Lipid biosynthesis and protein concentration respond uniquely to phosphate supply during leaf development in highly phosphorus-efficient Hakea prostrata

Enhancing photosynthetic phosphorus use efficiency through coordination of leaf phosphorus fractions, allocation, and anatomy during soybean domestication

Soybean domestication has significantly changed key agronomic traits, yet its impact on leaf photosynthetic phosphorus use efficiency (PPUE) and its underlying traits remains poorly known. Further information on this would be important to increase soybean P use efficiency. To address this gap, 48 soybean accessions (16 wild relatives, 16 landraces, and 16 cultivars) were used to compare leaf anatomical traits, foliar chemical P fractions, P allocation, and PPUE under two P levels. The results showed that the cultivars had higher area-based and mass-based photosynthesis rates, PPUE, metabolite P concentration, and its percentage of leaf total P, as well as a greater percentage of lipid P, nucleic acid P, and residual P. Conversely, wild relatives tended to have higher leaf P concentration, palisade:spongy thickness ratio, and concentrations of inorganic P, nucleic acid P, lipid P, and residual P. PPUE was negatively correlated with leaf inorganic P concentration and its percentage relative to leaf total P, while it was positively correlated with the concentration and percentage of metabolite P. We concluded that soybean domestication increased PPUE, as a result of both increased photosynthesis rate and decreased leaf P concentration; domestication reduced the palisade:spongy thickness ratio coupled with increased allocation of P to P-containing metabolites, thereby contributing to faster photosynthesis and higher PPUE. This study sheds light on the significance of leaf P allocation and anatomical traits affecting PPUE during soybean domestication, offering a mechanistic understanding to further enhance soybean P use efficiency.

Comparative transcriptomics pinpoints conserved and specific transcriptional responses to phosphorus limitation

Translating biological knowledge from Arabidopsis to crop species is important to advance agriculture and secure food production in the face of dwindling fertilizer resources and biotic and abiotic stresses. However, it is often not trivial to identify functional homologs (orthologs) of Arabidopsis genes in crops. Combining sequence and expression data can improve the correct prediction of orthologs. Here, we conducted a large-scale RNA sequencing based transcriptomics study of Arabidopsis, Medicago, Brachypodium, and Setaria grown side-by-side in phosphorus (P)-sufficient and P-limited conditions to generate comparable transcriptomics datasets. Comparison of top 200 P-limitation-induced genes in Arabidopsis revealed that ~80% of these genes have identifiable close homologs in the other three species but only ~50% retain their P-limitation response in the legume and grasses. Most of the hallmark genes of the P-starvation response were found conserved in all four species. This study reveals many known, novel, unannotated, conserved, and species-specific forms of regulation of the transcriptional P-starvation response. Identification and experimental verification of expressologs by independent RT-qPCR for P-limitation marker genes in Prunus showed the usefulness of comparative transcriptomics in pinpointing the functional orthologs in diverse crop species. This study provides an unprecedented resource for functional genomics and translational research to create P-efficient crops.

Life at the conservative end of the leaf economics spectrum: intergeneric variation in the allocation of phosphorus to biochemical fractions in species of Banksia (Proteaceae) and Hakea (Proteaceae)

In severely phosphorus (P)-impoverished environments, plants have evolved to use P very efficiently. Yet, it is unclear how P allocation in leaves contributes to their photosynthetic P-use efficiency (PPUE) and position along the leaf economics spectrum (LES). We address this question in 10 species of Banksia and Hakea, two highly P-efficient Proteaceae genera. We characterised traits in leaves of Banksia and Hakea associated with the LES: leaf mass per area, light-saturated photosynthetic rates, P and nitrogen concentrations, and PPUE. We also determined leaf P partitioning to five biochemical fractions (lipid, nucleic acid, metabolite, inorganic and residual P) and their possible association with the LES. For both genera, PPUE was negatively correlated with fractional allocation of P to lipids, but positively correlated with that to metabolites. For Banksia only, PPUE was negatively correlated with residual P, highlighting a strategy contrasting to that of Hakea. Phosphorus-allocation patterns significantly explained PPUE but were not linked to the resource acquisition vs resource conservation gradient defined by the LES. We conclude that distinct P-allocation patterns enable species from different genera to achieve high PPUE and discuss the implications of different P investments. We surmise that different LES axes representing different ecological strategies coexist in extremely P-impoverished environments.

