Cécile Julia

Screening for internal phosphorus utilisation efficiency: comparison of genotypes at equal shoot P content is critical

AimsProgress in improving the internal phosphorus utilisation efficiency of crops has been limited, which may be due to poor screening methods that allow differences in P uptake among genotypes grown in soil to mask genotypic differences in shoot biomass produced per unit of shoot P (PUE). We investigated alternative soil and hydroponic screening methods for their capacity to produce a consensus ranking of genotypes with regard to PUE.MethodsSix rice genotypes previously identified in hydroponic screening studies as being high, intermediate or low in PUE were screened using multi P rate hydroponic and soil-based experiments.ResultsComparisons made at each rate of soil-P supply produced estimates of PUE strongly biased by P uptake differences among genotypes. Using multiple-rate data to derive response functions per genotype showed that similar P content was achieved at different rates of P supply but that high-PUE genotypes clearly separated from intermediate- and low-PUE genotypes if equal P content was used. Ranking analysis suggested that results obtained from soil agreed well with those from the hydroponic study.ConclusionsPUE was significantly influenced by genotype and P supply, but there was no significant genotype x P supply interaction. Hence, we conclude that screening genotypes using hydroponics at one or two P supply levels is the most cost- and time effective means to screen large numbers of rice genotypes for PUE.

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

Summary 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.

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.

Morphological characterisation of Australian ex situ wild rice accessions and potential for identifying novel sources of tolerance to phosphorus deficiency

Australian wild rice (Oryza sp.) diverged in isolation from the progenitors of cultivated rice millions of years ago and is likely to harbour novel alleles of value to rice breeding programs. Since the separation from Gondwana 100 MYA, Australia has been characterised as a primarily dry continent with nutrient-impoverished soils. This has led to speculation that alleles conferring tolerance to nutritional deficiency and other abiotic stresses such as drought or heat may be present within Australian wild rice germplasm. Previous genetic studies suggested that representative ex situ accessions of O. meridionalis and O. rufipogon are distinct from Asian wild rice but the variation in morphological or nutrient stress traits within the Australian wild rice germplasm collections has not been investigated. We therefore characterised 14 key morphological traits at flowering and maturity in five O. meridionalisex situ genetic resource accessions in comparison with representatives of O. rufipogon, O. australiensis, O. officinalis and the O. sativa mega-variety IR64. The results were consistent with genetic analyses that indicate O. meridionalis and O. rufipogon are closely related. Variation in tolerance to phosphorus deficiency, a major nutritional constraint to global rice production, was identified among Australian wild rice accessions, although at the phenotypic level this did not appear to exceed levels of tolerance present in the O. sativa mega-variety IR64. More detailed genetic analysis may establish whether this has been achieved through convergence of distinct loci and alleles or whether recombination between the distinct lineages may generate novel transgressive levels of tolerance. We conclude that subsequent collections of Australian wild rice should be guided by underlying geological, topographical and other long-term environmental features which indicate locations known to be P deficient or exert selection pressure for adaptations to other abiotic stresses including salinity or drought stress.

Rice Adaptation Strategies in Response to Heat Stress at Flowering

The adaptation of flowering processes to heat is crucial for maintaining yields in rice growing systems. In the Senegal River valley, high temperatures cause sterility, a situation that has prompted the development of crop management sequences and predictive models that account for the ability of plants to escape (early anthesis time), avoid (panicle cooling through transpiration) or tolerate (presence of genes of interest) heat at flowering. Avoidance and tolerance are the main genetic improvement pathways, whereas cropping practices are adjusted to ensure escape and take advantage of suitable thermal periods.

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.