three clusters: GS1a, GS1b and GS2. The presence of genes encoding GS2 has been expanded to Cycadopsida gymnosperms, which suggests the origin of this gene in a common ancestor of Cycadopsida, Ginkgoopsida and angiosperms. GS1a genes have been identified in all gymnosperms, basal angiosperms and some Magnoliidae species. Previous studies in conifers and the gene expression profiles obtained in ginkgo and magnolia in the present work could explain the absence of GS1a in more recent angiosperm species (e.g. monocots and eudicots) as a result of the redundant roles of GS1a and GS2 in photosynthetic cells. Altogether, the results provide a better understanding of the evolution of plant GS isoenzymes and their physiological roles, which is valuable for improving crop nitrogen use efficiency and productivity. This new view of GS evolution in plants, including a new cytosolic GS group (GS1a), has important functional implications in the context of plant metabolism adaptation to global changes.

A Talk between Flavonoids and Hormones to Reorient the Growth of Gymnosperms

Plants reorient the growth of affected organs in response to the loss of gravity vector. In trees, this phenomenon has received special attention due to its importance for the forestry industry of conifer species. Sustainable management is a key factor in improving wood quality. It is of paramount importance to understand the molecular and genetic mechanisms underlying wood formation, together with the hormonal and environmental factors that affect wood formation and quality. Hormones are related to the modulation of vertical growth rectification. Many studies have resulted in a model that proposes differential growth in the stem due to unequal auxin and jasmonate allocation. Furthermore, many studies have suggested that in auxin distribution, flavonoids act as molecular controllers. It is well known that flavonoids affect auxin flux, and this is a new area of study to understand the intracellular concentrations and how these compounds can control the gravitropic response. In this review, we focused on different molecular aspects related to the hormonal role in flavonoid homeostasis and what has been done in conifer trees to identify molecular players that could take part during the gravitropic response and reduce low-quality wood formation.

ROS and Ions in Cell Signaling during Sexual Plant Reproduction

Pollen grain is a unique haploid organism characterized by two key physiological processes: activation of metabolism upon exiting dormancy and polar tube growth. In gymnosperms and flowering plants, these processes occur in different time frames and exhibit important features; identification of similarities and differences is still in the active phase. In angiosperms, the growth of male gametophyte is directed and controlled by its microenvironment, while in gymnosperms it is relatively autonomous. Recent reviews have detailed aspects of interaction between angiosperm female tissues and pollen such as interactions between peptides and their receptors; however, accumulated evidence suggests low-molecular communication, in particular, through ion exchange and ROS production, equally important for polar growth as well as for pollen germination. Recently, it became clear that ROS and ionic currents form a single regulatory module, since ROS production and the activity of ion transport systems are closely interrelated and form a feedback loop.

Cell wall composition strongly influences mesophyll conductance in gymnosperms

Cell wall thickness is widely recognized as one of the main determinants of mesophyll conductance to CO₂ (g_m ). However, little is known about the components that regulate effective CO₂ diffusivity in the cell wall (i.e. the ratio between actual porosity and tortuosity, the other two biophysical diffusion properties of cell walls). The aim of this study was to assess, at the interspecific level, potential relationships between cell wall composition, cell wall thickness (T_cw ) and g_m . Gymnosperms constitute an ideal group to deepen these relationships, as they present, on average, the thickest cell walls within spermatophytes. We characterized the foliar gas exchange, the morphoanatomical traits related with g_m , the leaf fraction constituted by cell walls and three main components of primary cell walls (hemicelluloses, cellulose and pectins) in seven gymnosperm species. We found that, although the relatively low g_m of gymnosperms was mainly determined by their elevated T_cw , g_m was also strongly correlated with cell wall composition, which presumably sets the final effective CO₂ diffusivity. The data presented here suggest that (i) differences in g_m are strongly correlated to the pectins to hemicelluloses and cellulose ratio in gymnosperms, and (ii) variations in cell wall composition may modify effective CO₂ diffusivity in the cell wall to compensate the negative impact of thickened walls. We speculate that higher relative pectin content allows higher g_m because pectins increase cell wall hydrophilicity and CO₂ molecules cross the wall dissolved in water.

Diel patterns of stem CO2 efflux vary among cycads, arborescent monocots, and woody eudicots and gymnosperms

The diel patterns of stem carbon dioxide efflux (E_s) were determined for cycads, monocots, and woody eudicot and gymnosperm tree species. Stem E_s at a height of 30-40 cm was measured every 2 h throughout 31-h campaigns. Our range of E_s was 1.5-4.0 µmol·m^-2·s^-1 for cycads, 1.0-3.5 µmol·m^-2·s^-1 for arborescent monocots, and 1.5-4.5 µmol·m^-2·s^-1 for woody eudicot and gymnosperm trees species. Time of day did not influence E_s of cycads or monocots. In contrast, the woody stems of eudicots and gymnosperms exhibited diurnal E_s that was 36% to 40% greater than nocturnal E_s. The established literature based on E_s of woody tree species cannot be used to estimate habitat carbon cycles in habitats which contain cycad or monocot trees. Time of day must be included for accuracy of research on E_s of woody tree species. Failures to account for the spatiotemporal differences of E_s may explain some of the disparity in outcomes of published stem respiration studies.

Biochemical characterization of diterpene synthases of Taiwania cryptomerioides expands the known functional space of specialized diterpene metabolism in gymnosperms

Taiwania cryptomerioides is a monotypic gymnosperm species, valued for the high decay resistance of its wood. This durability has been attributed to the abundance of terpenoids, especially the major diterpenoid metabolite ferruginol, with antifungal and antitermite activity. Specialized diterpenoid metabolism in gymnosperms primarily recruits bifunctional class-I/II diterpene synthases (diTPSs), whereas monofunctional class-II and class-I enzymes operate in angiosperms. In this study, we identified a previously unrecognized group of monofunctional diTPSs in T. cryptomerioides, which suggests a distinct evolutionary divergence of the diTPS family in this species. Specifically, five monofunctional diTPS functions not previously observed in gymnosperms were characterized, including monofunctional class-II enzymes forming labda-13-en-8-ol diphosphate (LPP, TcCPS2) and (+)-copalyl diphosphate (CPP, TcCPS4), and three class-I diTPSs producing biformene (TcKSL1), levopimaradiene (TcKSL3) and phyllocladanol (TcKSL5), respectively. Methyl jasmonate (MeJA) elicited the accumulation of levopimaradiene and the corresponding biosynthetic diTPS genes, TcCPS4 and TcKSL3, is consistent with a possible role in plant defense. Furthermore, TcCPS4 and TcKSL3 are likely to contribute to abietatriene biosynthesis via levopimaradiene as an intermediate in ferruginol biosynthesis in Taiwania. In conclusion, this study provides deeper insight into the functional landscape and molecular evolution of specialized diterpenoid metabolism in gymnosperms as a basis to better understand the role of these metabolites in tree chemical defense.

A Novel Hypothesis for the Role of Photosynthetic Physiology in Shaping Macroevolutionary Patterns

The fossil record and models of atmospheric concentrations of O2 and CO2 suggest that past shifts in plant ecological dominance often coincided with dramatic changes in Earth's atmospheric composition. This study tested the effects of past changes in atmospheric composition on the photosynthetic physiology of a limited range of early-diverging angiosperms (eight), gymnosperms (three), and ferns (two). We performed physiological measurements on all species and used the results to parameterize simulations of their photosynthetic paleophysiology using three independent modeling approaches. Unique physiological attributes were identified for the three evolutionary groups: angiosperm taxa displayed significantly higher mesophyll conductance (g m), yet their stomatal conductance (g s) was lower than that of ferns. Gymnosperm taxa displayed low g s and g m, but they partially offset their significant diffusional limitations on photosynthesis through their higher maximum Rubisco carboxylation rate. Despite their high total conductance to CO2, fern taxa lacked an optimized control of g s, which was reflected in their low intrinsic water use efficiency. Simulations of the photosynthetic physiology of ferns, angiosperms, and gymnosperms through Earth's history demonstrated that past fluctuations in O2 and CO2 concentrations may have resulted in significant shifts in the relative competitiveness of the three evolutionary groups. Although preliminary because of limited species sampling, these findings hint at a potential mechanistic basis for the observed broad temporal correlation between atmospheric change and shifts in plant evolutionary group-level richness observed in the fossil record and are presented as a framework to be tested with paleophotosynthetic proxies and through increased species sampling.