Dynamic interactions between SPX proteins, the ubiquitination machinery, and signalling molecules for stress adaptation at a whole-plant level

The plant macronutrient phosphorus is a scarce resource and plant-available phosphate is limiting in most soil types. Generally, a gene regulatory module called the phosphate starvation response (PSR) enables efficient phosphate acquisition by roots and translocation to other organs. Plants growing on moderate to nutrient-rich soils need to co-ordinate availability of different nutrients and repress the highly efficient PSR to adjust phosphate acquisition to the availability of other macro- and micronutrients, and in particular nitrogen. PSR repression is mediated by a small family of single SYG1/Pho81/XPR1 (SPX) domain proteins. The SPX domain binds higher order inositol pyrophosphates that signal cellular phosphorus status and modulate SPX protein interaction with PHOSPHATE STARVATION RESPONSE1 (PHR1), the central transcriptional regulator of PSR. Sequestration by SPX repressors restricts PHR1 access to PSR gene promoters. Here we focus on SPX4 that primarily acts in shoots and sequesters many transcription factors other than PHR1 in the cytosol to control processes beyond the classical PSR, such as nitrate, auxin, and jasmonic acid signalling. Unlike SPX1 and SPX2, SPX4 is subject to proteasomal degradation not only by singular E3 ligases, but also by SCF-CRL complexes. Emerging models for these different layers of control and their consequences for plant acclimation to the environment will be discussed.

Leaf phosphorus allocation to chemical fractions and its seasonal variation in south-western Australia is a species-dependent trait

South-western Australia is a global biodiversity hotspot and has some of the oldest and most phosphorus (P)-impoverished soils in the world. Proteaceae is one of the dominant P-efficient plant families there, but it is unknown how leaf P concentrations and foliar P allocation of Proteaceae and coexisting dominant plant families vary between seasons and habitats. To investigate this, we selected 18 species from Proteaceae, Myrtaceae and Fabaceae, six from each family, in two habitats from Alison Baird Reserve (32°1'19''S 15°58'52''E) in Western Australia. Total leaf P and nitrogen (N) concentrations, leaf mass per area, photosynthetic rate, pre-dawn leaf water potential and foliar P fractions were determined for each species both at the end of summer (March 2019 and early April 2020) and at the end of winter (September 2019). Soil P availability was also determined for each site. This is the very first study that focused on seasonal changes of foliar P fractions from different P-impoverished environments in three plant families. However, contrary to our expectation, we found little evidence for convergence of foliar P allocation within family, season or habitat. Each species exhibited a specific species-dependent pattern of foliar P allocation, and many species showed differences between seasons. Native plants in south-western Australia converged on a high photosynthetic P-use efficiency, but each species showed its own unique way associated with that outcome.

Lipidome Profiling of Phosphorus Deficiency-Tolerant Rice Cultivars Reveals Remodeling of Membrane Lipids as a Mechanism of Low P Tolerance

Plants have evolved various mechanisms for low P tolerance, one of which is changing their membrane lipid composition by remodeling phospholipids with non-phospholipids. The objective of this study was to investigate the remodeling of membrane lipids among rice cultivars under P deficiency. Rice (Oryza sativa L.) cultivars (Akamai, Kiyonishiki, Akitakomachi, Norin No. 1, Hiyadateine, Koshihikari, and Netaro) were grown in 0 (-P) and 8 (+P) mg P L^-1 solution cultures. Shoots and roots were collected 5 and 10 days after transplanting (DAT) in solution culture and subjected to lipidome profiling using liquid chromatography-mass spectrometry. Phosphatidylcholine (PC)34, PC36, phosphatidylethanolamine (PE)34, PE36, phosphatidylglycerol (PG)34, phosphatidylinositol (PI)34 were the major phospholipids and digalactosyldiacylglycerol (DGDG)34, DGDG36, 1,2-diacyl-3-O-alpha-glucuronosylglycerol (GlcADG)34, GlcADG36, monogalactosyldiacylglycerol (MGDG)34, MGDG36, sulfoquinovosyldiacylglycerol (SQDG)34 and SQDG36 were the major non-phospholipids. Phospholipids were lower in the plants that were grown under -P conditions than that in the plants that were grown under +P for all cultivars at 5 and 10 DAT. The levels of non-phospholipids were higher in -P plants than that in +P plants of all cultivars at 5 and 10 DAT. Decomposition of phospholipids in roots at 5 DAT correlated with low P tolerance. These results suggest that rice cultivars remodel membrane lipids under P deficiency, and the ability of remodeling partly contributes to low P tolerance.

