LEAF TIP NECROSIS1 plays a pivotal role in the regulation of multiple phosphate starvation responses in rice

OsSIZ1, a SUMO E3 Ligase Gene, is Involved in the Regulation of the Responses to Phosphate and Nitrogen in Rice

SIZ1-mediated SUMOylation regulates hormone signaling as well as abiotic and biotic stress responses in plants. Here, we investigated the expression profile of OsSIZ1 in rice using quantitative reverse transcription-PCR (qRT-PCR) and pOsSIZ1-GUS transgenic plants, and the function of OsSIZ1 in the responses to phosphate and nitrogen using a reverse genetics approach. OsSIZ1 is constitutively expressed throughout the vegetative and reproductive growth of rice, with stronger promoter activities in vascular bundles of culms. ossiz1 mutants had shorter primary roots and adventitious roots than wild-type plants, suggesting that OsSIZ1 is associated with the regulation of root system architecture. Total phosphorus (P) and phosphate (Pi) concentrations in both roots and shoots of ossiz1 mutants were significantly increased irrespective of Pi supply conditions compared with the wild type. Pi concentration in the xylem sap of ossiz1 mutants was significantly higher than that of the wild type under a Pi-sufficient growth regime. Total nitrogen (N) concentrations in the most detected tissues of ossiz1 mutants were significantly increased compared with the wild type. Analysis of mineral contents in ossiz1 mutants indicated that OsSIZ1 functions specifically in Pi and N responses, not those of other nutrients examined, in rice. Further, qRT-PCR analyses revealed that the expression of multiple genes involved in Pi starvation signaling and N transport and assimilation were altered in ossiz1 mutants. Together, these results suggested that OsSIZ1 may act as a regulator of the Pi (N)-dependent responses in rice.

Role of microRNAs involved in plant response to nitrogen and phosphorous limiting conditions

Plant microRNAs (miRNAs) are a class of small non-coding RNAs which target and regulate the expression of genes involved in several growth, development, and metabolism processes. Recent researches have shown involvement of miRNAs in the regulation of uptake and utilization of nitrogen (N) and phosphorus (P) and more importantly for plant adaptation to N and P limitation conditions by modifications in plant growth, phenology, and architecture and production of secondary metabolites. Developing strategies that allow for the higher efficiency of using both N and P fertilizers in crop production is important for economic and environmental benefits. Improved crop varieties with better adaptation to N and P limiting conditions could be a key approach to achieve this effectively. Furthermore, understanding on the interactions between N and P uptake and use and their regulation is important for the maintenance of nutrient homeostasis in plants. This review describes the possible functions of different miRNAs and their cross-talk relevant to the plant adaptive responses to N and P limiting conditions. In addition, a comprehensive understanding of these processes at molecular level and importance of biological adaptation for improved N and P use efficiency is discussed.

Integrative Comparison of the Role of the PHOSPHATE RESPONSE1 Subfamily in Phosphate Signaling and Homeostasis in Rice

Phosphorus (P), an essential macronutrient for all living cells, is indispensable for agricultural production. Although Arabidopsis (Arabidopsis thaliana) PHOSPHATE RESPONSE1 (PHR1) and its orthologs in other species have been shown to function in transcriptional regulation of phosphate (Pi) signaling and Pi homeostasis, an integrative comparison of PHR1-related proteins in rice (Oryza sativa) has not previously been reported. Here, we identified functional redundancy among three PHR1 orthologs in rice (OsPHR1, OsPHR2, and OsPHR3) using phylogenetic and mutation analysis. OsPHR3 in conjunction with OsPHR1 and OsPHR2 function in transcriptional activation of most Pi starvation-induced genes. Loss-of-function mutations in any one of these transcription factors (TFs) impaired root hair growth (primarily root hair elongation). However, these three TFs showed differences in DNA binding affinities and messenger RNA expression patterns in different tissues and growth stages, and transcriptomic analysis revealed differential effects on Pi starvation-induced gene expression of single mutants of the three TFs, indicating some degree of functional diversification. Overexpression of genes encoding any of these TFs resulted in partial constitutive activation of Pi starvation response and led to Pi accumulation in the shoot. Furthermore, unlike OsPHR2-overexpressing lines, which exhibited growth retardation under normal or Pi-deficient conditions, OsPHR3-overexpressing plants exhibited significant tolerance to low-Pi stress but normal growth rates under normal Pi conditions, suggesting that OsPHR3 would be useful for molecular breeding to improve Pi uptake/use efficiency under Pi-deficient conditions. We propose that OsPHR1, OsPHR2, and OsPHR3 form a network and play diverse roles in regulating Pi signaling and homeostasis in rice.