Global gene expression analysis in etiolated and de-etiolated seedlings in conifers

Plant life cycle begins with germination of seed below the ground. This is followed by seedling's development in the dark: skotomorphogenesis; and then a light-mediated growth: photomorphogenesis. After germination, hypocotyl grows rapidly to reach the sun, which involves elongation of shoot at the expense of root and cotyledons. Upon reaching ground level, seedling gets exposed to sunlight following a switch from the etiolated (skotomorphogenesis) to the de-etiolated (photomorphogenesis) stage, involving a series of molecular and physiological changes. Gymnosperms have evolved very differently and adopted diverse strategies as compared to angiosperms; with regards to response to light quality, conifers display a very mild high-irradiance response as compared to angiosperms. Absence of apical hook and synthesis of chlorophyll during skotomorphogenesis are two typical features in gymnosperms which differentiate them from angiosperms (dicots). Information regarding etiolation and de-etiolation processes are well understood in angiosperms, but these mechanisms are less explored in conifer species. It is, therefore, interesting to know how similar these processes are in conifers as compared to angiosperms. We performed a global expression analysis (RNA sequencing) on etiolated and de-etiolated seedlings of two economically important conifer species in Sweden to review the differentially expressed genes associated with the two processes. Based on the results, we propose that high levels of HY5 in conifers under DARK condition coupled with expression of few other genes associated with de-etiolation in angiosperms e.g. SPA, DET1 (lower expression under DARK) and CRY1 (higher expression under DARK), leads to partial expression of photomorphogenic genes in the DARK phenotype in conifers as displayed by absence of apical hook, opening of cotyledons and synthesis of chlorophyll.

Zygotic embryo cell wall responses to drying in three gymnosperm species differing in seed desiccation sensitivity

Plant cell walls (CWs) are dynamic in that they can change conformation during ontogeny and in response to various stresses. Though seeds are the main propagatory units of higher plants, little is known of the conformational responses of zygotic embryo CWs to drying. This study employed cryo-scanning electron microscopy to compare the effects of desiccation on zygotic embryo CW morphology across three gymnosperm species that were shown here to differ in seed desiccation sensitivity: Podocarpus henkelii (highly desiccation-sensitive), Podocarpus falcatus (moderately desiccation-sensitive), and Pinus elliottii (desiccation-tolerant). Fresh/imbibed (i.e. fresh Podocarpus at shedding and imbibed Pi. elliottii) embryos showed polyhedral cells with regular walls, typical of turgid cells with an intact plasmalemma. Upon desiccation to c. 0.05 g g^-1 (dry mass basis), CWs assumed an undulating conformation, the severity of which appeared to depend on the amount and type of dry matter accumulated. After desiccation, intercellular spaces between cortical cells in all species were comparably enlarged relative to those of fresh/imbibed embryos. After rehydration, meristematic and cotyledonary CWs of P. henkelii and meristematic CWs of P. falcatus remained slightly undulated, suggestive of plasmalemma and/or CW damage, while those of Pi. elliottii returned to their original conformation. Cell areas in dried-rehydrated P. henkelii root meristem and cotyledon were also significantly lower than those from fresh embryos, suggesting incomplete recovery, even though embryo water contents were comparable between the two states. Electrolyte leakage measurements suggest that the two desiccation-sensitive species incurred significant plasmalemma damage relative to the tolerant species upon desiccation, in agreement with the CW abnormalities observed in these species after rehydration. Immunocytochemistry studies revealed that of the four CW epitopes common to embryos of all three species, an increase in arabinan (LM6) upon desiccation and rehydration in desiccation-tolerant Pi. elliottii was the only difference, although this was not statistically significant. Seed desiccation sensitivity in species like P. henkelii and P. falcatus may therefore be partly based on the inability of the plasmalemma and consequently CWs of dried embryos to regain their original conformation following rehydration.

Phloem transport in gymnosperms: a question of pressure and resistance

Even in the highest trees, carbon is efficiently distributed from leaves to heterotrophic tissues like fruit, flowers and roots. This long-distance transport happens in the highly specialized sieve elements of the phloem. In gymnosperms, sieve element anatomy appears to be less suited for mass flow of phloem sap than that of angiosperms. This review covers available data on gymnosperm phloem to evaluate if it functions differently from that of angiosperms. Although current evidence suggests that, despite a higher pathway resistance, a single source-to-sink turgor pressure gradient can drive mass flow, several questions remain unanswered. These include how endoplasmic reticulum-complexes in sieve elements influence flow, as well as what the effect of symplasmic coupling along the whole phloem pathway could be.

Analysis of the WUSCHEL-RELATED HOMEOBOX gene family in Pinus pinaster: New insights into the gene family evolution

WUSCHEL-RELATED HOMEOBOX (WOX) genes are key players controlling stem cells in plants and can be divided into three clades according to the time of their appearance during plant evolution. Our knowledge of stem cell function in vascular plants other than angiosperms is limited, they separated from gymnosperms ca 300 million years ago and their patterning during embryogenesis differs significantly. For this reason, we have used the model gymnosperm Pinus pinaster to identify WOX genes and perform a thorough analysis of their gene expression patterns. Using transcriptomic data from a comprehensive range of tissues and stages of development we have shown three major outcomes: that the P. pinaster genome encodes at least fourteen members of the WOX family spanning all the major clades, that the genome of gymnosperms contains a WOX gene with no homologues in angiosperms representing a transitional stage between intermediate- and WUS-clade proteins, and that we can detect discrete WUS and WOX5 transcripts for the first time in a gymnosperm.

Phylogenetic and functional signals in gymnosperm ovular secretions

BACKGROUND AND AIMS

Gymnosperms are either wind-pollinated (anemophilous) or both wind- and insect-pollinated (ambophilous). Regardless of pollination mode, ovular secretions play a key role in pollen capture, germination and growth; they are likely also involved in pollinator reward. Little is known about the broad-scale diversity of ovular secretions across gymnosperms, and how these may relate to various reproductive functions. This study analyses the sugar and amino acid profiles of ovular secretions across a range of ambophilous (cycads and Gnetales) and anemophilous gymnosperms (conifers) to place them in an evolutionary context of their possible functions during reproduction.

METHODS

Ovular secretions from 13 species representing all five main lineages of extant gymnosperms were sampled. High-performance liquid chromatography techniques were used to measure sugar and amino acid content. Multivariate statistics were applied to assess whether there are significant differences in the chemical profiles of anemophilous and ambophilous species. Data were compared with published chemical profiles of angiosperm nectar. Chemical profiles were placed in the context of phylogenetic relationships.

KEY RESULTS

Total sugar concentrations were significantly higher in ovular secretions of ambophilous species than wind-pollinated taxa such as Pinaceae and Cupressophyta. Ambophilous species had lower amounts of total amino acids, and a higher proportion of non-protein amino acids compared with anemophilous lineages, and were also comparable to angiosperm nectar. Results suggest that early gymnosperms likely had ovular secretion profiles that were a mosaic of those associated with modern anemophilous and ambophilous species. Ginkgo, thought to be anemophilous, had a profile typical of ambophilous taxa, suggesting that insect pollination either exists in Gingko, but is undocumented, or that its ancestral populations were insect-pollinated.

CONCLUSIONS

Chemical profiles of ovular secretions of ambophilous gymnosperms show a clear signal of pollinator-driven selection, including higher levels of carbohydrates than anemophilous taxa, lower levels of amino acids, and the presence of specific amino acids, such as β-alanine, that are known to influence insect feeding behaviour and physiology.