Phosphorus fractions in leaves

Leaf phosphorus (P) comprises four major fractions: inorganic phosphate (P_i ), nucleic acids, phospholipids, P-containing metabolites and a residual fraction. In this review paper, we investigated whether allocation of P fractions varies among groups of terrestrial vascular plants, and is indicative of a species' strategy to use P efficiently. We found that as leaf total P concentration increases, the P_i fraction increases the most, without a plateau, while other fractions plateau. Variability of the concentrations of leaf P fractions is greatest among families > species(family) > regions > plant life forms. The percentage of total P allocated to nucleic acid-P (20-35%) and lipid-P (14-34%) varies less among families/species. High photosynthetic P-use efficiency is associated with low concentrations of all P fractions, and preferential allocation of P to metabolite-P and mesophyll cells. Sequential resorption of P from senescing leaves starts with P_i , followed by metabolite-P, and then other organic P fractions. Allocation of P to leaf P fractions varies with season. Leaf phytate concentrations vary considerably among species, associated with variation in photosynthesis and defence. Plasticity of P allocation to its fractions is important for acclimation to low soil P availability, and species-specific P allocation is needed for co-occurrence with other species.

Does Sulfoquinovosyl Diacylglycerol Synthase OsSQD1 Affect the Composition of Lipids in Rice Phosphate-Deprived Root?

Lipids are the essential components of the cell intracellular and plasma membranes. Sulfoquinovosyldiacylglycerol (SQDG) is a glycolipid; glycolipids can replace phospholipids in maintaining phosphate (Pi) homeostasis in plants which are undergoing Pi starvation. Sulfoquinovosyl diacylglycerol synthase 1 (OsSQD1) is a critical enzyme in the first step of catalyzation in the formation of SQDG in rice. In this study, the expression pattern of different zones in roots of OsSQD1 in response to different Pi conditions is examined, and it is found that OsSQD1 is highly expressed in lateral roots under Pi-sufficient and -deficient conditions. The root phenotype observation of different OsSQD1 transgenic lines suggests that the knockout/down of OsSQD1 inhibits the formation and growth of lateral roots under different Pi conditions. Additionally, the lipid concentrations in OsSQD1 transgenic line roots indicate that OsSQD1 knockout/down decreases the concentration of phospholipids and glycolipids in Pi-starved roots. The OsSQD1 mutation also changes the composition of different lipid species with different acyl chain lengths, mainly under Pi-deprived conditions. The relative transcript expression of genes relating to glycolipid synthesis and phospholipid degradation is estimated to help study the mechanism by which OsSQD1 exerts an influence on the alteration of lipid composition and concentration in Pi-starved roots. Moreover, in Pi-starved roots, the knockout of OsSQD1 decreases the unsaturated fatty acid content of phospholipids and glycolipids. To summarize, the present study demonstrates that OsSQD1 plays a key role in the maintenance of phospholipid and glycolipid composition in Pi-deprived rice roots, which may influence root growth and development under Pi-deprived conditions.

Differential aboveground-belowground adaptive strategies to alleviate N addition-induced P deficiency in two alpine coniferous forests

Increasing atmospheric nitrogen (N) deposition has resulted in phosphorus (P) limitation in multiple terrestrial ecosystems, yet how plants coordinate aboveground and belowground strategies to adapt to such P deficiency remains unclear. In this study, we conducted a field N fertilization experiment in two alpine coniferous plantations (Picea asperata Mast. and Pinus armandii Franch.) with different soil N availability on the eastern Tibetan Plateau of China, to examine N addition effects on plant nutrient limiting status and plant adaptive strategies corresponding to aboveground P conservation and belowground P acquisition. The results showed that N addition aggravated P deficiency in both plantations, as indicated by decreased needle P concentrations and increased N:P ratios, and that plant strategies for addressing such P deficiency differed in the two plantations with different initial soil N availabilities. In the P. asperata plantation with relatively high N availability, significantly enhanced needle phosphatase activity and shifts in P fraction allocation (downregulation of the structural P fraction and increased allocation to the residual P fraction) co-occurred with increased rhizosphere effects on phosphatase activity under N addition, indicating a synergistic strategy of aboveground P conservation and belowground P mining to alleviate P deficiency. In the P. armandii plantation with relatively low N availability, however, N addition only enhanced phosphatase activity and increased allocation to residual P fraction in the aboveground but had little effect on belowground P acquisition-associated traits, suggesting a decoupling relationship between aboveground P conservation and belowground P acquisition. This study highlights the vital significance of initial soil nutrient availability in regulating the coordination of aboveground and belowground strategic alternatives, emphasizing the need to integrate soil nutrient conditions for a holistic understanding of forest adaptation to anthropogenic N enrichment.

