Phosphate transporter OsPht1;8 in rice plays an important role in phosphorus redistribution from source to sink organs and allocation between embryo and endosperm of seeds

Bioengineering and management for efficient and sustainable utilization of phosphorus in crops

Phosphorus (P) is an essential macronutrient for plant growth, but low P availability in soils is also a primary constraint to crop production. To meet the increasing demands for food, P fertilizer applications have been increased, causing the accumulation of surplus P in soils, which has led to the frequency and magnitude of associated risk effects on agroecosystems. Finding solutions for efficient and sustainable crop P utilization is, therefore, an urgent priority. This review summarizes recent progress in bioengineering approaches to improving crop P efficiency and highlights that modifying root architecture in P-deficient soils and reducing P accumulation in grains in soils with P surplus could offer a way forward for improving P use efficiency.

Exploring the phosphorus-starch content balance mechanisms in maize grains using GWAS population and transcriptome data

Examining the connection between P and starch-related signals can help elucidate the balance between nutrients and yield. This study utilized 307 diverse maize inbred lines to conduct multi-year and multi-plot trials, aiming to explore the relationship among P content, starch content, and 100-kernel weight (HKW) of mature grains. A significant negative correlation was found between P content and both starch content and HKW, while starch content showed a positive correlation with HKW. The starch granules in grains with high-P and low-starch content (HPLS) were significantly smaller compared to grains with low-P high-starch content (LPHS). Additionally, mian04185-4 (HPLS) exhibited irregular and loosely packed starch granules. A significant decrease in ZmPHOs genes expression was detected in the HPLS line ZNC442 as compared to the LPHS line SCML0849, while no expression difference was observed in AGPase encoding genes between these two lines. The down-regulated genes in ZNC442 grains were enriched in nucleotide sugar and fatty acid anabolic pathways, while up-regulated genes were enriched in the ABC transporters pathway. An accelerated breakdown of fat as the P content increased was also observed. This implied that HPLS was resulted from elevated lipid decomposition and inadequate carbon sources. The GWAS analysis identified 514 significantly associated genes, out of which 248 were differentially expressed. Zm00001d052392 was found to be significantly associated with P content/HKW, exhibiting high expression in SCML0849 but almost no expression in ZNC442. Overall, these findings suggested new approaches for achieving a P-yield balance through the manipulation of lipid metabolic pathways in grains.

Microbe-dependent and independent nitrogen and phosphate acquisition and regulation in plants

Nitrogen (N) and phosphorus (P) are the most important macronutrients required for plant growth and development. To cope with the limited and uneven distribution of N and P in complicated soil environments, plants have evolved intricate molecular strategies to improve nutrient acquisition that involve adaptive root development, production of root exudates, and the assistance of microbes. Recently, great advances have been made in understanding the regulation of N and P uptake and utilization and how plants balance the direct uptake of nutrients from the soil with the nutrient acquisition from beneficial microbes such as arbuscular mycorrhiza. Here, we summarize the major advances in these areas and highlight plant responses to changes in nutrient availability in the external environment through local and systemic signals.

Phosphate Transporter OsPT4, Ubiquitinated by E3 Ligase OsAIRP2, Plays a Crucial Role in Phosphorus and Nitrogen Translocation and Consumption in Germinating Seed

Phosphorus (P) and nitrogen (N) are essential macronutrients necessary for plant growth and development. OsPT4 is a high-affinity phosphate (Pi) transporter that has a positive impact on nutrient uptake and seed development. In this study, the expression patterns of different Pi transporter genes in germinating seeds were determined, and the relative expression of OsPT4 was induced in Pi-deficient seeds and gradually increased with the passage of germination time. The analysis of P, N, Pi, and amino acid concentrations in germinating seeds of OsPT4 mutants showed that the OsPT4 mutation caused P and N retention and a continuous reduction in multiple amino acid concentrations in germinating seeds. Transcriptome analysis and qRT-PCR results also indicated that the OsPT4 mutation inhibits the expression of genes related to P and N transportation and amino acid synthesis in germinating seeds. In addition, the paraffin section and TUNEL assay of OsPT4 mutant germinating seeds suggests that OsPT4 mutation causes programmed cell death (PCD) delayed in the aleurone layer and inhibition of leaf outgrowth. Moreover, we also found that OsPT4 was ubiquitinated by OsAIRP2, which is a C3HC4-type RING E3 Ub ligase. Our studies illustrate that OsPT4 plays a crucial role in P and N collaborative translocation and consumption in germinating seeds. It also provides a theoretical basis for the molecules and physiological mechanisms of P and N cross-talk under suppressed Pi uptake conditions.

PIF1, a phytochrome-interacting factor negatively regulates drought tolerance and carotenoids biosynthesis in tobacco

The phytochrome-interacting factors (PIFs) function crucially in multiple physiological processes, but the biological functions of some PIFs remain elusive in some species. Here, a PIF transcription factor NtPIF1 was cloned and characterized in tobacco (Nicotiana tabacum L.). The transcript of NtPIF1 was significantly induced by drought stress treatments, and it localized in the nuclear. Knockout of NtPIF1 by CRISPR/Cas9 system led to the improved drought tolerance of tobacco with increased osmotic adjustment, antioxidant activity, photosynthetic efficiency and decreased water loss rate. On the contrary, NtPIF1-overexpression plants displays drought-sensitive phenotypes. In addition, NtPIF1 reduced the biosynthesis of abscisic acid (ABA) and its upstream carotenoids by regulating the expression of genes involved in ABA and carotenoids biosynthetic pathway upon drought stress. Electrophoretic mobility shift and dual-luciferase assays illustrated that, NtPIF1 directly bind to the E-box elements within the promoters of NtNCED3, NtABI5, NtZDS and Ntβ-LCY to repress their transcription. Overall, these data suggested that NtPIF1 negatively regulate tobacco adaptive response to drought stress and carotenoids biosynthesis; moreover, NtPIF1 has the potential to develop drought-tolerant tobacco plants using CRISPR/Cas9 system.