Land plants drive photorespiration as higher electron-sink: comparative study of post-illumination transient O2 -uptake rates from liverworts to angiosperms through ferns and gymnosperms

In higher plants, the electron-sink capacity of photorespiration contributes to alleviation of photoinhibition by dissipating excess energy under conditions when photosynthesis is limited. We addressed the question at which point in the evolution of photosynthetic organisms photorespiration began to function as electron sink and replaced the flavodiiron proteins which catalyze the reduction of O₂ at photosystem I in cyanobacteria. Algae do not have a higher activity of photorespiration when CO₂ assimilation is limited, and it can therefore not act as an electron sink. Using land plants (liverworts, ferns, gymnosperms, and angiosperms) we compared photorespiration activity and estimated the electron flux driven by photorespiration to evaluate its electron-sink capacity at CO₂ -compensation point. In vivo photorespiration activity was estimated by the simultaneous measurement of O₂ -exchange rate and chlorophyll fluorescence yield. All C3-plants leaves showed transient O₂ -uptake after actinic light illumination (post-illumination transient O₂ -uptake), which reflects photorespiration activity. Post-illumination transient O₂ -uptake rates increased in the order from liverworts to angiosperms through ferns and gymnosperms. Furthermore, photorespiration-dependent electron flux in photosynthetic linear electron flow was estimated from post-illumination transient O₂ -uptake rate and compared with the electron flux in photosynthetic linear electron flow in order to evaluate the electron-sink capacity of photorespiration. The electron-sink capacity at the CO₂ -compensation point also increased in the above order. In gymnosperms photorespiration was determined to be the main electron-sink. C3-C4 intermediate species of Flaveria plants showed photorespiration activity, which intermediate between that of C3- and C4-flaveria species. These results indicate that in the first land plants, liverworts, photorespiration started to function as electron sink. According to our hypothesis, the dramatic increase in partial pressure of O₂ in the atmosphere about 0.4 billion years ago made it possible to drive photorespiration with higher activity in liverworts.

Alternative electron transport mediated by flavodiiron proteins is operational in organisms from cyanobacteria up to gymnosperms

Photo-reduction of O₂ to water mediated by flavodiiron proteins (FDPs) represents a safety valve for the photosynthetic electron transport chain in fluctuating light. So far, the FDP-mediated O₂ photo-reduction has been evidenced only in cyanobacteria and the moss Physcomitrella; however, a recent phylogenetic analysis of transcriptomes of photosynthetic organisms has also revealed the presence of FDP genes in several nonflowering plant groups. What remains to be clarified is whether the FDP-dependent O₂ photo-reduction is actually operational in these organisms. We have established a simple method for the monitoring of FDP-mediated O₂ photo-reduction, based on the measurement of redox kinetics of P700 (the electron donor of photosystem I) upon dark-to-light transition. The O₂ photo-reduction is manifested as a fast re-oxidation of P700. The validity of the method was verified by experiments with transgenic organisms, namely FDP knock-out mutants of Synechocystis and Physcomitrella and transgenic Arabidopsis plants expressing FDPs from Physcomitrella. We observed the fast P700 re-oxidation in representatives of all green plant groups excluding angiosperms. Our results provide strong evidence that the FDP-mediated O₂ photo-reduction is functional in all nonflowering green plant groups. This finding suggests a major change in the strategy of photosynthetic regulation during the evolution of angiosperms.

Extremely thick cell walls and low mesophyll conductance: welcome to the world of ancient living!

Mesophyll conductance is thought to be an important photosynthetic limitation in gymnosperms, but they currently constitute the most understudied plant group in regard to the extent to which photosynthesis and intrinsic water use efficiency are limited by mesophyll conductance. A comprehensive analysis of leaf gas exchange, photosynthetic limitations, mesophyll conductance (calculated by three methods previously used for across-species comparisons), and the underlying ultra-anatomical, morphological and chemical traits in 11 gymnosperm species varying in evolutionary history was performed to gain insight into the evolution of structural and physiological controls on photosynthesis at the lower return end of the leaf economics spectrum. Two primitive herbaceous species were included in order to provide greater evolutionary context. Low mesophyll conductance was the main limiting factor of photosynthesis in the majority of species. The strongest sources of limitation were extremely thick mesophyll cell walls, high chloroplast thickness and variation in chloroplast shape and size, and the low exposed surface area of chloroplasts per unit leaf area. In gymnosperms, the negative relationship between net assimilation per mass and leaf mass per area reflected an increased mesophyll cell wall thickness, whereas the easy-to-measure integrative trait of leaf mass per area failed to predict the underlying ultrastructural traits limiting mesophyll conductance.

Genes Sufficient for Synthesizing Peptidoglycan are Retained in Gymnosperm Genomes, and MurE from Larix gmelinii can Rescue the Albino Phenotype of Arabidopsis MurE Mutation

The endosymbiotic theory states that plastids are derived from a single cyanobacterial ancestor that possessed a cell wall. Peptidoglycan (PG), the main component of the bacteria cell wall, gradually degraded during plastid evolution. PG-synthesizing Mur genes have been found to be retained in the genomes of basal streptophyte plants, although many of them have been lost from the genomes of angiosperms. The enzyme encoded by bacterial MurE genes catalyzes the formation of the UDP-N-acetylmuramic acid (UDP-MurNAc) tripeptide in bacterial PG biosynthesis. Knockout of the MurE gene in the moss Physcomitrella patens resulted in defects of chloroplast division, whereas T-DNA-tagged mutants of Arabidopsis thaliana for MurE revealed inhibition of chloroplast development but not of plastid division, suggesting that AtMurE is functionally divergent from the bacterial and moss MurE proteins. Here, we could identify 10 homologs of bacterial Mur genes, including MurE, in the recently sequenced genomes of Picea abies and Pinus taeda, suggesting the retention of the plastid PG system in gymnosperms. To investigate the function of gymnosperm MurE, we isolated an ortholog of MurE from the larch, Larix gmelinii (LgMurE) and confirmed its presence as a single copy per genome, as well as its abundant expression in the leaves of larch seedlings. Analysis with a fusion protein combining green fluorescent protein and LgMurE suggested that it localizes in chloroplasts. Cross-species complementation assay with MurE mutants of A. thaliana and P. patens showed that the expression of LgMurE cDNA completely rescued the albefaction defects in A. thaliana but did not rescue the macrochloroplast phenotype in P. patens. The evolution of plastid PG and the mechanism behind the functional divergence of MurE genes are discussed in the context of information about plant genomes at different evolutionary stages.

The correlations and sequence of plant stomatal, hydraulic, and wilting responses to drought

Climate change is expected to exacerbate drought for many plants, making drought tolerance a key driver of species and ecosystem responses. Plant drought tolerance is determined by multiple traits, but the relationships among traits, either within individual plants or across species, have not been evaluated for general patterns across plant diversity. We synthesized the published data for stomatal closure, wilting, declines in hydraulic conductivity in the leaves, stems, and roots, and plant mortality for 262 woody angiosperm and 48 gymnosperm species. We evaluated the correlations among the drought tolerance traits across species, and the general sequence of water potential thresholds for these traits within individual plants. The trait correlations across species provide a framework for predicting plant responses to a wide range of water stress from one or two sampled traits, increasing the ability to rapidly characterize drought tolerance across diverse species. Analyzing these correlations also identified correlations among the leaf and stem hydraulic traits and the wilting point, or turgor loss point, beyond those expected from shared ancestry or independent associations with water stress alone. Further, on average, the angiosperm species generally exhibited a sequence of drought tolerance traits that is expected to limit severe tissue damage during drought, such as wilting and substantial stem embolism. This synthesis of the relationships among the drought tolerance traits provides crucial, empirically supported insight into representing variation in multiple traits in models of plant and ecosystem responses to drought.