Low phosphorus induces differential metabolic responses in eucalyptus species improving nutrient use efficiency

Phosphorus (P) is a vital nutrient for plant growth. P availability is generally low in soils, and plant responses to low P availability need to be better understood. In a previous study, we studied the growth and physiological responses of 24 species to low P availability in the soil and verified of eucalypts, five (Eucalyptus acmenoides, E. grandis, E. globulus, E. tereticornis, and Corymbia maculata) contrasted regarding their efficiency and responsiveness to soil P availability. Here, we obtained the metabolomic and lipidomic profile of leaves, stems, and roots from these species growing under low (4.5 mg dm^-3) and sufficient (10.8 mg dm^-3) P in the soil. Disregarding the level of P in the soils, P allocation was always higher in the stems. However, when grown in the P-sufficient soil, the stems steadily were the largest compartment of the total plant P. Under low P, the relative contents of primary metabolites, such as amino acids, TCA cycle intermediates, organic acids and carbohydrates, changed differently depending on the species. Additionally, phosphorylated metabolites showed enhanced turnover or reductions. While photosynthetic efficiencies were not related to higher biomass production, A/Ci curves showed that reduced P availability increased the eucalypt species' Vcmax, Jmax and photosynthetic P-use efficiency. Plants of E. acmenoides increased galactolipids and sulfolipids in leaves more than other eucalypt species, suggesting that lipid remodelling can be a strategy to cope with the P shortage in this species. Our findings offer insights to understand genotypic efficiency among eucalypt species to accommodate primary metabolism under low soil P availability and eventually be used as biochemical markers for breeding programs.

Combining analyses of metabolite profiles and phosphorus fractions to explore high phosphorus utilization efficiency in maize

Phosphorus (P) limitation is a significant factor restricting crop production in agricultural systems, and enhancing the internal P utilization efficiency (PUE) of crops plays an important role in ensuring sustainable P use in agriculture. To better understand how P is remobilized to affect crop growth, we first screened P-efficient (B73 and GEMS50) and P-inefficient (Liao5114) maize genotypes at the same shoot P content, and then analyzed P pools and performed non-targeted metabolomic analyses to explore changes in cellular P fractions and metabolites in maize genotypes with contrasting PUE. We show that lipid P and nucleic acid P concentrations were significantly lower in lower leaves of P-efficient genotypes, and these P pools were remobilized to a major extent in P-efficient genotypes. Broad metabolic alterations were evident in leaves of P-efficient maize genotypes, particularly affecting products of phospholipid turnover and phosphorylated compounds, and the shikimate biosynthesis pathway. Taken together, our results suggest that P-efficient genotypes have a high capacity to remobilize lipid P and nucleic acid P and promote the shikimate pathway towards efficient P utilization in maize.

Phosphorus Acquisition and Utilization in Plants

Tremendous progress has been made on molecular aspects of plant phosphorus (P) nutrition, often without heeding information provided by soil scientists, ecophysiologists, and crop physiologists. This review suggests ways to integrate information from different disciplines. When soil P availability is very low, P-mobilizing strategies are more effective than mycorrhizal strategies. Soil parameters largely determine how much P roots can acquire from P-impoverished soil, and kinetic properties of P transporters are less important. Changes in the expression of P transporters avoid P toxicity. Plants vary widely in photosynthetic P-use efficiency, photosynthesis per unit leaf P. The challenge is to discover what the trade-offs are of different patterns of investment in P fractions. Less investment may save P, but are costs incurred? Are these costs acceptable for crops? These questions can be resolved only by the concerted action of scientists working at both molecular and physiological levels, rather than pursuing these problems independently.