Effects of different phosphorus levels on tiller bud development in hydroponic Phyllostachys edulis seedlings

An appropriate amount of phosphate fertilizer can improve the germination rate of bamboo buds and increase the bamboo shoot output. However, the underlying biological mechanisms of phosphate fertilizer in bamboo shoot development have not been systematically reported. Herein, the effects of low (LP, 1 μM), normal (NP, 50 μM) and high (HP, 1000 μM) phosphorus (P) on the growth and development of moso bamboo (Phyllostachys edulis) tiller buds were first investigated. Phenotypically, the seedling biomass, average number of tiller buds and bud height growth rate under the LP and HP treatments were significantly lower than those under the NP treatment. Next, the microstructure difference of tiller buds in the late development stage (S4) at three P levels was analyzed. The number of internode cells and vascular bundles were significantly lower in the LP treatments than in the NP treatments. The relative expression levels of eight P transport genes, eight hormone-related genes and four bud development genes at the tiller bud developmental stage (S2-S4) and the tiller bud re-tillering stage were analyzed with real-time polymerase chain reaction. The results showed that the expression trends for most P transport genes, hormone-related genes and bud development genes from S2 to S4 were diversified at different P levels, and the expression levels were also different at different P levels. In the tiller bud re-tillering stage, the expression levels of seven P transport genes and six hormone-related genes showed a downward trend with increasing P level. REV expression level decreased under LP and HP conditions. TB1 expression level increased under HP condition. Therefore, we conclude that P deficiency inhibits tiller bud development and re-tillering, and that P depends on the expression of REV and TB1 genes and auxin, cytokinin and strigolactones synthesis and transporter genes to mediate tiller bud development and re-tillering.

Molecular mechanisms and genetic improvement of low-phosphorus tolerance in rice

Phosphorus (P) is a macronutrient required for plant growth and reproduction. Orthophosphate (Pi), the preferred P form for plant uptake, is easily fixed in the soil, making it unavailable to plants. Limited phosphate rock resources, low phosphate fertilizer use efficiency and high demands for green agriculture production make it important to clarify the molecular mechanisms underlying plant responses to P deficiency and to improve plant phosphate efficiency in crops. Over the past 20 years, tremendous progress has been made in understanding the regulatory mechanisms of the plant P starvation response. Here, we systematically review current research on the mechanisms of Pi acquisition, transport and distribution from the rhizosphere to the shoot; Pi redistribution and reuse during reproductive growth; and the molecular mechanisms of arbuscular mycorrhizal symbiosis in rice (Oryza sativa L.) under Pi deficiency. Furthermore, we discuss several strategies for boosting P utilization efficiency and yield in rice.

Adaptive strategies of plants to conserve internal phosphorus under P deficient condition to improve P utilization efficiency

Phosphorus (P) is one of the limiting factors for plant growth and productivity due to its slow diffusion and immobilization in the soil which necessitates application of phosphatic fertilizers to meet the crop demand and obtain maximum yields. However, plants have evolved mechanisms to adapt to low P stress conditions either by increasing acquisition (alteration of belowground processes) or by internal inorganic P (Pi) utilization (cellular Pi homeostasis) or both. In this review, we have discussed the adaptive strategies that conserve the use of P and maintain cellular Pi homeostasis in the cytoplasm. These strategies involve modification in membrane lipid composition, flavanol/anthocyanin level, scavenging and reutilization of Pi adsorbed in cell wall pectin, remobilization of Pi during senescence by enzymes like RNases and purple acid phosphatases, alternative mitochondrial electron transport, and glycolytic pathways. The remobilization of Pi from senescing tissues and its internal redistribution to various cellular organelles is mediated by various Pi transporters. Although much efforts have been made to enhance P acquisition efficiency, an understanding of the physiological mechanisms conserving internal Pi and their manipulation would be useful for plants that can utilize P more efficiently to produce optimum growth per unit P uptake.

Systematic Identification and Expression Analysis of the Sorghum Pht1 Gene Family Reveals Several New Members Encoding High-Affinity Phosphate Transporters

Sorghum (Sorghum bicolor) is known to have a more robust capability of phosphorus uptake than many other cereal plants, which could be attributed to its phosphate transporter 1 (Pht1) that has a high phosphorus affinity. There are eleven SbPht1 genes in the sorghum genome, nine of which are expressed in sorghum roots or shoots in response to phosphorus deficiency (low-P). The molecular features of these nine genes were investigated by gene expression analysis, subcellular localization, and a yeast mutant complementation growth assay. They were found to be induced in response to low-P stress in root or shoot. All these SbPht1 proteins were found to be localized on the cell membrane, and SbPht1;8 was also detected in the endoplasmic reticulum. These SbPht1s were able to complement the yeast mutant EY917 that lacks all the functional phosphate transporters, and, among them, SbPht1;5, SbPht1;6 and SbPht1;8 could partially complement the yeast mutant strain EY917 in low-P conditions. Overall, these findings demonstrate that SbPht1;5, SbPht1;6, and SbPht1;8 are high-affinity phosphate transporters. SbPht1;5, in particular, is specifically involved in phosphorus uptake in the roots, whilst SbPht1;6 and SbPht1;8 are key players in both P uptake and P transport in response to low-P stress in sorghum.

Comparative analysis of combined phosphorus and drought stress-responses in two winter wheat

Phosphorus stress and drought stress are common abiotic stresses for wheat. In this study, two winter wheat varieties "Xindong20" and "Xindong23" were cultured in a hydroponic system using Hoagland nutrient solution and treated with drought stress under conventional (CP: 1.0 mmol/L) and low (LP: 0.05 mmol/L) phosphorus levels. Under drought stress, the root growth was better under LP than under CP. Under LP, root phosphorus content was increased by 94.2% in Xindong20 and decreased by 48.9% in Xindong23 at 3 d after re-watering, compared with those at 0 d under drought stress. However, the potassium (K) content was the highest among the four elements studied and the phosphorus (P) and calcium (Ca) content were reduced in the root of the two varieties. Under CP, the zinc (Zn) content was higher than that under LP in Xindong23. The GeneChip analysis showed that a total of 4,577 and 202 differentially expressed genes (DEGs) were detected from the roots of Xindong20 and Xindong23, respectively. Among them, 89.9% of DEGs were involved in organelles and vesicles in Xindong20, and 69.8% were involved in root anatomical structure, respiratory chain, electron transport chain, ion transport, and enzyme activity in Xindong23. Overall, LP was superior to CP in mitigating drought stress on wheat, and the regulatory genes were also different in the two varieties. Xindong20 had higher drought tolerance for more up-regulated genes involved in the responses compared to Xindong23.