Replace, reuse, recycle: improving the sustainable use of phosphorus by plants

The 'phosphorus problem' has recently received strong interest with two distinct strands of importance. The first is that too much phosphorus (P) is entering into waste water, creating a significant economic and ecological problem. Secondly, while agricultural demand for phosphate fertilizer is increasing to maintain crop yields, rock phosphate reserves are rapidly declining. Unravelling the mechanisms by which plants sense, respond to, and acquire phosphate can address both problems, allowing the development of crop plants that are more efficient at acquiring and using limited amounts of phosphate while at the same time improving the potential of plants and other photosynthetic organisms for nutrient recapture and recycling from waste water. In this review, we attempt to synthesize these important but often disparate parts of the debate in a holistic fashion, since solutions to such a complex problem require integrated and multidisciplinary approaches that address both P supply and demand. Rapid progress has been made recently in our understanding of local and systemic signalling mechanisms for phosphate, and of expression and regulation of membrane proteins that take phosphate up from the environment and transport it within the plant. We discuss the current state of understanding of such mechanisms involved in sensing and responding to phosphate stress. We also discuss approaches to improve the P-use efficiency of crop plants and future direction for sustainable use of P, including use of photosynthetic organisms for recapture of P from waste waters.

Interaction between carbon metabolism and phosphate accumulation is revealed by a mutation of a cellulose synthase-like protein, CSLF6

Phosphorus is an essential macronutrient for plant growth and development. However, the network that affects phosphate (Pi) accumulation in crops is not well established. It is reported here that OsCSLF6, a member of the cellulose synthase-like family (CSLF), which is found only in grasses, is involved in Pi accumulation. The oscslf6 mutants (oscslf6-1 and oscslf6-2) display Pi toxic symptoms and increased Pi accumulation in both roots and shoots under the Pi-sufficient condition, which correlate with the induced expression of Pi transporters in the knockout mutants. Consistent with the over-accumulation of Pi, a significant decrease in primary root length, adventitious root length, and adventitious root number were observed in the oscslf6 mutants when compared with the wild type (WT) under Pi-sufficient conditions. In addition, the sucrose (Suc) level was increased in the oscslf6 mutants and the expression of sucrose synthases (OsSUS4/5) and sucrose transporters (OsSUT1/2/4/OsSweet14) genes were also induced in the shoots of oscslf6 mutants, suggesting that OsCSLF6 may play a role in affecting Pi accumulation by affecting the level of carbon metabolism.

miRNA regulation of nutrient homeostasis in plants

Small RNAs including micro RNAs (miRNA) play an indispensable role in cell signaling mechanisms. Generally, miRNAs that are 20-24 nucleotides long bind to specific complementary transcripts, attenuating gene expression at the post-transcriptional level or via translational inhibition. In plants, miRNAs have emerged as the principal regulator of various stress responses, including low nutrient availability. It has been reported that miRNAs are vital for maintaining nutrient homeostasis in plants by regulating the expression of transporters that are involved in nutrient uptake and mobilization. The present review highlights the role of various miRNAs in several macro- or micronutrient deficiencies in plants. Understanding the regulation of different transporters by miRNAs will aid in elucidating the underlying molecular signal transduction mechanisms during nutritional stress. Recent findings regarding nutrient related-miRNAs and their gene regulation machinery may delineate a novel platform for improving the nutritional status of cereal grains or crop biofortification programs in the future.