Foliar nutrient allocation patterns in Banksia attenuata and Banksia sessilis differing in growth rate and adaptation to low-phosphorus habitats

BACKGROUND AND AIMS

Phosphorus (P) and nitrogen (N) are essential nutrients that frequently limit primary productivity in terrestrial ecosystems. Efficient use of these nutrients is important for plants growing in nutrient-poor environments. Plants generally reduce foliar P concentration in response to low soil P availability. We aimed to assess ecophysiological mechanisms and adaptive strategies for efficient use of P in Banksia attenuata (Proteaceae), naturally occurring on deep sand, and B. sessilis, occurring on shallow sand over laterite or limestone, by comparing the allocation of P among foliar P fractions.

METHODS

We carried out pot experiments with slow-growing B. attenuata, which resprouts after fire, and faster growing opportunistic B. sessilis, which is killed by fire, on substrates with different P availability using a randomized complete block design. We measured leaf P and N concentrations, photosynthesis, leaf mass per area, relative growth rate and P allocated to major biochemical fractions in B. attenuata and B. sessilis.

KEY RESULTS

The two species had similarly low foliar total P concentrations, but distinct patterns of P allocation to P-containing fractions. The foliar total N concentration of B. sessilis was greater than that of B. attenuata on all substrates. The foliar total P and N concentrations in both species decreased with decreasing P availability. The relative growth rate of both species was positively correlated with concentrations of both foliar nucleic acid P and total N, but there was no correlation with other P fractions. Faster growing B. sessilis allocated more P to nucleic acids than B. attenuata did, but other fractions were similar.

CONCLUSIONS

The nutrient allocation patterns in faster growing opportunistic B. sessilis and slower growing B. attenuata revealed different strategies in response to soil P availability which matched their contrasting growth strategy.

OCBIL theory examined: reassessing evolution, ecology and conservation in the world’s ancient, climatically buffered and infertile landscapes

OCBIL theory was introduced as a contribution towards understanding the evolution, ecology and conservation of the biological and cultural diversity of old, climatically buffered, infertile landscapes (OCBILs), especially in the Southern Hemisphere. The theory addresses some of the most intransigent environmental and cultural trends of our time – the ongoing decline of biodiversity and cultural diversity of First Nations. Here we reflect on OCBILs, the origins of the theory, and its principal hypotheses in biological, anthropological and conservation applications. The discovery that threatened plant species are concentrated in the Southwest Australian Floristic Region (SWAFR) on infertile, phosphorous-impoverished uplands within 500 km of the coast formed the foundational framework for OCBIL theory and led to the development of testable hypotheses that a growing literature is addressing. Currently, OCBILs are recognized in 15 Global Biodiversity Hotspots and eight other regions. The SWAFR, Greater Cape Floristic Region of South Africa and South America’s campos rupestres (montane grasslands) are those regions that have most comprehensively been investigated in the context of OCBIL theory. We summarize 12 evolutionary, ecological and cultural hypotheses and ten conservation-management hypotheses being investigated as recent contributions to the OCBIL literature.

Delayed greening in phosphorus-efficient Hakea prostrata (Proteaceae) is a photoprotective and nutrient-saving strategy

Hakea prostrata R.Br. (Proteaceae) shows a 'delayed greening' strategy of leaf development characterised by reddish young leaves that become green as they mature. This trait may contribute to efficient use of phosphorus (P) during leaf development by first investing P in the development of leaf structure followed by maturation of the photosynthetic machinery. In this study, we investigated the properties of delayed greening in a highly P-efficient species to enhance our understanding of the ecological significance of this trait as a nutrient-saving and photoprotective strategy. In glasshouse-grown plants, we assessed foliar pigments, fatty acids and nutrient composition across five leaf developmental stages. Young leaves had higher concentrations of anthocyanin, P, nitrogen (N), copper (Cu), xanthophyll-cycle pigments and saturated fatty acids than mature leaves. As leaves developed, the concentration of anthocyanins decreased, whereas that of chlorophyll and the double bond index of fatty acids increased. In mature leaves, ~60% of the fatty acids was α-linolenic acid (C18:3 n-3). Mature leaves also had higher concentrations of aluminium (Al), calcium (Ca) and manganese (Mn) than young leaves. We conclude that delayed greening in H. prostrata is a strategy that saves P as well as N and Cu through sequential allocation of these resources, first to cell production and structural development, and then to supplement chloroplast development. This strategy also protects young leaves against photodamage and oxidative stress during leaf expansion under high-light conditions.