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.

Mechanisms for improving phosphorus utilization efficiency in plants

BACKGROUND

Limitation of plant productivity by phosphorus (P) supply is widespread and will probably increase in the future. Relatively large amounts of P fertilizer are applied to sustain crop growth and development and to achieve high yields. However, with increasing P application, plant P efficiency generally declines, which results in greater losses of P to the environment with detrimental consequences for ecosystems.

SCOPE

A strategy for reducing P input and environmental losses while maintaining or increasing plant performance is the development of crops that take up P effectively from the soil (P acquisition efficiency) or promote productivity per unit of P taken up (P utilization efficiency). In this review, we describe current research on P metabolism and transport and its relevance for improving P utilization efficiency.

CONCLUSIONS

Enhanced P utilization efficiency can be achieved by optimal partitioning of cellular P and distributing P effectively between tissues, allowing maximum growth and biomass of harvestable plant parts. Knowledge of the mechanisms involved could help design and breed crops with greater P utilization efficiency.

Role of transporters in plant disease resistance

Plants being sessile have evolved numerous mechanisms to meet the changing environmental and growth conditions. Plant pathogens are responsible for devastating disease epidemics in many species. Transporter proteins are an integral part of plant growth and development, and several studies have documented their role in pathogen disease resistance. In this review, we analyze the studies on genome-wide identifications of plant transporters like sugars will eventually be exported transporters (SWEET), multidrug and toxic compound extrusion (MATE) transporters, ATP-binding cassette (ABC) transporters, natural resistance-associated macrophage proteins (NRAMP), and sugar transport proteins (STPs), all having a significant role in plant disease resistance. The mechanism of action of these transporters, their solute specificity, and the potential application of recent molecular biology approaches deploying these transporters for the development of disease-resistant plants are also discussed. The applications of genome editing tools, such as CRIPSR/Cas9, are also presented. Altogether the information included in this article gives a better understanding of the role of transporter proteins during plant-pathogen interaction.

A 1.84-Mb region on rice chromosome 2 carrying SPL4, SPL5 and MLO8 genes is associated with higher yield under phosphorus-deficient acidic soil

Phosphorus (P) deficiency is one of the major limiting factors for rice productivity with only one locus (PSTOL1) available for field based application. A biparental mapping population (F₆) derived from two P deficiency tolerant genotypes (Sahbhagi Dhan (SD) (PSTOL1⁺) and Chakhao Poreiton (CP) (PSTOL1^-)), in which, transcriptome data generated from our lab had previously shown existence of diverse mechanisms was used to identify novel regions for better yield under lowland acidic soils. Phenotyping at F₄, F₅ and F₆ generations revealed significant correlation between traits like tiller number at 30 days (TN 30), tiller number at 60 days (TN 60), filled grains (FG), percent spikelet fertility (SF%), panicle number (PN) and grain yield per panicle (GYPP) and also association with better yield/performance under low P acidic soil conditions. Through selected genotyping on a set of forty superior and inferior lines using SSR, candidate gene-based and SNP polymorphic markers, 5 genomic regions associated with various yield-related traits were identified. Marker trait association studies revealed 13 markers significantly associated with yield attributing traits and PUE under lowland acidic field conditions. Chi-square and regression analyses of markers run on the entire population identified seven and six markers for SF% and GYPP, respectively, and two for biological yield with positive allele derived from SD which constitute a novel 1.847-Mb region on chromosome 2 flanked by two markers RM12550 and PR9-2. Expression analysis of 7 candidate genes lying within this region across SD, CP and two low P susceptible rice genotypes has revealed that expression of four genes including SPL4, SPL5, ACA9 and MLO8 is significantly upregulated only in SD under low P conditions. In CP, there is low expression of MLO8 under low P conditions, whereas SPL4, SPL5 and Os02g08120 are downregulated. In the case of the two susceptible genotypes, there is no expression of Os02g08120 either in optimum or limiting conditions. Sequence data across a panel of 3024 rice genotypes also suggests that there is polymorphism for these differentially expressed genes. The genes and underlying markers identified on chromosome 2 will be key to imparting tolerance to low P in diverse genetic backgrounds and for marker-assisted selection for higher yield under lowland acidic conditions.

Maize ZmPT7 regulates Pi uptake and redistribution which is modulated by phosphorylation

Phosphorus, an essential mineral macronutrient, is a major constituent of fertilizers for maize (Zea mays L.) production. However, the molecular mechanisms of phosphate (Pi) acquisition in maize plants and its redistribution remain unclear. This study presents the functional characterization of ZmPT7 in Pi uptake and redistribution in maize. The ZmPT7 was expressed in roots and leaves, and induced during Pi starvation. The ZmPT7 complemented the Pi-uptake deficiency of yeast mutant phoΔnull and Arabidopsis mutant pht1;1Δ4Δ, indicating that ZmPT7 functioned as a Pi transporter. We generated zmpt7 mutants by CRISPR/Cas9 and ZmPT7-overexpressing lines. The zmpt7 mutants showed reduced, whereas the ZmPT7-overexpressing lines displayed increased Pi-uptake capacity and Pi redistribution from old to young leaves, demonstrating that ZmPT7 played central roles in Pi acquisition and Pi redistribution from old to young leaves. The ZmCK2 kinases phosphorylated ZmPT7 at Ser-521 in old maize leaves, which enhanced transport activity of ZmPT7. The Ser-520 of Arabidopsis AtPHT1;1, a conserved residue of ZmPT7 Ser-521, was also phosphorylated by AtCK2 kinase, and the mutation of Ser-520 to Glu (phosphorylation mimic) yielded enhanced transport activity of AtPHT1;1. Taken together, these results indicate that ZmPT7 plays important roles in Pi acquisition and redistribution, and its transport activity is modulated by phosphorylation.