Exposure of barley plants to low Pi leads to rapid changes in root respiration that correlate with specific alterations in amino acid substrates

The majority of inorganic phosphate (Pi ) stress studies in plants have focused on the response after growth has been retarded. Evidence from transcript analysis, however, shows that a Pi -stress specific response is initiated within minutes of transfer to low Pi and in crop plants precedes the expression of Pi transporters and depletion of vacuolar Pi reserves by days. In order to investigate the physiological and metabolic events during early exposure to low Pi in grain crops, we monitored the response of whole barley plants during the first hours following Pi withdrawal. Lowering the concentration of Pi led to rapid changes in root respiration and leaf gas exchange throughout the early phase of the light course. Combining amino and organic acid analysis with (15) N labelling we show a root-specific effect on nitrogen metabolism linked to specific substrates of respiration as soon as 1 h following Pi withdrawal; this explains the respiratory responses observed and was confirmed by stimulation of respiration by exogenous addition of these respiratory substrates to roots. The rapid adjustment of substrates for respiration in roots during short-term Pi -stress is highlighted and this could help guide roots towards Pi -rich soil patches without compromising biomass accumulation of the plant.

miRNAome analysis associated with anatomic and transcriptomic investigations reveal the polar exhibition of corky split vein in boron deficient Citrus sinensis

Corky split vein can develop under long-term boron deficient conditions in Citrus sinensis L. Osbeck cv. Newhall. This symptom only occurs in the upper rather than the lower epidermis of old leaves. Our previous study demonstrated that vascular hypertrophy was involved in the symptoms, and the 3rd developmental stage of corky split vein (BD3) was the critical stage for phenotype formation. Here, we performed an intensive study on the BD3 vein and its control sample (CK3 vein). A lignin test demonstrated that the lignin content in BD3 vein was approximately 1.7 times more than the CK3 vein. Anatomical investigation of the corky split vein indicated that the upper epidermis was destroyed by overgrowing vascular cells, and the increased lignin may contribute to vascular cell differentiation and wounding-induced lignification. In a subsequent small RNA sequencing of the BD3 and CK3 veins, 99 known miRNAs and 22 novel miRNAs were identified. Comparative profiling of these miRNAs demonstrated that the 57 known miRNAs and all novel miRNAs exhibited significant expression differences between the two small RNAs libraries of the BD3 and CK3 veins. Associated with our corresponding digital gene expression data, we propose that the decreased expression of two miRNAs, csi-miR156b and csi-miR164, which leads to the up-regulation of their target genes, SPLs (csi-miR156b-targeted) and CUC2 (csi-miR164-targeted), may promote vascular cell division and orderless stage transition in old leaves.

MicroRNA399 is involved in multiple nutrient starvation responses in rice

The increasing evidences have revealed that microRNAs (miRNAs) play significant role in nutrient stress response. Previously, miR399 was documented to be induced by phosphorus (P) starvation and involved in regulating P starvation responses. To further investigate the function of miR399 in rice (Oryza sativa L.), we performed GeneChip analysis with OsmiR399 over-expressing plants. Interestingly, our results showed that, besides P starvation responsive genes, the expression of a number of genes involved in iron (Fe), potassium (K), sodium (Na), and calcium (Ca) absorption was dramatically up-regulated in OsmiR399 over-expressing plants. Consistently, the concentrations of Fe, K, Na, and Ca were also increased in OsmiR399 over-expressing plants. The expression of OsmiR399 was also up-regulated by these nutrient starvations, respectively. Moreover, the loss-of-function of LTN1, the down-stream target of OsmiR399, also resulted in the increase of multiple metal elements and the up-regulation of the absorption related genes. These results indicated that OsmiR399 participates in the regulation of multiple nutrient starvation responses, which also gives new view on understanding the interaction among different nutrients mediated by miR399.

OsCYCP1;1, a PHO80 homologous protein, negatively regulates phosphate starvation signaling in the roots of rice (Oryza sativa L.)

