Transcriptional response of rice flag leaves to restricted external phosphorus supply during grain filling in rice cv. IR64.

Plant phosphorus (P) remobilisation during leaf senescence has fundamental implications for global P cycle fluxes. Hypothesising that genes involved in remobilisation of P from leaves during grain filling would show altered expression in response to P deprivation, we investigated gene expression in rice flag leaves at 8 days after anthesis (DAA) and 16 DAA in plants that received a continuous supply of P in the nutrient solution vs plants where P was omitted from the nutrient solution for 8 consecutive days prior to measurement. The transcriptional response to growth in the absence of P differed between the early stage (8 DAA) and the later stage (16 DAA) of grain filling. At 8 DAA, rice plants maintained production of energy substrates through upregulation of genes involved in photosynthesis. In contrast, at 16 DAA carbon substrates were produced by degradation of structural polysaccharides and over 50% of highly upregulated genes in P-deprived plants were associated with protein degradation and nitrogen/amino acid transport, suggesting withdrawal of P from the nutrient solution led to accelerated senescence. Genes involved in liberating inorganic P from the organic P compounds and vacuolar P transporters displayed differential expression depending on the stage of grain filling stage and timing of P withdrawal.

Phosphorus uptake commences at the earliest stages of seedling development in rice.

Seed phosphorus (P) reserves are essential for seedling development; however, we hypothesise that the quantity of P in seeds will lose importance in cultivars that rapidly acquire it via their roots. Our objective in this study was therefore to investigate the onset of seedling P uptake in rice (Oryza sativa). This was addressed through 33P-labelled supply and through measuring P depletion in combination with the detection of P transporter activity in the root tissue of three rice cultivars during early development. 33P supplied to roots 4 d after germination (DAG) was detected in shoots 2 d later, indicating that P was taken up and translocated to shoots during early seedling development. Measurements of P depletion from the growth medium indicated that uptake occurred even at 2 DAG when roots were only 3 cm long. By day 3, P depletion was rapid and P transporter activity was detected in roots, regardless of the levels of seed P reserves present. We conclude that P uptake commences at the earliest stages of seedling development in rice, that the amount taken up will be limited by root size, and that genotypes with more rapid root development should more rapidly complement seed-P reserves by root uptake.

Remobilisation of phosphorus fractions in rice flag leaves during grain filling: Implications for photosynthesis and grain yields.

Phosphorus (P) is translocated from vegetative tissues to developing seeds during senescence in annual crop plants, but the impact of this P mobilisation on photosynthesis and plant performance is poorly understood. This study investigated rice (Oryza sativa L.) flag leaf photosynthesis and P remobilisation in a hydroponic study where P was either supplied until maturity or withdrawn permanently from the nutrient solution at anthesis, 8 days after anthesis (DAA) or 16 DAA. Prior to anthesis, plants received either the minimum level of P in nutrient solution required to achieve maximum grain yield ('adequate P treatment'), or received luxury levels of P in the nutrient solution ('luxury P treatment'). Flag leaf photosynthesis was impaired at 16 DAA when P was withdrawn at anthesis or 8 DAA under adequate P supply but only when P was withdrawn at anthesis under luxury P supply. Ultimately, reduced photosynthesis did not translate into grain yield reductions. There was some evidence plants remobilised less essential P pools (e.g. Pi) or replaceable P pools (e.g. phospholipid-P) prior to remobilisation of P in pools critical to leaf function such as nucleic acid-P and cytosolic Pi. Competition for P between vegetative tissues and developing grains can impair photosynthesis when P supply is withdrawn during early grain filling. A reduction in the P sink strength of grains by genetic manipulation may enable leaves to sustain high rates of photosynthesis until the later stages of grain filling.

Phosphorus remobilization from rice flag leaves during grain filling: an RNA-seq study.