A genome-wide association study reveals the quantitative trait locus and candidate genes that regulate phosphate efficiency in a Vietnamese rice collection

The crucial role of phosphate (Pi) for plant alongside the expected depletion of non-renewable phosphate rock have created an urgent need for phosphate-efficient rice varieties. In this study, 157 greenhouse-grown Vietnamese rice landraces were treated under Pi-deficient conditions to discover the genotypic variation among biochemical traits, including relative efficiency of phosphorus use (REP), relative root to shoot weight ratio (RRSR), relative physiological phosphate use efficiency (RPPUE), and relative phosphate uptake efficiency (RPUpE). Plants were grown in Yoshida nutrient media with either a full (320 μM) or a low Pi supply (10 μM) over six weeks. This genome-wide association study led to the discovery of 31 significant single nucleotide polymorphisms, 18 quantitative trait loci (QTLs), and 85 candidate genes. A common QTL named qRPUUE9.16 was found among the three investigated traits. Some interesting candidate genes, such as PLASMA MEMBRANE PROTEIN1 (OsPM1), CALMODULIN-RELATED CALCIUM SENSOR PROTEIN 15 (OsCML15), phosphatases 2C (PP2C), STRESS-ACTIVATED PROTEIN KINASE (OsSAPK2), and GLYCEROPHOSPHORYL DIESTER PHOSPHODIESTERASES (GDPD13), were found strongly correlated to the Pi starvation. RNA sequencing transcriptomes revealed that 45 out of 85 candidate genes were significantly regulated under Pi starvation. Furthermore, nearly two-thirds of genotypes did not possess the OsPsTOL1 gene; however, no significant difference was observed in response to Pi deficiency between genotypes with or without this gene, suggesting that other QTLs in rice may resist Pi starvation. These results provide new information on the genetics of nutrient use efficiency in rice and may potentially assist with developing more phosphate-efficient rice plants.

Genome-Wide Identification and Expression Profile Analysis of the PHT1 Gene Family in Gossypium hirsutum and Its Two Close Relatives of Subgenome Donor Species

Phosphate transporter (PHT) is responsible for plant phosphorus (P) absorption and transport. PHT1 is a component of the high-affinity phosphate transporter system and plays pivotal roles in P absorption under P starvation conditions. However, in cotton, the number and identity of PHT1 genes that are crucial for P absorption from soil remain unclear. Here, genome-wide identification detected twelve PHT1 genes in Gossypium hirsutum and seven and eight PHT1 genes in two close relatives of the G. hirsutum genome-G. arboreum and G. raimondii, respectively. In addition, under low-phosphate treatment, the expressions of GaPHT1;3, GaPHT1;4, and GaPHT1;5 in roots were upregulated after 3 h of induction, and GhPHT1;3-At, GhPHT1;4-At, GhPHT1;5-At, GhPHT1;3-Dt, GhPHT1;4-Dt, and GhPHT1;5-Dt in the roots began to respond after 1 h of induction. Homologous pairs-GaPHT1;4 and GhPHT1;4-At; GaPHT1;5 and GhPHT1;5-At; GrPHT1;4 and GhPHT1;4-Dt, with GhPHT1;5-Dt and GhPHT1;5-At being syntenic-were all highly expressed in the roots under normal conditions. Among the genes highly expressed in the roots, GhPHT1;4-At, GhPHT1;5-At, GhPHT1;4-Dt and GhPHT1;5-Dt were continuously upregulated by P starvation. Therefore, it is concluded that these four genes might be key genes for P uptake in cotton roots. The results of this study provide insights into the mechanisms of P absorption and transport in cotton.

Improvement of nutrient use efficiency in rice: current toolbox and future perspectives

Modern agriculture relies heavily on chemical fertilizers, especially in terms of cereal production. The excess application of fertilizers not only increases production cost, but also causes severe environmental problems. As one of the major cereal crops, rice (Oryza sativa L.) provides the staple food for nearly half of population worldwide, especially in developing countries. Therefore, improving rice yield is always the priority for rice breeding. Macronutrients, especially nitrogen (N) and phosphorus (P), are two most important players for the grain yield of rice. However, with economic development and improved living standard, improving nutritional quality such as micronutrient contents in grains has become a new goal in order to solve the "hidden hunger." Micronutrients, such as iron (Fe), zinc (Zn), and selenium (Se), are critical nutritional elements for human health. Therefore, breeding the rice varieties with improved nutrient use efficiency (NUE) is thought to be one of the most feasible ways to increase both grain yield and nutritional quality with limited fertilizer input. In this review, we summarized the progresses in molecular dissection of genes for NUE by reverse genetics on macronutrients (N and P) and micronutrients (Fe, Zn, and Se), exploring natural variations for improving NUE in rice; and also, the current genetic toolbox and future perspectives for improving rice NUE are discussed.

Genotypic Differences in the Effect of P Fertilization on Phytic Acid Content in Rice Grain

Phytic acid (PA) prevents the absorption of minerals in the human intestine, and it is regarded as an antinutrient. Low PA rice is beneficial because of its higher Zn bioavailability and it is suggested that the gene expression level of myo-inositol 3-phosphate synthase 1 (INO1) in developing grain is a key factor to explain the genotypic difference in PA accumulation among natural variants of rice. P fertilization is also considered to affect the PA content, but it is not clear how it affects INO1 gene expression and the PA content in different genotypes. Here, we investigated the effect of P fertilization on the PA content in two contrasting rice genotypes, with low and high PA accumulation, respectively. Based on the results of the analysis of the PA content, inorganic P content, INO1 gene expression, and xylem sap inorganic P content, we concluded that the effect of P fertilization on PA accumulation in grain differed with the genotype, and it was regulated by multiple mechanisms.

The Rice Phosphate Transporter Protein OsPT8 Regulates Disease Resistance and Plant Growth

The absorption of nutrients and disease resistance are two indispensable physiological processes in plants; however, it is still largely unknown whether there is cross-talk between their molecular signaling pathways. In this study, we identified the rice OsPT8 protein, which is a member of the phosphate transporters (PTs) Pht1 family and also plays a role in rice disease resistance. The transcriptional level of OsPT8 is suppressed after infection with rice pathogens and treatment with pathogen-associated molecular patterns (PAMPs). Overexpression of OsPT8 suppresses rice disease resistance against the pathogens Magnaporthe oryzae and Xanthomonas oryzae pv. oryzae. Accordingly, the transcription level of resistance related genes, such as PAL and PBZ1, is inhibited in plants overexpressing OsPT8 (OsPT8-OX) after inoculation with these pathogens. In OsPT8-OX plants, PAMPs-triggered immunity (PTI) response genes, such as OsRac1 and SGT1, are suppressed during treatment with PAMPs chitin or flg22. Moreover, the typical response of PTI is suppressed after chitin or flg22 treatment. We also identified OsPT8 as an interactor of a rice mitogen-activated protein kinase BWMK1, which is a regulator of disease resistance. Under low phosphate (Pi) conditions, the OsPT8-OX plants display better agronomic traits than the control plants. However, the differences in development between OsPT8-OX and the control plants are reduced upon the increase of Pi concentration. These results demonstrate that OsPT8 regulates the transduction of Pi signaling for development and negatively regulates rice immunity.

