Iron Nutrition in Plants: Towards a New Paradigm?

Iron (Fe) is an essential micronutrient for plant growth and development. Fe availability affects crops' productivity and the quality of their derived products and thus human nutrition. Fe is poorly available for plant use since it is mostly present in soils in the form of insoluble oxides/hydroxides, especially at neutral to alkaline pH. How plants cope with low-Fe conditions and acquire Fe from soil has been investigated for decades. Pioneering work highlighted that plants have evolved two different strategies to mine Fe from soils, the so-called Strategy I (Fe reduction strategy) and Strategy II (Fe chelation strategy). Strategy I is employed by non-grass species whereas graminaceous plants utilize Strategy II. Recently, it has emerged that these two strategies are not fully exclusive and that the mechanism used by plants for Fe uptake is directly shaped by the characteristics of the soil on which they grow (e.g., pH, oxygen concentration). In this review, recent findings on plant Fe uptake and the regulation of this process will be summarized and their impact on our understanding of plant Fe nutrition will be discussed.

Temporal Control of Seed Development in Dicots: Molecular Bases, Ecological Impact and Possible Evolutionary Ramifications

In flowering plants, seeds serve as organs of both propagation and dispersal. The developing seed passes through several consecutive stages, following a conserved general outline. The overall time needed for a seed to develop, however, may vary both within and between plant species, and these temporal developmental properties remain poorly understood. In the present paper, we summarize the existing data for seed development alterations in dicot plants. For genetic mutations, the reported cases were grouped in respect of the key processes distorted in the mutant specimens. Similar phenotypes arising from the environmental influence, either biotic or abiotic, were also considered. Based on these data, we suggest several general trends of timing alterations and how respective mechanisms might add to the ecological plasticity of the families considered. We also propose that the developmental timing alterations may be perceived as an evolutionary substrate for heterochronic events. Given the current lack of plausible models describing timing control in plant seeds, the presented suggestions might provide certain insights for future studies in this field.

Thymol Chemotype Origanum vulgare L. Essential Oil as a Potential Selective Bio-Based Herbicide on Monocot Plant Species

Searching for new bio-based herbicides is crucial for decreasing chemical pollution, protecting the environment, and sustaining biodiversity. Origanum vulgare is considered a promising source of essential oil with herbicidal effect. The mode of action is not known. The present study focused on (1) comparison of phytotoxic activity of Origanum vulgare EO on monocot (Triticum aestivum and Hordeum vulgare) and dicot species (Lepidium sativum and Sinapis alba); (2) and evaluating other antimicrobial biological activities against phytopatogen bacteria (Clavibacter michiganensis, Pseudomonas syringae pv. phaseolicola, Pseudomonas savastanoi, and Xanthomonas campestris); antifungal activity against Monilinia fructicola, Aspergillus niger, Penicillium expansum, and Botrytis cinerea; cytotoxic activity and antioxidant activity. According to the GC/MS analyses, the EO belongs to the thymol chemotype O. vulgare with its high content of thymol (76%). Germination of all four species was not influenced by EO. The phytotoxic effect was statistically significant in the monocot species, while in the dicot species the opposite was observed-a stimulation effect, which was also statistically significant. Strong biological activity of O. vulgare EO was noted on all phytopatogen bacteria and fungi in the highest dose. Cytotoxic activity showed an IC₅₀ = 50.5 μg/mL. Antioxidant activity showed an IC₅₀ = 106.6 μg/mL after 45 min experimental time. Based on the presented results, it is possible to conclude that thymol chemotype O. vulgare essential oil could be potentially used as a herbicide with selective effects on monocot plant species.