OsSQD1 at the crossroads of phosphate and sulfur metabolism affects plant morphology and lipid composition in response to phosphate deprivation

In phosphate (Pi)-deprived Arabidopsis (Arabidopsis thaliana), phosphatidylglycerol (PG) is substituted by sulfolipid for maintaining Pi homeostasis. Sulfoquinovosyl diacylglycerol1 (AtSQD1) encodes a protein, which catalyzes uridine diphosphate glucose (UDPG) and sulfite (SO₃^2- ) to UDP-sulfoquinovose, which is a key component in the sulfolipid biosynthetic pathway. In this study, a reverse genetics approach was employed to decipher the function of the AtSQD1 homolog OsSQD1 in rice. Differential expressions of OsSQD1 in different tissue and response to -P and -S also detected, respectively. The in vitro protein assay and analysis suggests that OsSQD1 is a UDP-sulfoquinovose synthase. Transient expression analysis showed that OsSQD1 is located in the chloroplast. The analyses of the knockout (ossqd1) and knockdown (Ri1 and Ri2) mutants demonstrated reductions in Pi and total P concentrations, ³² Pi uptake rate, expression levels of Pi transporters and altered developmental responses of root traits, which were accentuated during Pi deficiency. The inhibitory effects of the OsSQD1 mutation were also evident in the development of reproductive tissue. Furthermore, OsSQD1 differently affects lipid composition under different Pi regime affects sulfur (S) homeostasis. Together, the study revealed that OsSQD1 affects Pi and S homeostasis, and lipid composition in response to Pi deprivation.

Effect of marker segregation distortion on high density linkage map construction and QTL mapping in Soybean (Glycine max L.)

Marker segregation distortion is a natural phenomenon. Severely distorted markers are usually excluded in the construction of linkage maps. We investigated the effect of marker segregation distortion on linkage map construction and quantitative trait locus (QTL) mapping. A total of 519 recombinant inbred lines of soybean from orthogonal and reciprocal crosses between LSZZH and NN493-1 were genotyped by specific length amplified fragment markers and seed linoleic acid content was measured in three environments. As a result, twenty linkage groups were constructed with 11,846 markers, including 1513 (12.77%) significantly distorted markers, on 20 chromosomes, and the map length was 2475.86 cM with an average marker-interval of 0.21 cM. The inclusion of distorted markers in the analysis was shown to not only improve the grouping of the markers from the same chromosomes, and the consistency of linkage maps with genome, but also increase genome coverage by markers. Combining genotypic data from both orthogonal and reciprocal crosses decreased the proportion of distorted markers and then improved the quality of linkage maps. Validation of the linkage maps was confirmed by the high collinearity between positions of markers in the soybean reference genome and in linkage maps and by the high consistency of 24 QTL regions in this study compared with the previously reported QTLs and lipid metabolism related genes. Additionally, linkage maps that include distorted markers could add more information to the outputs from QTL mapping. These results provide important information for linkage mapping, gene cloning and marker-assisted selection in soybean.

Responses of foliar phosphorus fractions to soil age are diverse along a 2 Myr dune chronosequence

Plants respond to soil phosphorus (P) availability by adjusting leaf P among inorganic P (Pi) and organic P fractions (nucleic acids, phospholipids, small metabolites and a residual fraction). We tested whether phylogenetically divergent plants in a biodiversity hotspot similarly adjust leaf P allocation in response to P limitation by sampling along a 2 Myr chronosequence in southwestern Australia where nitrogen (N) limitation transitions to P limitation with increasing soil age. Total P and N, and P allocated to five chemical fractions were determined for photosynthetic organs from Melaleuca systena (Myrtaceae), Acacia rostellifera (Fabaceae) and Hakea prostrata (Proteaceae). Soil characteristics were also determined. Acacia rostellifera maintained phyllode total P and N concentrations at c. 0.5 and 16 mg g^-1 DW, respectively, with a constant P-allocation pattern along the chronosequence. H. prostrata leaves allocated less P to Pi, phospholipids and nucleic acids with increasing soil age, while leaf N concentration was constant. M. systena had the greatest variation in allocating leaf P, whereas leaf N concentration decreased 20% along the chronosequence. Variation in P-allocation patterns was only partially conserved among species along the chronosequence. Such variation could have an impact on species distribution and contribute to species richness in P-limited environments.