Characterization of the loss-of-function mutant NH101 for yield under phosphate deficiency from EMS-induced mutants of rice variety Nagina22

In earlier studies at IIRR, Hyderabad, screening of ∼2000 EMS mutants of the rice variety Nagina22 (N22) resulted in the identification of 11 loss-of-function mutants with zero grain yield in Pi-deprived soil under field condition. Among these mutants, NH101 was selected for comparative analyses with N22 for various morphophysiological and/or molecular traits during growth in a hydroponic system (7 d) and in a pot soil (50% flowering) under different Pi regime. The total length of the seminal and adventitious roots, agronomic traits (panicle length and unfilled spikelet/panicle), activities of the antioxidant enzymes (SOD, POD, and APX), and the relative expression levels of the genes involved in the maintenance of Pi homeostasis (MPH) i.e., OsPHR2, SPX1/2 OsPT4, 6, and 8 showed significant increase in the Pi-deprived mutant compared with N22. Whereas, some of the traits showed significant reduction in NH101 than N22 such as number of tillers and filled spikelets/panicle, yield, contents of Pi and externally secreted APase, activity of CAT, and the relative expression levels of MPH genes i.e., OsmiR399a, OsPHO1;2, OsIPS1, OsPAP10a, OsPT2, 9, and 10. The study highlighted wide spectrum differential effects of the mutation in NH101 on various traits that play important roles governing the maintenance of Pi homeostasis. This mutant thus provides a rich repository of genetic material amenable for the identification of the genes that are pivotal for Pi use efficiency.

Genome-wide association study dissects yield components associated with low-phosphorus stress tolerance in maize

Phosphorus deficiency in soil is a worldwide constraint threatening maize production. Through a genome-wide association study, we identified molecular markers and associated candidate genes and molecular pathways for low-phosphorus stress tolerance. Phosphorus deficiency in soils will severely affect maize (Zea mays L.) growth and development, thus decreasing the final yield. Deciphering the genetic basis of yield-related traits can benefit our understanding of maize tolerance to low-phosphorus stress. However, considering that yield-related traits should be evaluated under field condition with large populations rather than under hydroponic condition at a single-plant level, searching for appropriate field experimental sites and target traits for low-phosphorus stress tolerance is still very challenging. In this study, a genome-wide association analysis using two natural populations was performed to detect candidate genes in response to low-phosphorus stress at two experimental sites representative of different climate and soil types. In total, 259 candidate genes were identified and these candidate genes are mainly involved in four major pathways: transcriptional regulation, reactive oxygen scavenging, hormone regulation, and remodeling of cell wall. Among these candidate genes, 98 showed differential expression by transcriptome data. Based on a haplotype analysis of grain number under phosphorus deficiency condition, the positive haplotypes with favorable alleles across five loci increased grain number by 42% than those without favorable alleles. For further verifying the feasibility of genomic selection for improving maize low-phosphorus tolerance, we also validated the predictive ability of five genomic selection methods and suggested that moderate-density SNPs were sufficient to make accurate predictions for low-phosphorus tolerance traits. All these results will facilitate elucidating genetic basis of maize tolerance to low-phosphorus stress and improving marker-assisted selection efficiency in breeding process.

Cellular and Subcellular Phosphate Transport Machinery in Plants

Phosphorus (P) is an essential element required for incorporation into several biomolecules and for various biological functions; it is, therefore, vital for optimal growth and development of plants. The extensive research on identifying the processes underlying the uptake, transport, and homeostasis of phosphate (Pi) in various plant organs yielded valuable information. The transport of Pi occurs from the soil into root epidermal cells, followed by loading into the root xylem vessels for distribution into other plant organs. Under conditions of Pi deficiency, Pi is also translocated from the shoot to the root via the phloem. Vacuoles act as a storage pool for extra Pi, enabling its delivery to the cytosol, a process which plays an important role in the homeostatic control of cytoplasmic Pi levels. In mitochondria and chloroplasts, Pi homeostasis regulates ATP synthase activity to maintain optimal ATP levels. Additionally, the endoplasmic reticulum functions to direct Pi transporters and Pi toward various locations. The intracellular membrane potential and pH in the subcellular organelles could also play an important role in the kinetics of Pi transport. The presented review provides an overview of Pi transport mechanisms in subcellular organelles, and also discusses how they affect Pi balancing at cellular, tissue, and whole-plant levels.

Key acclimation responses to phosphorus deficiency in maize plants are influenced by exogenous nitric oxide

Improving phosphorus (P) acquisition and utilization in crops is of great importance in order to achieve a good plant nutritional state and maximize biomass production while minimizing the addition of fertilizers, and the concomitant risk of eutrophication. This study explores to which extent key processes involved in P-acquisition, and other acclimation mechanisms to low P supply in maize (Zea mays L.) plants, are affected by the addition of a nitric oxide (NO) donor (S-nitrosoglutathione, GSNO). Plants grown in a complete culture solution were exposed to four treatments performed by the combination of two P levels (0 and 0.5 mM), and two GSNO levels (0 and 0.1 mM), and responses to P-deprivation were then studied. Major plant responses related to P-deprivation were affected by the presence of the NO donor. In roots, the activity of acid phosphatases was significantly increased in P-depleted plants simultaneously exposed to GSNO. Acidification of the culture solution also increased in plants that had been grown in the presence of the NO donor. Furthermore, the potential capability displayed by roots of P-deprived plants for P-uptake, was higher in the plants that had been treated with GSNO. These results indicate that exogenous NO addition affects fundamental acclimation responses of maize plants to P scarcity, particularly and positively those that help plants to sustain P-acquisition under low P availability.