Calcium plus magnesium indicates digestibility: the significance of the second major axis of plant chemical variation for ecological processes

Plant variation in nutrient concentrations encompasses two major axes. The first is connected to nitrogen (N) and phosphorus (P), reflects growth rate and has been designated as the leaf economics spectrum (LES) while the second follows the gradient in calcium (Ca) and magnesium (Mg) and mirrors cell structural differences. Here, we tested in grasslands whether the sum Ca + Mg concentrations is a better indicator of digestibility than LES constituents. Structural equation modelling revealed that the total effect size of N (0.30) on digestibility was much lower than that of Ca + Mg (0.58). The N effect originated predominantly from sampling date (biomass ageing), while the Ca + Mg effect largely from phylogenetic composition (proportion of monocots). Thus, plant variation in partially substitutable divalent cations seems to play a significant role in biomass digestion by ruminants. This finding contests, together with litter decomposition studies, the prominent role of the LES for understanding both fundamental ecological processes.

Conservation analysis of long non-coding RNAs in plants

Long non-coding RNAs (lncRNAs) are gene regulators that have vital roles in development and adaptation to the environment in eukaryotes. However, the structural and evolutionary analyses of plant lncRNAs are limited. In this study, we performed an analysis of lncRNAs in five monocot and five dicot species. Our results showed that plant lncRNA genes were generally shorter and had fewer exons than protein-coding genes. The numbers of lncRNAs were positively correlated with the numbers of protein-coding genes in different plant species, despite a high range of variation. Sequence conservation analysis showed that the majority of lncRNAs had high sequence conservation at the intra-species and sub-species levels, reminiscent of protein-coding genes. At the inter-species level, a subset of lncRNAs were highly diverged at the nucleotide level, but conserved by position. Interestingly, we found that plant lncRNAs have identical splicing signals, and those which can form precursors or targets of miRNAs have a conservative identity in different species. We also revealed that most of the lowly expressed lncRNAs were tissue-specific, while those highly conserved were constitutively transcribed. Meanwhile, we characterized a subset of rice lncRNAs that were co-expressed with their adjacent protein-coding genes, suggesting they may play cis-regulatory roles. These results will contribute to understanding the biological significance and evolution of lncRNAs in plants.

The role of WOX genes in flower development

BACKGROUND

WOX (Wuschel-like homeobOX) genes form a family of plant-specific HOMEODOMAIN transcription factors, the members of which play important developmental roles in a diverse range of processes. WOX genes were first identified as determining cell fate during embryo development, as well as playing important roles in maintaining stem cell niches in the plant. In recent years, new roles have been identified in plant architecture and organ development, particularly at the flower level.

SCOPE

In this review, the role of WOX genes in flower development and flower architecture is highlighted, as evidenced from data obtained in the last few years. The roles played by WOX genes in different species and different flower organs are compared, and differential functional recruitment of WOX genes during flower evolution is considered.

CONCLUSIONS

This review compares available data concerning the role of WOX genes in flower and organ architecture among different species of angiosperms, including representatives of monocots and eudicots (rosids and asterids). These comparative data highlight the usefulness of the WOX gene family for evo-devo studies of floral development.

Novel insights from non-conserved microRNAs in plants

Plant microRNAs (miRNAs), a class of small non-coding regulatory RNAs, are canonically 20-24 nucleotides in length and bind to complementary target RNA sequences, guiding target attenuation via mRNA degradation or translation inhibition. Of the annotated miRNA families, evolutionarily conserved families have been well known to extensively regulate analogous targets and play critical roles in plant development and adaptation to adverse environments. By contrast, majority of these families that are merely present in a specific lineage or in a few closely related species have not been well functionally explored until recently. The fast-growing progresses being made in the actions of non-conserved miRNAs nowadays in diverse plant species may represent a highly promising research field in future. This review thereby summarizes the emerging advances in our understanding of the biogenesis, associated effectors, modes to targets, and biological functions of plant non-conserved miRNAs. In addition, it outlines the regulatory units recently discovered between conserved miRNAs and their alternative targets.

Auxin polar transport in stamen formation and development: how many actors?