Molecular mechanisms underpinning phosphorus-use efficiency in rice

Orthophosphate (H₂ PO₄^- , Pi) is an essential macronutrient integral to energy metabolism as well as a component of membrane lipids, nucleic acids, including ribosomal RNA, and therefore essential for protein synthesis. The Pi concentration in the solution of most soils worldwide is usually far too low for maximum growth of crops, including rice. This has prompted the massive use of inefficient, polluting, and nonrenewable phosphorus (P) fertilizers in agriculture. We urgently need alternative and more sustainable approaches to decrease agriculture's dependence on Pi fertilizers. These include manipulating crops by (a) enhancing the ability of their roots to acquire limiting Pi from the soil (i.e. increased P-acquisition efficiency) and/or (b) increasing the total biomass/yield produced per molecule of Pi acquired from the soil (i.e. increased P-use efficiency). Improved P-use efficiency may be achieved by producing high-yielding plants with lower P concentrations or by improving the remobilization of acquired P within the plant so as to maximize growth and biomass allocation to developing organs. Membrane lipid remodelling coupled with hydrolysis of RNA and smaller P-esters in senescing organs fuels P remobilization in rice, the world's most important cereal crop.

Phosphorus nutrition of Populus × canescens reflects adaptation to high P-availability in the soil

Phosphorus (P) constitutes one of five macronutrients essential for plant growth and development due to the central function of phosphate in energy metabolism, inheritance and metabolic control. In many ecosystems, plant available soil-P gets limited by soil aging. Hence, plants have developed adaptation strategies to cope with such limitation by an efficient plant and ecosystem internal P-cycling during annual growth. The natural floodplain habitat of fast-growing Populus × canescens is characterized by high soil-P availability. It was thus expected that the P-nutrition of P. × canescens had adapted to this conditions. Therefore, different P-fractions in different twig tissues were investigated during two annual growth cycles. The P-nutrition of P. × canescens markedly differs from that of European beech grown at low soil-P availability (Netzer F, Schmid C, Herschbach C, Rennenberg H (2017) Phosphorus-nutrition of European beech (Fagus sylvatica L.) during annual growth depends on tree age and P-availability in the soil. Environ Exp Bot 137:194-207). This was mainly due to a lack of tree internal P-cycling during annual growth indicated by the absence of P-storage and remobilization in twig bark and wood. Hence, strategies to economize P-nutrition and to prevent P-losses had not developed. This fits with the fast-growth strategy of P. × canescens at unrestricted P-availability. Hence, the P-nutrition strategy of P. × canescens can be seen as an evolutionary adaptation to its natural growth habitat.

Tight control of sulfur assimilation: an adaptive mechanism for a plant from a severely phosphorus-impoverished habitat

Hakea prostrata (Proteaceae) has evolved in extremely phosphorus (P)-impoverished habitats. Unlike species that evolved in P-richer environments, it tightly controls its nitrogen (N) acquisition, matching its low protein concentration, and thus limiting its P requirement for ribosomal RNA (rRNA). Protein is a major sink for sulfur (S), but the link between low protein concentrations and S metabolism in H. prostrata is unknown, although this is pivotal for understanding this species' supreme adaptation to P-impoverished soils. Plants were grown at different sulfate supplies for 5 wk and used for nutrient and metabolite analyses. Total S content in H. prostrata was unchanged with increasing S supply, in sharp contrast with species that typically evolved in environments where P is not a major limiting nutrient. Unlike H. prostrata, other plants typically store excess available sulfate in vacuoles. Like other species, S-starved H. prostrata accumulated arginine, lysine and O-acetylserine, indicating S deficiency. Hakea prostrata tightly controls its S acquisition to match its low protein concentration and low demand for rRNA, and thus P, the largest organic P pool in leaves. We conclude that the tight control of S acquisition, like that of N, helps H. prostrata to survive in P-impoverished environments.