Evolution of Xylan Substitution Patterns in Gymnosperms and Angiosperms: Implications for Xylan Interaction with Cellulose

The interaction between cellulose and xylan is important for the load-bearing secondary cell wall of flowering plants. Based on the precise, evenly spaced pattern of acetyl and glucuronosyl (MeGlcA) xylan substitutions in eudicots, we recently proposed that an unsubstituted face of xylan in a 2-fold helical screw can hydrogen bond to the hydrophilic surfaces of cellulose microfibrils. In gymnosperm cell walls, any role for xylan is unclear, and glucomannan is thought to be the important cellulose-binding polysaccharide. Here, we analyzed xylan from the secondary cell walls of the four gymnosperm lineages (Conifer, Gingko, Cycad, and Gnetophyta). Conifer, Gingko, and Cycad xylan lacks acetylation but is modified by arabinose and MeGlcA. Interestingly, the arabinosyl substitutions are located two xylosyl residues from MeGlcA, which is itself placed precisely on every sixth xylosyl residue. Notably, the Gnetophyta xylan is more akin to early-branching angiosperms and eudicot xylan, lacking arabinose but possessing acetylation on alternate xylosyl residues. All these precise substitution patterns are compatible with gymnosperm xylan binding to hydrophilic surfaces of cellulose. Molecular dynamics simulations support the stable binding of 2-fold screw conifer xylan to the hydrophilic face of cellulose microfibrils. Moreover, the binding of multiple xylan chains to adjacent planes of the cellulose fibril stabilizes the interaction further. Our results show that the type of xylan substitution varies, but an even pattern of xylan substitution is maintained among vascular plants. This suggests that 2-fold screw xylan binds hydrophilic faces of cellulose in eudicots, early-branching angiosperm, and gymnosperm cell walls.

A dynamic leaf gas-exchange strategy is conserved in woody plants under changing ambient CO2 : evidence from carbon isotope discrimination in paleo and CO2 enrichment studies

Rising atmospheric [CO2 ], ca , is expected to affect stomatal regulation of leaf gas-exchange of woody plants, thus influencing energy fluxes as well as carbon (C), water, and nutrient cycling of forests. Researchers have proposed various strategies for stomatal regulation of leaf gas-exchange that include maintaining a constant leaf internal [CO2 ], ci , a constant drawdown in CO2 (ca  - ci ), and a constant ci /ca . These strategies can result in drastically different consequences for leaf gas-exchange. The accuracy of Earth systems models depends in part on assumptions about generalizable patterns in leaf gas-exchange responses to varying ca . The concept of optimal stomatal behavior, exemplified by woody plants shifting along a continuum of these strategies, provides a unifying framework for understanding leaf gas-exchange responses to ca . To assess leaf gas-exchange regulation strategies, we analyzed patterns in ci inferred from studies reporting C stable isotope ratios (δ(13) C) or photosynthetic discrimination (∆) in woody angiosperms and gymnosperms that grew across a range of ca spanning at least 100 ppm. Our results suggest that much of the ca -induced changes in ci /ca occurred across ca spanning 200 to 400 ppm. These patterns imply that ca  - ci will eventually approach a constant level at high ca because assimilation rates will reach a maximum and stomatal conductance of each species should be constrained to some minimum level. These analyses are not consistent with canalization toward any single strategy, particularly maintaining a constant ci . Rather, the results are consistent with the existence of a broadly conserved pattern of stomatal optimization in woody angiosperms and gymnosperms. This results in trees being profligate water users at low ca , when additional water loss is small for each unit of C gain, and increasingly water-conservative at high ca , when photosystems are saturated and water loss is large for each unit C gain.

Drought responses of two gymnosperm species with contrasting stomatal regulation strategies under elevated [CO2] and temperature

Future climate regimes characterized by rising [CO2], rising temperatures and associated droughts may differentially affect tree growth and physiology. However, the interactive effects of these three factors are complex because elevated [CO2] and elevated temperature may generate differential physiological responses during drought. To date, the interactive effects of elevated [CO2] and elevated temperature on drought-induced tree mortality remain poorly understood in gymnosperm species that differ in stomatal regulation strategies. Water relations and carbon dynamics were examined in two species with contrasting stomatal regulation strategies: Pinus radiata D. Don (relatively isohydric gymnosperm; regulating stomata to maintain leaf water potential above critical thresholds) and Callitris rhomboidea R. Br (relatively anisohydric gymnosperm; allowing leaf water potential to decline as the soil dries), to assess response to drought as a function of [CO2] and temperature. Both species were grown in two [CO2] (C(a) (ambient, 400 μl l(-1)) and C(e) (elevated, 640 μl l(-1))) and two temperature (T(a) (ambient) and T(e) (ambient +4 °C)) treatments in a sun-lit glasshouse under well-watered conditions. Drought plants were then exposed to a progressive drought until mortality. Prior to mortality, extensive xylem cavitation occurred in both species, but significant depletion of non-structural carbohydrates was not observed in either species. Te resulted in faster mortality in P. radiata, but it did not modify the time-to-mortality in C. rhomboidea. C(e) did not delay the time-to-mortality in either species under drought or T(e) treatments. In summary, elevated temperature (+4 °C) had greater influence than elevated [CO2] (+240 μl l(-1)) on drought responses of the two studied gymnosperm species, while stomatal regulation strategies did not generally affect the relative contributions of hydraulic failure and carbohydrate depletion to mortality under severe drought.

Slower phloem transport in gymnosperm trees can be attributed to higher sieve element resistance

In trees, carbohydrates produced in photosynthesizing leaves are transported to roots and other sink organs over distances of up to 100 m inside a specialized transport tissue, the phloem. Angiosperm and gymnosperm trees have a fundamentally different phloem anatomy with respect to cell size, shape and connectivity. Whether these differences have an effect on the physiology of carbohydrate transport, however, is not clear. A meta-analysis of the experimental data on phloem transport speed in trees yielded average speeds of 56 cm h(-1) for angiosperm trees and 22 cm h(-1) for gymnosperm trees. Similar values resulted from theoretical modeling using a simple transport resistance model. Analysis of the model parameters clearly identified sieve element (SE) anatomy as the main factor for the significantly slower carbohydrate transport speed inside the phloem in gymnosperm compared with angiosperm trees. In order to investigate the influence of SE anatomy on the hydraulic resistance, anatomical data on SEs and sieve pores were collected by transmission electron microscopy analysis and from the literature for 18 tree species. Calculations showed that the hydraulic resistance is significantly higher in the gymnosperm than in angiosperm trees. The higher resistance is only partially offset by the considerably longer SEs of gymnosperms.

Stomatal and mesophyll conductances to CO₂ in different plant groups: underrated factors for predicting leaf photosynthesis responses to climate change?

The climate change conditions predicted for the end of the current century are expected to have an impact on the performance of plants under natural conditions. The variables which are foreseen to have a larger effect are increased CO2 concentration and temperature. Although it is generally considered CO2 assimilation rate could be increased by the increasing levels of CO2, it has been reported in previous studies that acclimation to high CO2 results in reductions of physiological parameters involved in photosynthesis, like the maximum carboxylation rate (Vc,max), stomatal conductance (gs) and mesophyll conductance to CO2 (gm). On the one hand, most of the previous modeling efforts have neglected the potential role played by the acclimation of gm to high CO2 and temperature. On the other hand, the effect of climate change on plant clades other than angiosperms, like ferns, has received little attention, and there are no studies evaluating the potential impact of increasing CO2 and temperature on these species. In this study we predicted responses of several representative species among angiosperms, gymnosperms and ferns to increasing CO2 and temperature. Our results show that species with lower photosynthetic capacity - such as some ferns and gymnosperms - would be proportionally more favored under these foreseen environmental conditions. The main reason for this difference is the lower diffusion limitation imposed by gs and gm in plants having high capacity for photosynthesis among the angiosperms, which reduces the positive effect of increasing CO2. However, this apparent advantage of low-diffusion species would be canceled if the two conductances - gs and gm - acclimate and are down regulated to high CO2, which is basically unknown, especially for gymnosperms and ferns. Hence, for a better understanding of different plant responses to future climate, studies are urged in which the actual photosynthetic response/acclimation to increased CO2 and temperature of ferns, gymnosperms and other under-evaluated plant groups is assessed.