In flowering plants, proper development of stamens, the male reproductive organs, is required for successful sexual reproduction. In Arabidopsis thaliana normally six stamen primordia arise in the third whorl of floral organs and subsequently differentiate into stamen filaments and anthers, where male meiosis occurs, thus ending the early developmental phase. This early phase is followed by a late developmental phase, which consists of a rapid elongation of stamen filaments coordinated with anther dehiscence and pollen maturation, and terminates with mature pollen grain release at anthesis. Increasing evidence suggests that auxin transport is necessary for both early and late phases of stamen development. It has been shown that different members of PIN (PIN-FORMED) family are involved in the early phase, whereas members of both PIN and P-glycoproteins of the ABCB (PGP) transporter families are required during the late developmental phase. In this review we provide an overview of the increasing knowledge on auxin transporters involved in Arabidopsis stamen formation and development and we discuss their role and functional conservation across plant species.

Old AIMs of the exocyst: evidence for an ancestral association of exocyst subunits with autophagy-associated Atg8 proteins

In a recent addendum, Oren Tzfadia and Gad Galili (PSB 2014; 9:e26732) showed that several Arabidopsis exocyst subunits possess consensus Atg8-interacting motifs (AIMs), which may mediate their interaction with the autophagy-associated Atg8 protein, providing thus a mechanistic base for participation of exocyst (sub)complexes in autophagy. However, the bioinformatically identified AIMs are short peptide motifs that may occur by chance. We thus performed an exhaustive search in a large collection of plant exocyst-derived sequences from our previous bioinformatic study and found that AIMs are over-represented among exocyst subunits of all lineages examined, including moss and club moss, compared with a representative sample of the Arabidopsis proteome. This is consistent with the proposed exocyst AIMs being biologically meaningful and evolutionarily ancient. Moreover, among the numerous EXO70 paralogs, the monocot-specific EXO70F clade appears to be exempt from the general AIM enrichment, suggesting a modification of the autophagy connection in a subset of exocyst variants.

We are good to grow: dynamic integration of cell wall architecture with the machinery of growth

Despite differences in cell wall composition between the type I cell walls of dicots and most monocots and the type II walls of commelinid monocots, all flowering plants respond to the same classes of growth regulators in the same tissue-specific way and exhibit the same growth physics. Substantial progress has been made in defining gene families and identifying mutants in cell wall-related genes, but our understanding of the biochemical basis of wall extensibility during growth is still rudimentary. In this review, we highlight insights into the physiological control of cell expansion emerging from genetic functional analyses, mostly in Arabidopsis and other dicots, and a few examples of genes of potential orthologous function in grass species. We discuss examples of cell wall architectural features that impact growth independent of composition, and progress in identifying proteins involved in transduction of growth signals and integrating their outputs in the molecular machinery of wall expansion.

Modeling and phylogeny analysis of bread wheat MnSOD

Superoxide dismutase (SOD) acts as first line of defense against oxidative and genetic stress. Manganese superoxide dismutase (MnSOD), found in mitochondria or peroxisomes, contains Mn(III) at the active site. Therefore, it is of interest to study MnSOD from bread wheat (a grain crop). However, a structure model is not yet solved for bread wheat MnSOD. Hence, we describe the structure model of bread wheat MnSOD developed using homology model. The model provides molecular insight to metal binding molecular function towards the understanding of oxidative stress resistance in plants. The distinction of bread wheat (a monocot) MnSOD from dicots is also shown using phylogenetic analysis.