The physiology and molecular regulation of phosphorus (P) remobilization from vegetative tissues to grains during grain filling is poorly understood, despite the pivotal role it plays in the global P cycle. To test the hypothesis that a subset of genes involved in the P starvation response are involved in remobilization of P from flag leaves to developing grains, we conducted an RNA-seq analysis of rice flag leaves during the preremobilization phase (6 DAA) and when the leaves were acting as a P source (15 DAA). Several genes that respond to phosphate starvation, including three purple acid phosphatases (OsPAP3, OsPAP9b and OsPAP10a), were significantly up-regulated at 15 DAA, consistent with a role in remobilization of P from flag leaves during grain filling. A number of genes that have not been implicated in the phosphate starvation response, OsPAP26, SPX-MFS1 (a putative P transporter) and SPX-MFS2, also showed expression profiles consistent with involvement in P remobilization from senescing flag leaves. Metabolic pathway analysis using the KEGG system suggested plastid membrane lipid synthesis is a critical process during the P remobilization phase. In particular, the up-regulation of OsPLDz2 and OsSQD2 at 15 DAA suggested phospholipids were being degraded and replaced by other lipids to enable continued cellular function while liberating P for export to developing grains. Three genes associated with RNA degradation that have not previously been implicated in the P starvation response also showed expression profiles consistent with a role in P mobilization from senescing flag leaves.

Phosphorus uptake, partitioning and redistribution during grain filling in rice.

BACKGROUNDS AND AIMS: In cultivated rice, phosphorus (P) in grains originates from two possible sources, namely exogenous (post-flowering root P uptake from soil) or endogenous (P remobilization from vegetative parts) sources. This study investigates P partitioning and remobilization in rice plants throughout grain filling to resolve contributions of P sources to grain P levels in rice. METHODS: Rice plants (Oryza sativa 'IR64') were grown under P-sufficient or P-deficient conditions in the field and in hydroponics. Post-flowering uptake, partitioning and re-partitioning of P was investigated by quantifying tissue P levels over the grain filling period in the field conditions, and by employing 33P isotope as a tracer in the hydroponic study. KEY RESULTS: Post-flowering P uptake represented 40-70 % of the aerial plant P accumulation at maturity. The panicle was the main P sink in all studies, and the amount of P potentially remobilized from vegetative tissues to the panicle during grain filling was around 20 % of the total aerial P measured at flowering. In hydroponics, less than 20 % of the P tracer taken up at 9 d after flowering (DAF) was found in the above-ground tissues at 14 DAF and half of it was partitioned to the panicle in both P treatments. CONCLUSIONS: The results demonstrate that P uptake from the soil during grain filling is a critical contributor to the P content in grains in irrigated rice. The P tracer study suggests that the mechanism of P loading into grains involves little direct transfer of post-flowering P uptake to the grain but rather substantial mobilization of P that was previously taken up and stored in vegetative tissues.

The knowns and unknowns of phosphorus loading into grains, and implications for phosphorus efficiency in cropping systems.

Inefficient use of phosphorus (P) in agriculture adds to production costs, increases the risk of eutrophication of waterways, and contributes to the rapid depletion of the world's non-renewable rock phosphate supplies. The removal of large quantities of P from fields in harvested grains is a major driver in the global P cycle, but opportunities exist to reduce the amount of P in harvested grains through plant breeding. Using rice (Oryza sativa L.) as a model crop, we examine our current understanding of the process of P loading into grain and its regulation by genetic and environmental factors. We expose a dearth of knowledge on the physiological processes involved in loading P into grains, poor resolution of the genes and networks involved in P mobilization from vegetative tissues to grains, and limited understanding of genetic versus environmental contributions to variation in grain P concentrations observed among genotypes. We discuss potential breeding strategies and highlight key research gaps that should be addressed to facilitate these breeding approaches. Given the strong economic and environmental incentives for a low grain P trait, we suggest that some of the investment and resources currently directed to determining the molecular regulation of P starvation responses in model plant species should be diverted to resolving the physiology, genetics, and molecular regulation of P loading into cereal grains.