Characterization of the rice NLA family reveals a key role for OsNLA1 in phosphate homeostasis

BACKGROUND

Phosphate (Pi), an essential mineral nutrient for plant development and reproduction, is one of the main components of fertilizers in modern agriculture. Previous research demonstrated that AtNLA1 mediates ubiquitination of Pi transporters in the plasma membrane and triggers their endocytosis and degradation in Arabidopsis. In this study, we researched the function of NLA homologous proteins in Pi homeostasis in rice.

FINDINGS

Two OsNLA homologs from rice (Oryza sativa L.) were identified by bioinformatics and phylogenetic analysis and designated OsNLA1 and OsNLA2. The OsNLA1 clustered with Arabidopsis AtNLA1, was expressed higher than OsNLA2 and was transcriptionally repressed under Pi-deficient condition. Loss-of-function of OsNLA1 caused P overaccumulation and growth inhibitions in both root and shoot under Pi-sufficient condition. Furthermore, mutation of OsNLA1 affected expression of Pi tranporters and root hair development under Pi-sufficient and/or Pi-deficient conditions.

CONCLUSIONS

OsNLA1 plays a key role in maintaining phosphate homeostasis in rice.

Functional characterization of the PHT1 family transporters of foxtail millet with development of a novel Agrobacterium-mediated transformation procedure

Phosphate is an essential nutrient for plant growth and is acquired from the environment and distributed within the plant in part through the action of phosphate transporters of the PHT1 family. Foxtail millet (Setaria italica) is an orphan crop essential to the food security of many small farmers in Asia and Africa and is a model system for other millets. A novel Agrobacterium-mediated transformation and direct plant regeneration procedure was developed from shoot apex explants and used to downregulate expression of 3 members of the PHT1 phosphate transporter family SiPHT1;2 SiPHT1;3 and SiPHT1;4. Transformants were recovered with close to 10% efficiency. The downregulation of individual transporters was confirmed by RT-PCR. Downregulation of individual transporters significantly reduced the total and inorganic P contents in shoot and root tissues and increased the number of lateral roots and root hairs showing they have non-redundant roles. Downregulation of SiPHT1;2 had the strongest effect on total and inorganic P in shoot and root tissues. Complementation experiments in S. cerevisiae provide evidence for the ability of SiPHT1;1, 1;2, 1;3, 1;7 and 1;8 to function as high affinity Pi transporters. This work will aid development of improved millet varieties for global food security.

Engineering crop nutrient efficiency for sustainable agriculture

Increasing crop yields can provide food, animal feed, bioenergy feedstocks and biomaterials to meet increasing global demand; however, the methods used to increase yield can negatively affect sustainability. For example, application of excess fertilizer can generate and maintain high yields but also increases input costs and contributes to environmental damage through eutrophication, soil acidification and air pollution. Improving crop nutrient efficiency can improve agricultural sustainability by increasing yield while decreasing input costs and harmful environmental effects. Here, we review the mechanisms of nutrient efficiency (primarily for nitrogen, phosphorus, potassium and iron) and breeding strategies for improving this trait, along with the role of regulation of gene expression in enhancing crop nutrient efficiency to increase yields. We focus on the importance of root system architecture to improve nutrient acquisition efficiency, as well as the contributions of mineral translocation, remobilization and metabolic efficiency to nutrient utilization efficiency.

Identification of putative QTLs for seedling stage phosphorus starvation response in finger millet (Eleusine coracana L. Gaertn.) by association mapping and cross species synteny analysis

A germplasm assembly of 128 finger millet genotypes from 18 countries was evaluated for seedling-stage phosphorus (P) responses by growing them in P sufficient (Psuf) and P deficient (Pdef) treatments. Majority of the genotypes showed adaptive responses to low P condition. Based on phenotype behaviour using the best linear unbiased predictors for each trait, genotypes were classified into, P responsive, low P tolerant and P non-responsive types. Based on the overall phenotype performance under Pdef, 10 genotypes were identified as low P tolerants. The low P tolerant genotypes were characterised by increased shoot and root length and increased root hair induction with longer root hairs under Pdef, than under Psuf. Association mapping of P response traits using mixed linear models revealed four quantitative trait loci (QTLs). Two QTLs (qLRDW.1 and qLRDW.2) for low P response affecting root dry weight explained over 10% phenotypic variation. In silico synteny analysis across grass genomes for these QTLs identified putative candidate genes such as Ser-Thr kinase and transcription factors such as WRKY and basic helix-loop-helix (bHLH). The QTLs for response under Psuf were mapped for traits such as shoot dry weight (qHSDW.1) and root length (qHRL.1). Putative associations of these QTLs over the syntenous regions on the grass genomes revealed proximity to cytochrome P450, phosphate transporter and pectin methylesterase inhibitor (PMEI) genes. This is the first report of the extent of phenotypic variability for P response in finger millet genotypes during seedling-stage, along with the QTLs and putative candidate genes associated with P starvation tolerance.

A phosphoproteomic landscape of rice (Oryza sativa) tissues

Protein phosphorylation is an important posttranslational modification that regulates various plant developmental processes. Here, we report a comprehensive, quantitative phosphoproteomic profile of six rice tissues, including callus, leaf, root, shoot meristem, young panicle and mature panicle from Nipponbare by employing a mass spectrometry (MS)-based, label-free approach. A total of 7171 unique phosphorylation sites in 4792 phosphopeptides from 2657 phosphoproteins were identified, of which 4613 peptides were differentially phosphorylated (DP) among the tissues. Motif-X analysis revealed eight significantly enriched motifs, such as [sP], [Rxxs] and [tP] from the rice phosphosites. Hierarchical clustering analysis divided the DP peptides into 63 subgroups, which showed divergent spatial-phosphorylation patterns among tissues. These clustered proteins are functionally related to nutrition uptake in roots, photosynthesis in leaves and tissue differentiation in panicles. Phosphorylations were specific in the tissues where the target proteins execute their functions, suggesting that phosphorylation might be a key mechanism to regulate the protein activity in different tissues. This study greatly expands the rice phosphoproteomic dataset, and also offers insight into the regulatory roles of phosphorylation in tissue development and functions.