Reproduction of Meloidogyne marylandi and M. incognita on several Poaceae

The susceptibility of 22 plant species to Meloidogyne marylandi and M. incognita was examined in three greenhouse experiments. Inoculum of M. marylandi was eggs from cultures maintained on Zoysia matrella "Cavalier" or Cynodon dactylon x C. trasvaalensis "Tifdwarf". Inoculum of M. incognita was eggs from cultures maintained on Solanum lycopersicum 'Rutgers'. In each host test the inoculum density was 2,000 nematode eggs/pot. None of the three dicot species tested (Gossypium hirsutum, Arachis hypogaea, and S. lycopersicum) were hosts for M. marylandi but, as expected, M. incognita had high levels of reproduction on G. hirsutum and S. lycopersicum. Meloidogyne marylandi reproduced on all of the 19 grass species (Poaceae) tested but reproduction varied greatly (P = 0.05) among these hosts. The following grasses were identified for the first time as hosts for M. marylandi: Buchloe dactyloides (buffalograss), Echinochloa colona (jungle rice), Eragostis curvula (weeping lovegrass), Paspalum dilatatum (dallisgrass), P. notatum (bahiagrass), Sorghastrum, nutans (indiangrass), Tripsacum dactyloides (eastern gamagrass), and Zoysia matrella (zoysiagrass). No reproduction of M. incognita was observed on B. dactyloides, Cyndon dactylon (common bermudagrass), E. curvula, P. vaginatum (seashore paspalum), S. nutans, T. dactyloides, Z. matrella or Z. japonica. Reproduction of M. incognita was less than reproduction of M. marylandi on the other grass species, except for the Zea mays inbred line B73 on which M. incognita had greater reproduction than did M. marylandi (P = 0.05) and Stenotaphrum secundatum (St. Augustinegrass) on which M. incognita and M. marylandi had similar levels of reproduction.

In tropical lowland rain forests monocots have tougher leaves than dicots, and include a new kind of tough leaf

BACKGROUND AND AIMS

There has been little previous work on the toughness of the laminae of monocots in tropical lowland rain forest (TLRF) despite the potential importance of greater toughness in inhibiting herbivory by invertebrates. Of 15 monocot families with >100 species in TLRF, eight have notably high densities of fibres in the lamina so that high values for toughness are expected.

METHODS

In north-eastern Australia punch strength was determined with a penetrometer for both immature leaves (approx. 30 % final area on average) and fully expanded, fully toughened leaves. In Singapore and Panama, fracture toughness was determined with an automated scissors apparatus using fully toughened leaves only.

KEY RESULTS

In Australia punch strength was, on average, 7x greater in shade-tolerant monocots than in neighbouring dicots at the immature stage, and 3x greater at the mature stage. In Singapore, shade-tolerant monocots had, on average, 1.3x higher values for fracture toughness than neighbouring dicots. In Panama, both shade-tolerant and gap-demanding monocots were tested; they did not differ in fracture toughness. The monocots had markedly higher values than the dicots whether shade-tolerant or gap-demanding species were considered.

CONCLUSIONS

It is predicted that monocots will be found to experience lower rates of herbivory by invertebrates than dicots. The tough monocot leaves include both stiff leaves containing relatively little water at saturation (e.g. palms), and leaves which lack stiffness, are rich in water at saturation and roll readily during dry weather or even in bright sun around midday (e.g. gingers, heliconias and marants). Monocot leaves also show that it is possible for leaves to be notably tough throughout the expansion phase of development, something never recorded for dicots. The need to broaden the botanist's mental picture of a 'tough leaf' is emphasized.

Monocot leaves are eaten less than dicot leaves in tropical lowland rain forests: correlations with toughness and leaf presentation

BACKGROUND AND AIMS

In tropical lowland rain forest (TLRF) the leaves of most monocots differ from those of most dicots in two ways that may reduce attack by herbivores. Firstly, they are tougher. Secondly, the immature leaves are tightly folded or rolled until 50-100 % of their final length. It was hypothesized that (a) losses of leaf area to herbivorous invertebrates are generally greatest during leaf expansion and smaller for monocots than for dicots, and (b) where losses after expansion are appreciable any difference between monocots and dicots then is smaller than that found during expansion.