Phosphorus is one of the most essential and limiting nutrients in all living organisms, thus the organisms have evolved complicated and precise regulatory mechanisms for phosphorus acquisition, storage and homeostasis. In the budding yeast, Saccharomyces cerevisiae, the modification of PHO4 by the PHO80 and PHO85 complex is a core regulation system. However, the existence and possible functions in phosphate signaling of the homologs of the PHO80 and PHO85 components in plants has yet to be determined. Here we describe the identification of a family of seven PHO80 homologous genes in rice named OsCYCPs. Among these, the OsCYCP1;1 gene was able to partially rescue the pho80 mutant strain of yeast. The OsCYCP1;1 protein was predominantly localized in the nucleus, and was ubiquitously expressed throughout the whole plant and during the entire growth period of rice. Consistent with the negative role of PHO80 in phosphate signaling in yeast, OsCYCP1;1 expression was reduced by phosphate starvation in the roots. This reduction was dependent on PHR2, the central regulator of phosphate signaling in rice. Overexpression and suppression of the expression of OsCYCP1;1 influenced the phosphate starvation signaling response. The inducible expression of phosphate starvation inducible and phosphate transporter genes was suppressed in the OsCYCP1;1 overexpression lines and was relatively enhanced in the OsCYCP1;1 RNAi plants by phosphate starvation. Together, these results demonstrate the role of PHO80 homologs in the phosphate starvation signaling pathway in rice.

Root architecture responses: in search of phosphate

Soil phosphate represents the only source of phosphorus for plants and, consequently, is its entry into the trophic chain. This major component of nucleic acids, phospholipids, and energy currency of the cell (ATP) can limit plant growth because of its low mobility in soil. As a result, root responses to low phosphate favor the exploration of the shallower part of the soil, where phosphate tends to be more abundant, a strategy described as topsoil foraging. We will review the diverse developmental strategies that can be observed among plants by detailing the effect of phosphate deficiency on primary and lateral roots. We also discuss the formation of cluster roots: an advanced adaptive strategy to cope with low phosphate availability observed in a limited number of species. Finally, we will put this work into perspective for future research directions.

Challenges of modifying root traits in crops for agriculture

Roots play an essential role in the acquisition of water and minerals from soils. Measuring crop root architecture and assaying for changes in function can be challenging, but examples have emerged showing that modifications to roots result in higher yield and increased stress tolerance. In this review, we focus mainly on the molecular genetic advances that have been made in altering root system architecture and function in crop plants, as well as phenotyping methods. The future for the modification of crop plant roots looks promising based on recent advances, but there are also important challenges ahead.

Regulation of plant root system architecture: implications for crop advancement

Root system architecture (RSA) plays a major role in plant fitness, crop performance, and grain yield yet only recently has this role been appreciated. RSA describes the spatial arrangement of root tissue within the soil and is therefore crucial to nutrient and water uptake. Recent studies have identified many of the genetic and environmental factors influencing root growth that contribute to RSA. Some of the identified genes have the potential to limit crop loss caused by environmental extremes and are currently being used to confer drought tolerance. It is hypothesized that manipulating these and other genes that influence RSA will be pivotal for future crop advancements worldwide.

MicroRNA-mediated surveillance of phosphate transporters on the move

Phosphate (Pi), which is indispensable for the structural and metabolic needs of plants, is acquired and translocated by Pi transporters. Deciphering the regulatory network of Pi signaling and homeostasis that involves the control of Pi transporters trafficking to, and their activity at, the plasma membrane provides insight into how plants adapt to environmental changes in Pi availability. Here, we review recent studies that revealed the involvement of microRNA399-PHOSPHATE 2 (PHO2) and microR827-NITROGEN LIMITATION ADAPTATION (NLA) modules in mediating the ubiquitination and degradation of PHOSPHATE TRANSPORTER 1 (PHT1) and/or PHOSPHATE 1 (PHO1). These discoveries show that miRNAs are an effective way for plants to monitor the turnover of Pi transporters in the membrane system by modulating the functioning of the membrane-associated ubiquitin machinery.