Gymnosperms have increased capacity for electron leakage to oxygen (Mehler and PTOX reactions) in photosynthesis compared with angiosperms

Oxygen plays an important role in photosynthesis by participating in a number of O2-consuming reactions. O2 inhibits CO2 fixation by stimulating photorespiration, thus reducing plant production. O2 interacts with photosynthetic electron transport in the chloroplasts' thylakoids in two main ways: by accepting electrons from PSI (Mehler reaction); and by accepting electrons from reduced plastoquinone (PQ) mediated by the plastid terminal oxidase (PTOX). In this study, we show, using 101 plant species, that there is a difference in the potential for photosynthetic electron flow to O2 between angiosperms and gymnosperms. We found, from measurements of Chl fluorescence and leaf absorbance at 830 nm, (i) that electron outflow from PSII, as determined by decay kinetics of Chl fluorescence after application of a saturating light pulse, is more rapid in gymnosperms than in angiosperms; (ii) that the reaction center Chl of PSI (P700) is rapidly and highly oxidized in gymnosperms during induction of photosynthesis; and (iii) that these differences are dependent on oxygen. Finally, rates of O2 uptake measured by mass spectrometry in the absence of photorespiration were significantly promoted by illumination in dark-adapted leaves of gymnosperms, but not in those of angiosperms. The light-stimulated O2 uptake was around 10% of the maximum O2 evolution in gymnosperms and 1% in angiosperms. These results suggest that gymnosperms have increased capacity for electron leakage to oxygen in photosynthesis compared with angiosperms. The involvement of the Mehler reaction and PTOX in the electron flow to O2 is discussed.

Proton-dependent coniferin transport, a common major transport event in differentiating xylem tissue of woody plants

Lignin biosynthesis is an essential physiological activity of vascular plants if they are to survive under various environmental stresses on land. The biosynthesis of lignin proceeds in the cell wall by polymerization of precursors; the initial step of lignin polymerization is the transportation of lignin monomers from the cytosol to the cell wall, which is critical for lignin formation. There has been much debate on the transported form of the lignin precursor, either as free monolignols or their glucosides. In this study, we performed biochemical analyses to characterize the membrane transport mechanism of lignin precursors using angiosperms, hybrid poplar (Populus sieboldii × Populus grandidentata) and poplar (Populus sieboldii), as well gymnosperms, Japanese cypress (Chamaecyparis obtusa) and pine (Pinus densiflora). Membrane vesicles prepared from differentiating xylem tissues showed clear ATP-dependent transport activity of coniferin, whereas less than 4% of the coniferin transport activity was seen for coniferyl alcohol. Bafilomycin A1 and proton gradient erasers markedly inhibited coniferin transport in hybrid poplar membrane vesicles; in contrast, vanadate had no effect. Cis-inhibition experiments suggested that this transport activity was specific for coniferin. Membrane fractionation of hybrid poplar microsomes demonstrated that transport activity was localized to the tonoplast- and endomembrane-rich fraction. Differentiating xylem of Japanese cypress exhibited almost identical transport properties, suggesting the involvement of a common endomembrane-associated proton/coniferin antiport mechanism in the lignifying tissues of woody plants, both angiosperms and gymnosperms.

Conserved function of core clock proteins in the gymnosperm Norway spruce (Picea abies L. Karst)

From studies of the circadian clock in the plant model species Arabidopsis (Arabidopsis thaliana), a number of important properties and components have emerged. These include the genes CIRCADIAN CLOCK ASSOCIATED 1 (CCA1), GIGANTEA (GI), ZEITLUPE (ZTL) and TIMING OF CAB EXPRESSION 1 (TOC1 also known as PSEUDO-RESPONSE REGULATOR 1 (PRR1)) that via gene expression feedback loops participate in the circadian clock. Here, we present results from ectopic expression of four Norway spruce (Picea abies) putative homologs (PaCCA1, PaGI, PaZTL and PaPRR1) in Arabidopsis, their flowering time, circadian period length, red light response phenotypes and their effect on endogenous clock genes were assessed. For PaCCA1-ox and PaZTL-ox the results were consistent with Arabidopsis lines overexpressing the corresponding Arabidopsis genes. For PaGI consistent results were obtained when expressed in the gi2 mutant, while PaGI and PaPRR1 expressed in wild type did not display the expected phenotypes. These results suggest that protein function of PaCCA1, PaGI and PaZTL are at least partly conserved compared to Arabidopsis homologs, however further studies are needed to reveal the protein function of PaPRR1. Our data suggest that components of the three-loop network typical of the circadian clock in angiosperms were present before the split of gymnosperms and angiosperms.

Rate of belowground carbon allocation differs with successional habit of two afromontane trees

BACKGROUND

Anthropogenic disturbance of old-growth tropical forests increases the abundance of early successional tree species at the cost of late successional ones. Quantifying differences in terms of carbon allocation and the proportion of recently fixed carbon in soil CO(2) efflux is crucial for addressing the carbon footprint of creeping degradation.

METHODOLOGY

We compared the carbon allocation pattern of the late successional gymnosperm Podocarpus falcatus (Thunb.) Mirb. and the early successional (gap filling) angiosperm Croton macrostachyus Hochst. es Del. in an Ethiopian Afromontane forest by whole tree (13)CO(2) pulse labeling. Over a one-year period we monitored the temporal resolution of the label in the foliage, the phloem sap, the arbuscular mycorrhiza, and in soil-derived CO(2). Further, we quantified the overall losses of assimilated (13)C with soil CO(2) efflux.

PRINCIPAL FINDINGS

(13)C in leaves of C. macrostachyus declined more rapidly with a larger size of a fast pool (64% vs. 50% of the assimilated carbon), having a shorter mean residence time (14 h vs. 55 h) as in leaves of P. falcatus. Phloem sap velocity was about 4 times higher for C. macrostachyus. Likewise, the label appeared earlier in the arbuscular mycorrhiza of C. macrostachyus and in the soil CO(2) efflux as in case of P. falcatus (24 h vs. 72 h). Within one year soil CO(2) efflux amounted to a loss of 32% of assimilated carbon for the gap filling tree and to 15% for the late successional one.

CONCLUSIONS

Our results showed clear differences in carbon allocation patterns between tree species, although we caution that this experiment was unreplicated. A shift in tree species composition of tropical montane forests (e.g., by degradation) accelerates carbon allocation belowground and increases respiratory carbon losses by the autotrophic community. If ongoing disturbance keeps early successional species in dominance, the larger allocation to fast cycling compartments may deplete soil organic carbon in the long run.

Is iron phloem mobile during senescence in trees? A reinvestigation of Rissmüller's finding of 1874

As long as 130 years ago Rissmüller reported substantial retranslocation of iron (Fe) from beech leaves (Fagus sylvatica L.) shortly before leaf fall. This rather limited report on Fe retranslocation via the phloem in plants was the reason for this research to study changes in Fe content in individual beech leaves in more detail during the vegetative period. Besides Fe, other micronutrients and particularly Ca and K, well known to differ substantially in phloem mobility, were analysed as mineral nutrient markers. In addition to beech, other deciduous and evergreen species of Angiosperms and Gymnosperms were also studied. As expected, there was no evidence of Ca retranslocation from senescent leaves, while K as a phloem mobile mineral nutrient was retranslocated in fall in deciduous but not in evergreen trees. There was no indication to support Rissmüller's finding of Fe retranslocation in any of the different species studied. From these results, we conclude that natural leaf senescence of trees during late season does not induce retranslocation of Fe and other micronutrients. Possible reasons for the absence of a distinct retranslocation of Fe in the species studied during late season senescence are the lack of a sink activity, as for example the development of seeds in annual plant species (e.g., cereals), or the presence of a root system still active enough to provide Fe and other mineral nutrients for plant demand, and both factors have to be considered in further studies. Reviewing the data in the literature on Fe and Zn retranslocation during senescence, we conclude that in principle both micronutrients are potentially phloem mobile. However, various prerequisites are needed for the occurrence of phloem mobility which were absent in the plant species studied. Regardless of this conclusion, we recommend that in general early published research data need a critical re-evaluation.

Pollen tube reuses intracellular components of nucellar cells undergoing programmed cell death in Pinus densiflora

Through the process known as programmed cell death (PCD), nucelli of Pinus densiflora serve as the transmitting tissue for growth of the pollen tube. We sought to clarify the processes of degradation of nucellar cell components and their transport to the pollen tube during PCD in response to pollen tube penetration of such nucelli. Stimulated by pollination, synthesis of large amounts of starch grains occurred in cells in a wide region of the nucellus, but as the pollen tube penetrated the nucellus, starch grains were degraded in amyloplasts of nucellar cells. In cells undergoing PCD, electron-dense vacuoles with high membrane contrast appeared, assumed a variety of autophagic structures, expanded, and ultimately collapsed and disappeared. Vesicles and electron-dense amorphous materials were released inside the thickened walls of cells undergoing PCD, and those vesicles and materials reaching the pollen tube after passing through the extracellular matrix were taken into the tube by endocytosis. These results show that in PCD of nucellar cells, intracellular materials are degraded in amyloplasts and vacuoles, and some of the degraded material is supplied to the pollen tube by vesicular transport to support tube growth.