Transport and homeostasis of potassium and phosphate: limiting factors for sustainable crop production

Potassium (K) and phosphate (Pi) are both macronutrients essential for plant growth and crop production, but the unrenewable resources of phosphorus rock and potash have become limiting factors for food security. One critical measure to help solve this problem is to improve nutrient use efficiency (NUE) in plants by understanding and engineering genetic networks for ion uptake, translocation, and storage. Plants have evolved multiple systems to adapt to various nutrient conditions for growth and production. Within the NUE networks, transport proteins and their regulators are the primary players for maintaining nutrient homeostasis and could be utilized to engineer high NUE traits in crop plants. A large number of publications have detailed K+ and Pi transport proteins in plants over the past three decades. Meanwhile, the discovery and validation of their regulatory mechanisms are fast-track topics for research. Here, we provide an overview of K+ and Pi transport proteins and their regulatory mechanisms, which participate in the uptake, translocation, storage, and recycling of these nutrients in plants.

Roles, Regulation, and Agricultural Application of Plant Phosphate Transporters

Phosphorus (P) is an essential mineral nutrient for plant growth and development. Low availability of inorganic phosphate (orthophosphate; Pi) in soil seriously restricts the crop production, while excessive fertilization has caused environmental pollution. Pi acquisition and homeostasis depend on transport processes controlled Pi transporters, which are grouped into five families so far: PHT1, PHT2, PHT3, PHT4, and PHT5. This review summarizes the current understanding on plant PHT families, including phylogenetic analysis, function, and regulation. The potential application of Pi transporters and the related regulatory factors for developing genetically modified crops with high phosphorus use efficiency (PUE) are also discussed in this review. At last, we provide some potential strategies for developing high PUE crops under salt or drought stress conditions, which can be valuable for improving crop yields challenged by global scarcity of water resources and increasing soil salinization.

Genome-wide Identification, Characterization, and Expression Analysis of PHT1 Phosphate Transporters in Wheat

The PHT1 family of phosphate (Pi) transporters mediates phosphorus (P) uptake and re-mobilization in plants. A genome-wide sequence analysis of PHT1 genes in wheat (Triticum aestivum) was conducted, and their expression locations and responses to P availability were further investigated. We cloned 21 TaPHT1 genes from the homologous alleles at TaPHT1.1 to 1.10 through screening a BAC library and amplifying genomic sequences. The TaPHT1 transporters were clustered into five branches in the phylogenetic tree of PHT1 proteins, and the TaPHT1 genes from a given branch shared high similarities in sequences, expression locations, and responses to P availability. The seven tested PHT1 genes all showed Pi-transport activity in yeast (Saccharomyces cerevisiae) cells grown under both low Pi and high Pi conditions. The expression of TaPHT1.1/1.9, 1.2, and 1.10 were root specific. The expression of these TaPHT1 genes at flowering positively correlated with P uptake after stem elongation across three P application rates and two wheat varieties in a field experiment. Therefore, modification of PHT1 expression may improve P use efficiency in a broad regime of P availability.

Characterization of six PHT1 members in Lycium barbarum and their response to arbuscular mycorrhiza and water stress

Phosphorus (P) is vitally important for most plant processes. However, the P available to plants is present in the soil in the form of inorganic phosphate (Pi), and is often present in only limited amounts. Water stress further reduces Pi availability. Previous studies have highlighted the important roles of members of the PHOSPHATE TRANSPORTER 1 (PHT1) family and arbuscular mycorrhizal (AM) associations for Pi acquisition by plants growing in various environments. In order to understand the Pi uptake of Lycium barbarumL., a drought-tolerant ligneous species belonging to the Solanaceae family, we cloned and characterized six L. barbarum genes encoding transporter proteins belonging to the PHT1 family, and investigated their transcriptional response to AM associations and water stress. The six cloned PHT1 genes of L. barbarum had a similar evolutionary history to that of PHT1 genes found in other Solanaceae species. Three of these genes (LbPT3, LbPT4 and LbPT5) were AM-induced; the other three genes (LbPT1, LbPT2 and LbPT7) played distinct roles in Pi acquisition, translocation and remobilization in roots and leaves. AM-induced PHT1 genes maintained their function under water stress, while moderate and severe water stress upregulated non-AM-induced PHT1 genes in roots and leaves, respectively. Moreover, although LbPT1 was upregulated in AM roots under water stress, LbPT2 and LbPT7 were inhibited in AM roots, which suggested that an AM association satisfied the demand for Pi in roots under water stress and that LbPT1 may play a role in translocating Pi from roots to shoots in this situation.

Tissue specific transcript profiling of wheat phosphate transporter genes and its association with phosphate allocation in grains

Approaches enabling efficient phosphorus utilization in crops are of great importance. In cereal crop like wheat, utilization of inorganic phosphate (Pi) is high and mature grains are the major sink for Pi utilization and storage. Research that addresses the importance of the Pi homeostasis in developing grains is limited. In an attempt to understand the Pi homeostasis in developing wheat grains, we identified twelve new phosphate transporters (PHT), these are phyologentically well distributed along with the members reported from Arabidopsis and rice. Enhanced expression of PHT1-subfamily genes was observed in roots subjected to the Pi starvation suggesting their active role in Pi homeostasis. Differential expression patterns of all the PHT genes during grain filling stages suggested their importance in the filial tissues. Additionally, high accumulation of Pi and total P in aleurone correlates well with the expression of TaPHTs and other phosphate starvation related genes. Tissue specific transcript accumulation of TaPHT1.1, TaPHT1.2, TaPHT1.4 in aleurone; TaPHT3.1 in embryo and TaPHT4.2 in the endosperm was observed. Furthermore, their transcript abundance was affected in low phytate wheat grains. Altogether, this study helps in expanding the knowledge and prioritize the candidate wheat Pi-transporters to modulate the Pi homeostasis in cereal grains.

Identification and expression analysis of OsLPR family revealed the potential roles of OsLPR3 and 5 in maintaining phosphate homeostasis in rice

BACKGROUND

Phosphorus (P), an essential macronutrient, is often limiting in soils and affects plant growth and development. In Arabidopsis thaliana, Low Phosphate Root1 (LPR1) and its close paralog LPR2 encode multicopper oxidases (MCOs). They regulate meristem responses of root system to phosphate (Pi) deficiency. However, the roles of LPR gene family in rice (Oryza sativa) in maintaining Pi homeostasis have not been elucidated as yet.