Control of phosphate homeostasis through gene regulation in crops

Phosphorus (P) is an essential yet frequently deficient element in plants. Maintenance of phosphate (Pi) homeostasis is crucial for crop production. In comparison with the model plant Arabidopsis, crops face wider ranges and larger fluctuations in P supply from the soil environment, and thus develop more complicated strategies to improve Pi acquisition and utilization efficiency. Undergirding these strategies, there are numerous genes involved in alternative metabolism pathways that are regulated by complex Pi signaling networks. In this review, we intend to summarize the recent advances in crops on control of Pi homeostasis through gene regulation from Pi acquisition and mobilization within plants, as well as activation of rhizosphere P and P uptake through symbiotic associations.

Molecular dissection of complex agronomic traits of rice: a team effort by Chinese scientists in recent years

Rice is a staple food for more than half of the worldwide population and is also a model species for biological studies on monocotyledons. Through a team effort, Chinese scientists have made rapid and important progresses in rice biology in recent years. Here, we briefly review these advances, emphasizing on the regulatory mechanisms of the complex agronomic traits that affect rice yield and grain quality. Progresses in rice genome biology and genome evolution have also been summarized.

Are rice (Oryza sativa L.) phosphate transporters regulated similarly by phosphate and arsenate? A comprehensive study

Rice is one of the most important staple foods worldwide, but it often contains inorganic arsenic, which is toxic and gives rise to severe health problems. Rice plants take up arsenate As(V) via the phosphate transport pathways, though it is not known how As(V), as compared to phosphate, modifies the expression of phosphate transporters (PTs). Therefore, the impact of As(V) or phosphate (Pi) on the gene expression of PTs and several Pi signaling regulators was investigated. Rice plants were grown on medium containing different As(V) or Pi concentrations. Growth was evaluated and the expression of tested genes was quantified at different time points, using quantitative RT-PCR (qPCR). The As and P content in plants was determined using inductively coupled plasma mass spectrometry (ICP-MS). As(V) elicited diverse and opposite responses of different PTs in roots and shoots, while Pi triggered a more shallow and uniform transcriptional response in several tested genes. Only a restricted set of genes, including PT2, PT3, PT5 and PT13 and two SPX-MFS family members, was particularly responsive to As(V). Despite some common reactions, the responses of the analyzed genes were predominantly ion-specific. The possible reasons and consequences are discussed.

Identification of boron-deficiency-responsive microRNAs in Citrus sinensis roots by Illumina sequencing

BACKGROUND

Boron (B)-deficiency is a widespread problem in many crops, including Citrus. MicroRNAs (miRNAs) play important roles in nutrient deficiencies. However, little is known on B-deficiency-responsive miRNAs in plants. In this study, we first identified miRNAs and their expression pattern in B-deficient Citrus sinensis roots by Illumina sequencing in order to identify miRNAs that might be involved in the tolerance of plants to B-deficiency.

RESULTS

We isolated 52 (40 known and 12 novel) up-regulated and 82 (72 known and 10 novel) down-regulated miRNAs from B-deficient roots, demonstrating remarkable metabolic flexibility of roots, which might contribute to the tolerance of plants to B-deficiency. A model for the possible roles of miRNAs in the tolerance of roots to B-deficiency was proposed. miRNAs might regulate the adaptations of roots to B-deficiency through following several aspects: (a) inactivating reactive oxygen species (ROS) signaling and scavenging through up-regulating miR474 and down-regulating miR782 and miR843; (b) increasing lateral root number by lowering miR5023 expression and maintaining a certain phenotype favorable for B-deficiency-tolerance by increasing miR394 expression; (c) enhancing cell transport by decreasing the transcripts of miR830, miR5266 and miR3465; (d) improving osmoprotection (miR474) and regulating other metabolic reactions (miR5023 and miR821). Other miRNAs such as miR472 and miR2118 in roots increased in response to B-deficiency, thus decreasing the expression of their target genes, which are involved in disease resistance, and hence, the disease resistance of roots.