Symplasmic transport and phloem loading in gymnosperm leaves

Despite more than 130 years of research, phloem loading is far from being understood in gymnosperms. In part this is due to the special architecture of their leaves. They differ from angiosperm leaves among others by having a transfusion tissue between bundle sheath and the axial vascular elements. This article reviews the somewhat inaccessible and/or neglected literature and identifies the key points for pre-phloem transport and loading of photoassimilates. The pre-phloem pathway of assimilates is structurally characterized by a high number of plasmodesmata between all cell types starting in the mesophyll and continuing via bundle sheath, transfusion parenchyma, Strasburger cells up to the sieve elements. Occurrence of median cavities and branching indicates that primary plasmodesmata get secondarily modified and multiplied during expansion growth. Only functional tests can elucidate whether this symplasmic pathway is indeed continuous for assimilates, and if phloem loading in gymnosperms is comparable with the symplasmic loading mode in many angiosperm trees. In contrast to angiosperms, the bundle sheath has properties of an endodermis and is equipped with Casparian strips or other wall modifications that form a domain border for any apoplasmic transport. It constitutes a key point of control for nutrient transport, where the opposing flow of mineral nutrients and photoassimilates has to be accommodated in each single cell, bringing to mind the principle of a revolving door. The review lists a number of experiments needed to elucidate the mode of phloem loading in gymnosperms.

Evidence of a general 2/3-power law of scaling leaf nitrogen to phosphorus among major plant groups and biomes

Scaling relations among plant traits are both cause and consequence of processes at organ-to-ecosystem scales. The relationship between leaf nitrogen and phosphorus is of particular interest, as both elements are essential for plant metabolism; their limited availabilities often constrain plant growth, and general relations between the two have been documented. Herein, we use a comprehensive dataset of more than 9300 observations of approximately 2500 species from 70 countries to examine the scaling of leaf nitrogen to phosphorus within and across taxonomical groups and biomes. Power law exponents derived from log-log scaling relations were near 2/3 for all observations pooled, for angiosperms and gymnosperms globally, and for angiosperms grouped by biomes, major functional groups, orders or families. The uniform 2/3 scaling of leaf nitrogen to leaf phosphorus exists along a parallel continuum of rising nitrogen, phosphorus, specific leaf area, photosynthesis and growth, as predicted by stoichiometric theory which posits that plants with high growth rates require both high allocation of phosphorus-rich RNA and a high metabolic rate to support the energy demands of macromolecular synthesis. The generality of this finding supports the view that this stoichiometric scaling relationship and the mechanisms that underpin it are foundational components of the living world. Additionally, although abundant variance exists within broad constraints, these results also support the idea that surprisingly simple rules regulate leaf form and function in terrestrial ecosystems.

Stable oil bodies sheltered by a unique caleosin in cycad megagametophytes

Stable oil bodies of smaller sizes and higher thermostability were isolated from mature cycad (Cycas revoluta) megagametophytes compared with those isolated from sesame seeds. Immunological cross-recognition revealed that cycad oil bodies contained a major protein of 27 kDa, tentatively identified as caleosin, while oleosin, the well-known structural protein, was apparently absent. Mass spectrometric analysis showed that the putative cycad caleosin possessed a tryptic fragment of 15 residues matching to that of a theoretical moss caleosin. A complete cDNA fragment encoding this putative caleosin was obtained by PCR cloning using a primer designed according to the tryptic peptide and another one designed according to a highly conservative region among diverse caleosins. The identification of this clone was subsequently confirmed by immunodetection and MALDI-MS analyses of its recombinant fusion protein over-expressed in Escherichia coli and the native form from cycad oil bodies. Stable artificial oil bodies were successfully constituted with triacylglycerol, phospholipid and the recombinant fusion protein containing the cycad caleosin. These results suggest that stable oil bodies in cycad megagametophytes are mainly sheltered by a unique structural protein caleosin.

Nectar and pollination drops: how different are they?

BACKGROUND

Pollination drops and nectars (floral nectars) are secretions related to plant reproduction. The pollination drop is the landing site for the majority of gymnosperm pollen, whereas nectar of angiosperm flowers represents a common nutritional resource for a large variety of pollinators. Extrafloral nectars also are known from all vascular plants, although among the gymnosperms they are restricted to the Gnetales. Extrafloral nectars are not generally involved in reproduction but serve as 'reward' for ants defending plants against herbivores (indirect defence).

SCOPE

Although very different in their task, nectars and pollination drops share some features, e.g. basic chemical composition and eventual consumption by animals. This has led some authors to call these secretions collectively nectar. Modern techniques that permit chemical analysis and protein characterization have very recently added important information about these sugary secretions that appear to be much more than a 'reward' for pollinating (floral nectar) and defending animals (extrafloral nectar) or a landing site for pollen (pollination drop).

CONCLUSIONS

Nectar and pollination drops contain sugars as the main components, but the total concentration and the relative proportions are different. They also contain amino acids, of which proline is frequently the most abundant. Proteomic studies have revealed the presence of common functional classes of proteins such as invertases and defence-related proteins in nectar (floral and extrafloral) and pollination drops. Invertases allow for dynamic rearrangement of sugar composition following secretion. Defence-related proteins provide protection from invasion by fungi and bacteria. Currently, only few species have been studied in any depth. The chemical composition of the pollination drop must be investigated in a larger number of species if eventual phylogenetic relationships are to be revealed. Much more information can be provided from further proteomic studies of both nectar and pollination drop that will contribute to the study of plant reproduction and evolution.

Analysis of the soluble cell wall proteome of gymnosperms

We analyzed the cell wall proteome of lignifying suspension cell cultures (SCCs) from four gymnosperms that differ in evolution degree. This analysis showed the presence of "peptide sequence tags" (PSTs) corresponding to glucan endo-1,3-beta-D-glucosidase, xyloglucan-endotrans-glucosylase/hydrolase, chitinases, thaumatin-like proteins and proteins involved in lignin/lignan biosynthesis, such as dirigent-like proteins and peroxidases. Surprisingly, and given the abundance of peroxidases in the cell wall proteome of these gymnosperms, PSTs corresponding to peroxidases were only detected in tryptic fragments of the cell wall proteome of Cycas revoluta. The current lack of knowledge regarding C. revoluta peroxidases led us to purify, characterize and partially sequence the peroxidases responsible for lignin biosynthesis in this species. This yielded three peroxidase-enriched fractions: CrPrx 1, CrPrx 2 and CrPrx 3. Analyses of tryptic peptides of CrPrx 2 (32kDa) and CrPrx 3 (26kDa) suggest that CrPrx 3 arises from CrPrx 2 by protein truncation, and that CrPrx 3 apparently constitutes a post-translational modification of CrPrx 2. That CrPrx 2 and CrPrx 3 are apparently the same enzyme was also deduced from the similarity between the k(cat) shown by both peroxidases for the three monolignols. These results emphasize the analogies between the cell wall proteome of gymnosperms and angiosperms, the complexity of the peroxidase proteome, and the difficulties involved in establishing fine structure-function relationships.