METHODS

At six sites on four continents, estimates were made of lamina area loss from the four most recently mature leaves of focal monocots and of the nearest dicot shoot. Measurements of leaf mass per unit area, and the concentrations of water and nitrogen were made for many of the species. In Panama, the losses from monocots (palms) and dicots were also measured after placing fully expanded palm leaflets and whole dicot leaves on trails of leaf-cutter ants.

KEY RESULTS

At five of six sites monocots experienced significantly smaller leaf area loss than dicots. The results were not explicable in terms of leaf mass per unit area, or concentrations of water or nitrogen. At only one site was the increase in loss from first to fourth mature leaf significant (also large and the same in monocots and dicots), but the losses sustained during expansion were much smaller in the monocots. In the leaf-cutter ant experiment, losses were much smaller for palms than for dicots.

CONCLUSIONS

The relationship between toughness and herbivory is complex; despite the negative findings of some recent authors for dicots we hypothesize that either greater toughness or late folding can protect monocot leaves against herbivorous insects in tropical lowland rain forest, and that the relative importance varies widely with species. The difficulties of establishing unequivocally the roles of leaf toughness and leaf folding or rolling in a given case are discussed.

Widespread occurrence of a covalent linkage between xyloglucan and acidic polysaccharides in suspension-cultured angiosperm cells

BACKGROUND AND AIMS

Covalent linkages between xyloglucan and rhamnogalacturonan-I (RG-I) have been reported in the primary cell walls of cultured Rosa cells and may contribute to wall architecture. This study investigated whether this chemical feature is general to angiosperms or whether Rosa is unusual. *

METHODS

Xyloglucan was alkali-extracted from the walls of l-[1-3H]arabinose-fed suspension-cultured cells of Arabidopsis, sycamore, rose, tomato, spinach, maize and barley. The polysaccharide was precipitated with 50 % ethanol and subjected to anion-exchange chromatography in 8 m urea. Eluted fractions were Driselase-digested, yielding [3H]isoprimeverose (diagnostic of [3H]xyloglucan). The Arabidopsis cells were also fed [6-14C]glucuronic acid, and radiolabelled pectins were extracted with ammonium oxalate. *

KEY RESULTS

[3H]Xyloglucan was detected in acidic (galacturonate-containing) as well as non-anionic polysaccharide fractions. The proportion of the [3H]isoprimeverose units that were in anionic fractions was: Arabidopsis, 45 %; sycamore, 60 %; rose, 44 %; tomato, 75 %; spinach, 70 %; maize, 50 %; barley, 70 %. In Arabidopsis cultures fed d-[6-(14)C]glucuronate, 20 % of the (galacturonate-14C)-labelled pectins were found to hydrogen-bond to cellulose, a characteristic normally restricted to hemicelluloses such as xyloglucan. *

CONCLUSIONS

Alkali-stable, anionic complexes of xyloglucan (reported in the case of Rosa to be xyloglucan-RG-I covalent complexes) are widespread in the cell walls of angiosperms, including gramineous monocots.

The effect of plant species on soil nitrogen mineralization

1 To ascertain the influence of different plant species on nitrogen (N) cycling, we performed a long-term garden experiment with six grasses and five dicots with different potential growth rates, that are adapted to habitats with different nutrient supplies. We measured in situ N mineralization and nitrification of the soil under monocultures of each species during the fourth year of the experiment.2 We focused on the effects of the different species on (i) annual net N mineralization; (ii) the seasonal pattern of N mineralization; and (iii) the fraction of the total N mineralization that is nitrified. Our hypothesis was that plant species of nutrient-rich habitats would enhance the N mineralization compared with species of nutrient-poor habitats.3 The results demonstrate a strong influence of the species on net N mineralization and net nitrification, both of which fluctuated strongly during the year. Overall, species from high fertility habitats increased N mineralization and nitrification more than species from low fertility habitats. About 90% of the mineralized ammonium was oxidized to nitrate. There was no significant difference in this proportion in the plots of species from nutrient-rich, moderate and nutrient-poor habitats.