CONCLUSIONS

Our work demonstrates the possible roles of miRNAs and related mechanisms in the response of plant roots to B-deficiency.

miR444a has multiple functions in the rice nitrate-signaling pathway

Nitrate (NO3-) is a key signaling molecule in plant metabolism and development, in addition to its role as a nutrient. It has been shown previously in Arabidopsis that ANR1, a MADS-box transcription factor, is a major component in the NO3--signaling pathway that triggers lateral root growth and that miR444, which is specific to monocots, targets four genes that are homologous to ANR1 in rice. Here, we show that miR444a plays multiple roles in the rice NO3--signaling pathway - not only in root development, but also involving nitrate accumulation and even Pi -starvation responses. miR444a overexpression resulted in reduced rice lateral root elongation, but promoted rice primary and adventitious root growth, in a nitrate-dependent manner. In addition, overexpression of miR444a improved nitrate accumulation and expression of nitrate transporter genes under high nitrate concentration conditions, but reduced the remobilization of nitrate from old leaves to young leaves thus affecting the plant's ability to adapt to nitrogen-limitating conditions. Intriguingly, we found that Pi starvation strongly induced miR444 accumulation in rice roots and that overexpression of miR444a altered Pi -starvation-induced root architecture and enhanced Pi accumulation and expression of three Pi transporter genes. We further provide evidence that miR444a is involved in the interaction between the NO3--signaling and Pi -signaling pathways in rice. Taken together, our observations demonstrated that miR444a plays multiple roles in the rice NO3--signaling pathway in nitrate-dependent root growth, nitrate accumulation and phosphate-starvation responses.

Recent progress on molecular breeding of rice in China

Molecular breeding of rice for high yield, superior grain quality, and strong environmental adaptability is crucial for feeding the world's rapidly growing population. The increasingly cloned quantitative trait loci and genes, genome variations, and haplotype blocks related to agronomically important traits in rice have provided a solid foundation for direct selection and molecular breeding, and a number of genes have been successfully introgressed into mega varieties of rice. Here we summarize China's great achievements in molecular breeding of rice in the following five traits: high yield, biotic stress resistance, abiotic stress resistance, quality and physiology. Further, the prospect of rice breeding by molecular design is discussed.

Meta-analysis and candidate gene mining of low-phosphorus tolerance in maize

Plants with tolerance to low-phosphorus (P) can grow better under low-P conditions, and understanding of genetic mechanisms of low-P tolerance can not only facilitate identifying relevant genes but also help to develop low-P tolerant cultivars. QTL meta-analysis was conducted after a comprehensive review of the reports on QTL mapping for low-P tolerance-related traits in maize. Meta-analysis produced 23 consensus QTL (cQTL), 17 of which located in similar chromosome regions to those previously reported to influence root traits. Meanwhile, candidate gene mining yielded 215 genes, 22 of which located in the cQTL regions. These 22 genes are homologous to 14 functionally characterized genes that were found to participate in plant low-P tolerance, including genes encoding miR399s, Pi transporters and purple acid phosphatases. Four cQTL loci (cQTL2-1, cQTL5-3, cQTL6-2, and cQTL10-2) may play important roles for low-P tolerance because each contains more original QTL and has better consistency across previous reports.

OsPT2, a phosphate transporter, is involved in the active uptake of selenite in rice

• Selenite is a predominant form of selenium (Se) available to plants, especially in anaerobic soils, but the molecular mechanism of selenite uptake by plants is not well understood. • ltn1, a rice mutant previously shown to have increased phosphate (Pi) uptake, was found to exhibit higher selenite uptake than the wild-type in both concentration- and time-dependent selenite uptake assays. Respiratory inhibitors significantly inhibited selenite uptake in the wildtype and the ltn1 mutant, indicating that selenite uptake was coupled with H(+) and energy-dependent. Selenite uptake was greatly enhanced under Pi-starvation conditions, suggesting that Pi transporters are involved in selenite uptake. • OsPT2, the most abundantly expressed Pi transporter in the roots, is also significantly up-regulated in ltn1 and dramatically induced by Pi starvation. OsPT2-overexpressing and knockdown plants displayed significantly increased and decreased rates of selenite uptake, respectively, suggesting that OsPT2 plays a crucial role in selenite uptake. Se content in rice grains also increased significantly in OsPT2-overexpressing plants. • These data strongly demonstrate that selenite and Pi share similar uptake mechanisms and that OsPT2 is involved in selenite uptake, which provides a potential strategy for breeding Se-enriched rice varieties.