Discrete shoot and root stem cell-promoting WUS/WOX5 functions are an evolutionary innovation of angiosperms

The morphologically diverse bodies of seed plants comprising gymnosperms and angiosperms, which separated some 350 Ma, grow by the activity of meristems containing stem cell niches. In the dicot model Arabidopsis thaliana, these are maintained by the stem cell-promoting functions of WUS and WUSCHEL-related homeobox 5 (WOX5) in the shoot and the root, respectively. Both genes are members of the WOX gene family, which has a monophyletic origin in green algae. The establishment of the WOX gene phylogeny from basal land plants through gymnosperms to basal and higher angiosperms reveals three major branches: a basal clade consisting of WOX13-related genes present in some green algae and throughout all land plant genomes, a second clade containing WOX8/9/11/12 homologues, and a modern clade restricted to seed plants. The analysis of the origin of the modern branch in two basal angiosperms (Amborella trichopoda and Nymphaea jamesoniana) and three gymnosperms (Pinus sylvestris, Ginkgo biloba, and Gnetum gnemon) shows that all members of the modern clade consistently found in monocots and dicots exist at the base of the angiosperm lineage, including WUS and WOX5 orthologues. In contrast, our analyses identify a single WUS/WOX5 homologue in all three gymnosperm genomes, consistent with a monophyletic origin in the last common ancestor of gymnosperms and angiosperms. Phylogenetic data, WUS- and WOX5-specific evolutionary signatures, as well as the expression pattern and stem cell-promoting function of the single gymnosperm WUS/WOX5 pro-orthologue in Arabidopsis indicate a gene duplication event followed by subfunctionalization at the base of angiosperms.

Serine proteinase inhibitors in seeds of Cycas siamensis and other gymnosperms

Seeds of 32 species selected from two of the four major groups of gymnosperms, the ancient Cycadales and the economically important Coniferales, were analysed for inhibitors (I) of the serine proteinases trypsin (T), chymotrypsin (C), subtilisin (S) and elastase (E) using isoelectric focusing (IEF) combined with gelatin replicas. Subtilisin inhibitors were detected in 17 species, being particularly active in the Cycadales. Several species of the genera Cephalotaxus, Pseudotsuga and Cycas contained inhibitors active against elastase while strong CSTIs and CSIs were also present in Cycas pectinata and C. siamensis. No inhibitors were detected in seeds of Chamaecyparis, Thuja, Abies, Larix, Picea and Pinus spp. Serine proteinase inhibitors were purified from seeds of C. siamensis by affinity chromatography using trypsin and chymotrypsin, IEF and SDS-PAGE. Several CSTI components with M(r) ranging from 4000 to 18,000 were partially sequenced using Edman degradation and mass spectrometry. Most of the sequences were similar to a hypothetical protein encoded by an mRNA from sporophylls of C. rumphii which in turn was similar to Kunitz-type proteinase inhibitors from flowering plants. Analysis of expressed sequence tag (EST) databases confirmed the presence of mRNAs encoding Kunitz-type inhibitors in the Cycadales and Coniferales and also demonstrated their presence in a third major group of gymnosperms, the Ginkgoales. This is the first report of Kunitz-type serine proteinase inhibitors from plants other than Angiosperms.

Anatomical traits associated with absorption and mycorrhizal colonization are linked to root branch order in twenty-three Chinese temperate tree species

* Different portions of tree root systems play distinct functional roles, yet precisely how to distinguish roots of different functions within the branching fine-root system is unclear. * Here, anatomy and mycorrhizal colonization was examined by branch order in 23 Chinese temperate tree species of both angiosperms and gymnosperms forming ectomycorrhizal and arbuscular-mycorrhizal associations. * Different branch orders showed marked differences in anatomy. First-order roots exhibited primary development with an intact cortex, a high mycorrhizal colonization rate and a low stele proportion, thus serving absorptive functions. Second and third orders had both primary and secondary development. Fourth and higher orders showed mostly secondary development with no cortex or mycorrhizal colonization, and thus have limited role in absorption. Based on anatomical traits, it was estimated that c. 75% of the fine-root length was absorptive, and 68% was mycorrhizal, averaged across species. * These results showed that: order predicted differences in root anatomy in a relatively consistent manner across species; anatomical traits associated with absorption and mycorrhizal colonization occurred mainly in the first three orders; the single diameter class approach may have overestimated absorptive root length by 25% in temperate forests.

Pollination drop in Juniperus communis: response to deposited material

BACKGROUND AND AIMS

The pollination drop is a liquid secretion produced by the ovule and exposed outside the micropyle. In many gymnosperms, pollen lands on the surface of the pollination drop, rehydrates and enters the ovule as the drop retracts. The objective of this work was to study the formation of the pollination drop in Juniperus communis, its carbohydrate composition and the response to deposition of conspecific pollen, foreign pollen and other particulate material, in an attempt to clarify the mechanism of pollination drop retraction.

METHODS

Branches with female cones close to pollination drop secretion were collected. On the first day of pollination drop exposure, an eyelash mounted on a wooden stick with paraffin was used to collect pollen or silica gel particles, which were then deposited by contact with the drop. Volume changes in pollination drops were measured by using a stereomicroscope with a micrometer eyepiece 3 h after deposition. The volume of non-pollinated control drops was also recorded. On the first day of secretion, drops were also collected for sugar analysis by high-performance liquid chromatography.

KEY RESULTS

The pollination drop persisted for about 12 d if not pollinated, and formed again after removal for up to four consecutive days. After pollination with viable conspecific pollen, the drop retracted quickly and did not form again. Partial withdrawal occurred after deposition of other biological and non-biological material. Fructose was the dominant sugar; glucose was also present but at a much lower percentage.

CONCLUSIONS

Sugar analysis confirmed the general trend of fructose dominance in gymnosperm pollination drops. Complete pollination drop withdrawal appears to be triggered by a biochemical mechanism resulting from interaction between pollen and drop constituents. The results of particle deposition suggest the existence of a non-specific, particle-size-dependent mechanism that induces partial pollination drop withdrawal. These results suggest that the non-specific response may decrease the probability of pollen landing on the drop, reducing pollination efficiency.

Dynamic variation in sapwood specific conductivity in six woody species

Our goals were to quantify how non-embolism-inducing pressure gradients influence trunk sapwood specific conductivity (k(s)) and to compare the impacts of constant and varying pressure gradients on k(s) with KCl and H2O as the perfusion solutions. We studied six woody species (three conifers and three angiosperms) which varied in pit membrane structure, pit size and frequency of axial water transport across pits (long versus short conduits). Both stepwise ("steady") and nonlinear continuous ("non-steady") decreases in the pressure gradient led to decreased k(s) in all species but white oak (Quercus garryana Dougl. ex Hook), a ring-porous and long-vesseled angiosperm. In one diffuse-porous angiosperm (red alder, Alnus rubra Bong.) and two conifers (western red cedar, Thuja plicata Donn. ex D. Don, and Douglas-fir, Pseudotsuga menziesii (Mirb.) Franco), k(s) was 10-30% higher under steady pressure gradients than under non-steady pressure gradients, and a decrease in the pressure gradient from 0.15 to 0.01 MPa m(-1) caused a 20-42% decrease in k(s). In another diffuse-porous angiosperm (maple, Acer macrophyllum Pursh) and in a third coniferous species (western hemlock, Tsuga heterophylla (Raf.) Sarg), there was no difference between k(s) measured under steady and non-steady pressure gradients. With the exception of western red cedar, a conifer with simple pit membranes, the differences in k(s) between low and high pressure gradients tended to be lower in the conifers than in the diffuse-porous angiosperms. In Douglas-fir, western red cedar and the diffuse-porous angiosperms, k(s) was higher when measured with KCl than with H2O. In white oak, there were no differences in k(s) whether measured under steady or non-steady pressure gradients, or when xylem was perfused with KCl or H2O. The species differences in the behavior of k(s) suggest that elasticity of the pit membrane was the main factor causing k(s) to be disproportionate to the pressure gradient and to the different pressure regimes. The results imply that, if nonlinearities in pressure-flux relationships are ignored when modeling tree water relations in vivo, large errors will result in the predictions of tree water status and its impact on stomatal control of transpiration and photosynthesis.