RESULTS

Here, the identification and expression analysis for the homologs of LPR1/2 in rice were carried out. Five homologs, hereafter referred to as OsLPR1-5, were identified in rice, which are distributed on chromosome1 over a range of 65 kb. Phylogenetic analysis grouped OsLPR1/3/4/5 and OsLPR2 into two distinct sub-clades with OsLPR3 and 5 showing close proximity. Quantitative real-time RT-PCR (qRT-PCR) analysis revealed higher expression levels of OsLPR3-5 and OsLPR2 in root and shoot, respectively. Deficiencies of different nutrients ie, P, nitrogen (N), potassium (K), magnesium (Mg) and iron (Fe) exerted differential and partially overlapping effects on the relative expression levels of the members of OsLPR family. Pi deficiency (-P) triggered significant increases in the relative expression levels of OsLPR3 and 5. Strong induction in the relative expression levels of OsLPR3 and 5 in osphr2 suggested their negative transcriptional regulation by OsPHR2. Further, the expression levels of OsLPR3 and 5 were either attenuated in ossiz1 and ospho2 or augmented in rice overexpressing OsSPX1.

CONCLUSIONS

The results from this study provided insights into the evolutionary expansion and a likely functional divergence of OsLPR family with potential roles of OsLPR3 and 5 in the maintenance of Pi homeostasis in rice.

Phosphate Uptake and Allocation - A Closer Look at Arabidopsis thaliana L. and Oryza sativa L

This year marks the 20th anniversary of the discovery and characterization of the two Arabidopsis PHT1 genes encoding the phosphate transporter in Arabidopsis thaliana. So far, multiple inorganic phosphate (Pi) transporters have been described, and the molecular basis of Pi acquisition by plants has been well-characterized. These genes are involved in Pi acquisition, allocation, and/or signal transduction. This review summarizes how Pi is taken up by the roots and further distributed within two plants: A. thaliana and Oryza sativa L. by plasma membrane phosphate transporters PHT1 and PHO1 as well as by intracellular transporters: PHO1, PHT2, PHT3, PHT4, PHT5 (VPT1), SPX-MFS and phosphate translocators family. We also describe the role of the PHT1 transporters in mycorrhizal roots of rice as an adaptive strategy to cope with limited phosphate availability in soil.

Complex Regulation of Plant Phosphate Transporters and the Gap between Molecular Mechanisms and Practical Application: What Is Missing?

It has been almost 25 years since the first report of the gene encoding a high-affinity phosphate transporter (PT), PHO84, in yeast. Since then, an increasing number of yeast PHO84 homologs as well as other genes encoding proteins with phosphate (Pi) transport activities have been identified and functionally characterized in diverse plant species. Great progress has been made also in deciphering the molecular mechanism underlying the regulation of the abundance and/or activity of these genes and their products. The regulatory genes affect plant Pi homeostasis commonly through direct or indirect regulation of the abundance of PTs at different levels. However, little has been achieved in the use of PTs for developing genetically modified crops with high phosphorus use efficiency (PUE). This might be a consequence of overemphasizing Pi uptake from the rhizosphere and lack of knowledge about the roles of PTs in Pi transport and recycling within the plant that are required to optimize PUE. Here, we mainly focused on the genes encoding proteins with Pi transport activities and the emerging understanding of their regulation at the transcriptional, post-transcriptional, translational, and post-translational levels. In addition, we propose potential strategies for effective use of PTs in improving plant growth and development.

Involvement of OsPht1;4 in phosphate acquisition and mobilization facilitates embryo development in rice

Phosphate (Pi) transporters mediate acquisition and transportation of Pi within plants. Here, we investigated the functions of OsPht1;4 (OsPT4), one of the 13 members of the Pht1 family in rice. Quantitative real-time RT-PCR analysis revealed strong expression of OsPT4 in roots and embryos, and OsPT4 promoter analysis using reporter genes confirmed these findings. Analysis using rice protoplasts showed that OsPT4 localized to the plasma membrane. OsPT4 complemented a yeast mutant defective in Pi uptake, and also facilitated increased accumulation of Pi in Xenopus oocytes. Further, OsPT4 genetically modified (GM) rice lines were generated by knockout/knockdown or over-expression of OsPT4. Pi concentrations in roots and shoots were significantly lower and higher in knockout/knockdown and over-expressing plants, respectively, compared to wild-type under various Pi regimes. (33) Pi uptake translocation assays corroborated the altered acquisition and mobilization of Pi in OsPT4 GM plants. We also observed effects of altered expression levels of OsPT4 in GM plants on the concentration of Pi, the size of the embryo, and several attributes related to seed development. Overall, our results suggest that OsPT4 encodes a plasma membrane-localized Pi transporter that facilitates acquisition and mobilization of Pi, and also plays an important role in development of the embryo in rice.

Comparative Morphophysiological Analyses and Molecular Profiling Reveal Pi-Efficient Strategies of a Traditional Rice Genotype

Phosphate (Pi) deficiency severely affects crop yield. Modern high yielding rice genotypes are sensitive to Pi deficiency whereas traditional rice genotypes are naturally compatible with low Pi ecosystems. However, the underlying molecular mechanisms for low Pi tolerance in traditional genotypes remain largely elusive. To delineate the molecular mechanisms for low Pi tolerance, two contrasting rice genotypes, Dular (low Pi tolerant), and PB1 (low Pi sensitive), have been selected. Comparative morphophysiological, global transcriptome and lipidome analyses of root and shoot tissues of both genotypes grown under Pi deficient and sufficient conditions revealed potential low Pi tolerance mechanisms of the traditional genotype. Most of the genes associated with enhanced internal Pi utilization (phospholipid remobilization) and modulation of root system architecture (RSA) were highly induced in the traditional rice genotype, Dular. Higher reserves of phospholipids and greater accumulation of galactolipids under low Pi in Dular indicated it has more efficient Pi utilization. Furthermore, Dular also maintained greater root growth than PB1 under low Pi, resulting in larger root surface area due to increased lateral root density and root hair length. Genes involved in enhanced low Pi tolerance of the traditional genotype can be exploited to improve the low Pi tolerance of modern high yielding rice cultivars.