Characterization of the Wheat Leaf Metabolome during Grain Filling and under Varied N-Supply

Progress in improving crop growth is an absolute goal despite the influence multifactorial components have on crop yield and quality. An Avalon × Cadenza doubled-haploid wheat mapping population was used to study the leaf metabolome of field grown wheat at weekly intervals during the time in which the canopy contributes to grain filling, i.e., from anthesis to 5 weeks post-anthesis. Wheat was grown under four different nitrogen supplies reaching from residual soil N to a luxury over-fertilization (0, 100, 200, and 350 kg N ha^-1). Four lines from a segregating doubled haploid population derived of a cross of the wheat elite cvs. Avalon and Cadenza were chosen as they showed pairwise differences in either N utilization efficiency (NUtE) or senescence timing. 108 annotated metabolites of primary metabolism and ions were determined. The analysis did not provide genotype specific markers because of a remarkable stability of the metabolome between lines. We speculate that the reason for failing to identify genotypic markers might be due to insufficient genetic diversity of the wheat parents and/or the known tendency of plants to keep metabolome homeostasis even under adverse conditions through multiple adaptations and rescue mechanism. The data, however, provided a consistent catalogue of metabolites and their respective responses to environmental and developmental factors and may bode well for future systems biology approaches, and support plant breeding and crop improvement.

Acclimation of the crucifer Eutrema salsugineum to phosphate limitation is associated with constitutively high expression of phosphate-starvation genes

Eutrema salsugineum, a halophytic relative of Arabidopsis thaliana, was subjected to varying phosphate (Pi) treatments. Arabidopsis seedlings grown on 0.05 mm Pi displayed shortened primary roots, higher lateral root density and reduced shoot biomass allocation relative to those on 0.5 mm Pi, whereas Eutrema seedlings showed no difference in lateral root density and shoot biomass allocation. While a low Fe concentration mitigated the Pi deficiency response for Arabidopsis, Eutrema root architecture was unaltered, but adding NaCl increased Eutrema lateral root density almost 2-fold. Eutrema and Arabidopsis plants grown on soil without added Pi for 4 weeks had low shoot and root Pi content. Pi-deprived, soil-grown Arabidopsis plants were stunted with senescing older leaves, whereas Eutrema plants were visually indistinguishable from 2.5 mm Pi-supplemented plants. Genes associated with Pi starvation were analysed by RT-qPCR. EsIPS2, EsPHT1;4 and EsPAP17 showed up-regulated expression in Pi-deprived Eutrema plants, while EsPHR1, EsWRKY75 and EsRNS1 showed no induction. Absolute quantification of transcripts indicated that PHR1, WRKY75 and RNS1 were expressed at higher levels in Eutrema plants relative to those in Arabidopsis regardless of external Pi. The low phenotypic plasticity Eutrema displays to Pi supply is consistent with adaptation to chronic Pi deprivation in its extreme natural habitat.

Natural genetic variation for morphological and molecular determinants of plant growth and yield

The rates of increase in yield of the main commercial crops have been steadily falling in many areas worldwide. This generates concerns because there is a growing demand for plant biomass due to the increasing population. Plant yield should thus be improved in the context of climate change and decreasing natural resources. It is a major challenge which could be tackled by improving and/or altering light-use efficiency, CO2 uptake and fixation, primary metabolism, plant architecture and leaf morphology, and developmental plant processes. In this review, we discuss some of the traits which could lead to yield increase, with a focus on how natural genetic variation could be harnessed. Moreover, we provide insights for advancing our understanding of the molecular aspects governing plant growth and yield, and propose future avenues for improvement of crop yield. We also suggest that knowledge accumulated over the last decade in the field of molecular physiology should be integrated into new ideotypes.

Phosphorus nutrition in Proteaceae and beyond

Proteaceae in southwestern Australia have evolved on some of the most phosphorus-impoverished soils in the world. They exhibit a range of traits that allow them to both acquire and utilize phosphorus highly efficiently. This is in stark contrast with many model plants such as Arabidopsis thaliana and crop species, which evolved on soils where nitrogen is the major limiting nutrient. When exposed to low phosphorus availability, these plants typically exhibit phosphorus-starvation responses, whereas Proteaceae do not. This Review explores the traits that account for the very high efficiency of acquisition and use of phosphorus in Proteaceae, and explores which of these traits are promising for improving the phosphorus efficiency of crop plants.