Molecular mechanisms underlying phosphate sensing, signaling, and adaptation in plants

As an essential plant macronutrient, the low availability of phosphorus (P) in most soils imposes serious limitation on crop production. Plants have evolved complex responsive and adaptive mechanisms for acquisition, remobilization and recycling of phosphate (Pi) to maintain P homeostasis. Spatio-temporal molecular, physiological, and biochemical Pi deficiency responses developed by plants are the consequence of local and systemic sensing and signaling pathways. Pi deficiency is sensed locally by the root system where hormones serve as important signaling components in terms of developmental reprogramming, leading to changes in root system architecture. Root-to-shoot and shoot-to-root signals, delivered through the xylem and phloem, respectively, involving Pi itself, hormones, miRNAs, mRNAs, and sucrose, serve to coordinate Pi deficiency responses at the whole-plant level. A combination of chromatin remodeling, transcriptional and posttranslational events contribute to globally regulating a wide range of Pi deficiency responses. In this review, recent advances are evaluated in terms of progress toward developing a comprehensive understanding of the molecular events underlying control over P homeostasis. Application of this knowledge, in terms of developing crop plants having enhanced attributes for P use efficiency, is discussed from the perspective of agricultural sustainability in the face of diminishing global P supplies.

Fine characterization of OsPHO2 knockout mutants reveals its key role in Pi utilization in rice

Previous research using forward genetics approaches demonstrated that OsPHO2 regulates multiple phosphate-starvation responses in rice. In this work, we finely characterized two independent OsPHO2 knockout rice mutants under inorganic phosphate (Pi)-sufficient conditions. The ospho2 mutants exhibited defects in growth and reproductive development in the whole growing period. The cells in the elongation zone of ospho2 seedling roots were much shorter than those of the wild type. The phosphorus concentration in the blades of ospho2 mutants was 5.8-fold higher than those of wild-type plants, whereas it was only slightly higher in the sheaths, culms, spikelets, and seeds. Furthermore, Pi levels in the ospho2 mutants were highest in the oldest leaf and lowest in the youngest leaf, whereas there was no significant difference in the corresponding leaves of wild-type plants. These results suggest that ospho2 mutant phenotype results from a partial defect in Pi translocation and remobilization in the shoot of rice. This study thus provides evidence that OsPHO2, which functions at the downstream of OsPHF1, modulates Pi utilization by regulating the expression of Pht1 transporters in rice.

Classification and interaction modes of 40 rice E2 ubiquitin-conjugating enzymes with 17 rice ARM-U-box E3 ubiquitin ligases

Rice, a monocot model crop, contains at least 48 putative E2 ubiquitin (Ub)-conjugating enzymes. Based on homology comparisons with 40 Arabidopsis E2 proteins and 35 human E2s, 48 rice E2s were classified into 15 different groups. Yeast two-hybrid analyses using the U-box-domain regions of armadillo (ARM)-U-box E3 Ub-ligases and the Ub-conjugating (UBC) domains of E2s showed that, among 40 rice E2s, 11 E2s accounted for 70% of the interactions with 17 ARM-U-box E3s. Thus, a single E2 could interact with multiple ARM-U-box E3s, suggesting the presence of E2 hubs for E2-E3 interactions in rice. Rice SPL11 ARM-U-box E3 displayed distinct self-ubiquitination patterns, including poly-ubiquitination, mono-ubiquitination, or no ubiquitination, depending on different E2 partners. This suggests that the mode of ubiquitination of SPL11 E3 is critically influenced by individual E2s.