Characterization of the expression patterns of LEAFY/FLORICAULA and NEEDLY orthologs in female and male cones of the conifer genera Picea, Podocarpus, and Taxus: implications for current evo-devo hypotheses for gymnosperms

The identity of genes causally implicated in the development and evolutionary origin of reproductive characters in gymnosperms is largely unknown. Working within the framework of plant evolutionary developmental biology, here we have cloned, sequenced, performed phylogenetic analyses upon and tested the expression patterns of LEAFY/FLORICAULA and NEEDLY orthologs in reproductive structures from selected species of the conifer genera Picea, Podocarpus, and Taxus. Contrary to expectations based on previous assessments, expression of LFY/FLO and NLY in cones of these taxa was found to occur simultaneously in a single reproductive axis, initially overlapping but later in mutually exclusive primordia and/or groups of developing cells in both female and male structures. These observations directly affect the status of the "mostly male theory" for the origin of the angiosperm flower. On the other hand, comparative spatiotemporal patterns of the expression of these genes suggest a complex genetic regulatory network of cone development, as well as a scheme of functional divergence for LFY/FLO with respect to NLY homologs in gymnosperms, both with clear heterochronic aspects. Results presented in this study contribute to the understanding of the molecular-genetic basis of morphological evolution in conifer cones, and may aid in establishing a foundation for gymnosperm-specific, testable evo-devo hypotheses.

Structural motifs of syringyl peroxidases predate not only the gymnosperm-angiosperm divergence but also the radiation of tracheophytes

* The most distinctive variation in the monomer composition of lignins in vascular land plants is that found between the two main groups of seed plants. Thus, while gymnosperm lignins are typically composed of guaiacyl (G) units, angiosperm lignins are largely composed of similar levels of G and syringyl (S) units. * However, and contrary to what might be expected, peroxidases isolated from basal (Cycadales and Ginkgoales) and differentially evolved (Coniferales and Gnetales) gymnosperms are also able to oxidize S moieties, and this ability is independent of the presence or absence of S-type units in their lignins. * The results obtained led us to look at the protein database to search for homologies between gymnosperm peroxidases and true eudicot S-peroxidases, such as the Zinnia elegans peroxidase. * The findings showed that certain structural motifs characteristic of eudicot S-peroxidases (certain amino acid sequences and beta-sheet secondary structures) predate the gymnosperm-angiosperm divergence and the radiation of tracheophytes, since they are found not only in peroxidases from basal gymnosperms, ferns and lycopods, but also in peroxidases from the moss Physcomitrella patens (Bryopsida) and the liverwort Marchantia polymorpha (Marchantiopsida), which, as typical of bryophytes, do not have xylem tissue nor lignins.

Purification and biological effects of Araucaria angustifolia (Araucariaceae) seed lectin

This paper describes the purification and characterization of a new N-acetyl-d-glucosamine-specific lectin from Araucaria angustifolia (AaL) seeds (Araucariaceae) and its anti-inflammatory and antibacterial activities. AaL was purified using a combination of affinity chromatography on a chitin column and ion exchange chromatography on Sephacel-DEAE. The pure protein has 8.0kDa (SDS-PAGE) and specifically agglutinates rabbit erythrocytes, effect that was independent of the presence of divalent cations and was inhibited after incubation with glucose and N-acetyl-d-glucosamine. AaL showed antibacterial activity against Gram-negative and Gram-positive strains, shown by scanning electron microscopy. AaL, intravenously injected into rats, showed anti-inflammatory effect, via carbohydrate site interaction, in the models of paw edema and peritonitis. This lectin can be used as a tool for studying bacterial infections and inflammatory processes.

Neurotoxic flying foxes as dietary items for the Chamorro people, Marianas Islands

Fanihi -- flying foxes (Pteropus mariannus mariannus, Pteropodidae) -- are a highly salient component of the traditional Chamorro diet. A neurotoxic, non-protein amino acid, beta-methylamino-l-alanine (BMAA) accumulates in flying foxes, which forage on the seeds of Cycas micronesica (Cycadaceae) in Guam's forests. BMAA occurs throughout flying fox tissues both as a free amino acid and in a protein-bound form. It is not destroyed by cooking. Protein-bound BMAA also remains in cycad flour which has been washed and prepared by the Chamorro people as tortillas, dumplings, and thickened soups. Other animals that forage on cycad seeds may also provide BMAA inputs into the traditional Chamorro diet.

Seasonal patterns of leaf gas exchange and water relations in dry rain forest trees of contrasting leaf phenology

Diurnal and seasonal patterns of leaf gas exchange and water relations were examined in tree species of contrasting leaf phenology growing in a seasonally dry tropical rain forest in north-eastern Australia. Two drought-deciduous species, Brachychiton australis (Schott and Endl.) A. Terracc. and Cochlospermum gillivraei Benth., and two evergreen species, Alphitonia excelsa (Fenzal) Benth. and Austromyrtus bidwillii (Benth.) Burret. were studied. The deciduous species had higher specific leaf areas and maximum photosynthetic rates per leaf dry mass in the wet season than the evergreens. During the transition from wet season to dry season, total canopy area was reduced by 70-90% in the deciduous species and stomatal conductance (g(s)) and assimilation rate (A) were markedly lower in the remaining leaves. Deciduous species maintained daytime leaf water potentials (Psi(L)) at close to or above wet season values by a combination of stomatal regulation and reduction in leaf area. Thus, the timing of leaf drop in deciduous species was not associated with large negative values of daytime Psi(L) (greater than -1.6 MPa) or predawn Psi(L) (greater than -1.0 MPa). The deciduous species appeared sensitive to small perturbations in soil and leaf water status that signalled the onset of drought. The evergreen species were less sensitive to the onset of drought and g(s) values were not significantly lower during the transitional period. In the dry season, the evergreen species maintained their canopies despite increasing water-stress; however, unlike Eucalyptus species from northern Australian savannas, A and g(s) were significantly lower than wet season values.

Changes in cell wall composition associated with maturation in the gymnosperm Araucaria angustifolia

A general structural characterization and an investigation on the dynamics of formation of cell wall polysaccharides was performed, using plantlets stem samples from a typical gymnosperm from southern Brazil, Araucaria angustifolia, as experimental model. Microscopic examination and monosaccharide composition of plantlet segments at different heights were carried out to show the representative portions of stem cell wall development. The plantlets were divided in portions (tip, middle and base) which were submitted to sequential extractions. The extraction with water gave rise to large amounts of pectic material in the three portions and more highly substituted pectins occurred in the tip portion of the stems. Increase in alkali concentration extracted, respectively, higher amounts of xyloglucan structurally similar to those from dicotyledons. However, oligosaccharides containing galactose and fucose where found in higher amounts in base than tip portion. The changes in cell wall composition suggest that the development in gymnosperm cell walls follow the same key events as found in dicotyledon walls (type I).

Contrasting seasonal leaf habits of canopy trees between tropical dry-deciduous and evergreen forests in Thailand

We compared differences in leaf properties, leaf gas exchange and photochemical properties between drought-deciduous and evergreen trees in tropical dry forests, where soil nutrients differed but rainfall was similar. Three canopy trees (Shorea siamensis Miq., Xylia xylocarpa (Roxb.) W. Theob. and Vitex peduncularis Wall. ex Schauer) in a drought-deciduous forest and a canopy tree (Hopea ferrea Lanessan) in an evergreen forest were selected. Soil nutrient availability is lower in the evergreen forest than in the deciduous forest. Compared with the evergreen tree, the deciduous trees had shorter leaf life spans, lower leaf masses per area, higher leaf mass-based nitrogen (N) contents, higher leaf mass-based photosynthetic rates (mass-based P(n)), higher leaf N-based P(n), higher daily maximum stomatal conductance (g(s)) and wider conduits in wood xylem. Mass-based P(n) decreased from the wet to the dry season for all species. Following onset of the dry season, daily maximum g(s) and sensitivity of g(s) to leaf-to-air vapor pressure deficit remained relatively unchanged in the deciduous trees, whereas both properties decreased in the evergreen tree during the dry season. Photochemical capacity and non-photochemical quenching (NPQ) of photosystem II (PSII) also remained relatively unchanged in the deciduous trees even after the onset of the dry season. In contrast, photochemical capacity decreased and NPQ increased in the evergreen tree during the dry season, indicating that the leaves coped with prolonged drought by down-regulating PSII. Thus, the drought-avoidant deciduous species were characterized by high N allocation for leaf carbon assimilation, high water use and photoinhibition avoidance, whereas the drought-tolerant evergreen was characterized by low N allocation for leaf carbon assimilation, conservative water use and photoinhibition tolerance.