Phosphate nutrition: improving low-phosphate tolerance in crops

Phosphorus is an essential nutrient that is required for all major developmental processes and reproduction in plants. It is also a major constituent of the fertilizers required to sustain high-yield agriculture. Levels of phosphate--the only form of phosphorus that can be assimilated by plants--are suboptimal in most natural and agricultural ecosystems, and when phosphate is applied as fertilizer in soils, it is rapidly immobilized owing to fixation and microbial activity. Thus, cultivated plants use only approximately 20-30% of the applied phosphate, and the rest is lost, eventually causing water eutrophication. Recent advances in the understanding of mechanisms by which wild and cultivated species adapt to low-phosphate stress and the implementation of alternative bacterial pathways for phosphorus metabolism have started to allow the design of more effective breeding and genetic engineering strategies to produce highly phosphate-efficient crops, optimize fertilizer use, and reach agricultural sustainability with a lower environmental cost. In this review, we outline the current advances in research on the complex network of plant responses to low-phosphorus stress and discuss some strategies used to manipulate genes involved in phosphate uptake, remobilization, and metabolism to develop low-phosphate-tolerant crops, which could help in designing more efficient crops.

Auxin response factor (OsARF12), a novel regulator for phosphate homeostasis in rice (Oryza sativa)

Phosphorus (P) is crucial nutrient element for crop growth and development. However, the network pathway regulating homeostasis of phosphate (Pi) in crops has many molecular breeding unknowns. Here, we report that an auxin response factor, OsARF12, functions in Pi homeostasis. Measurement of element content, quantitative reverse transcription polymerase chain reaction analysis and acid phosphatases (APases) activity assay showed that the osarf12 mutant and osarf12/25 double mutant with P-intoxicated phenotypes had higher P concentrations, up-regulation of the Pi transporter encoding genes and increased APase activity under Pi-sufficient/-deficient (+Pi/-Pi, 0.32/0 mM NaH2 PO4) conditions. Transcript analysis revealed that Pi-responsive genes--Phosphate starvation (OsIPS)1 and OsIPS2, SYG1/Pho81/XPR1(OsSPX1), Sulfoquinovosyldiacylglycerol 2 (OsSQD2), R2R3 MYB transcription factor (OsMYB2P-1) and Transport Inhibitor Response1 (OsTIR1)--were more abundant in the osarf12 and osarf12/25 mutants under +Pi/-Pi conditions. Knockout of OsARF12 also influenced the transcript abundances of the OsPHR2 gene and its downstream components, such as OsMiR399j, OsPHO2, OsMiR827, OsSPX-MFS1 and OsSPX-MFS2. Results from -Pi/1-naphthylphthalamic acid (NPA) treatments, and auxin reporter DR5::GUS staining suggest that root system alteration and Pi-induced auxin response were at least partially controlled by OsARF12. These findings enrich our understanding of the biological functions of OsARF12, which also acts in regulating Pi homeostasis.

Adventitious roots and lateral roots: similarities and differences

In addition to its role in water and nutrient uptake, the root system is fundamentally important because it anchors a plant to its substrate. Although a wide variety of root systems exist across different species, all plants have a primary root (derived from an embryonic radicle) and different types of lateral roots. Adventitious roots, by comparison, display the same functions as lateral roots but develop from aerial tissues. In addition, they not only develop as an adaptive response to various stresses, such as wounding or flooding, but also are a key limiting component of vegetative propagation. Lateral and adventitious roots share key elements of the genetic and hormonal regulatory networks but are subject to different regulatory mechanisms. In this review, we discuss the developmental processes that give rise to lateral and adventitious roots and highlight knowledge acquired over the past few years about the mechanisms that regulate adventitious root formation.

Parallel analysis of RNA ends enhances global investigation of microRNAs and target RNAs of Brachypodium distachyon

BACKGROUND

The wild grass Brachypodium distachyon has emerged as a model system for temperate grasses and biofuel plants. However, the global analysis of miRNAs, molecules known to be key for eukaryotic gene regulation, has been limited in B. distachyon to studies examining a few samples or that rely on computational predictions. Similarly an in-depth global analysis of miRNA-mediated target cleavage using parallel analysis of RNA ends (PARE) data is lacking in B. distachyon.

RESULTS

B. distachyon small RNAs were cloned and deeply sequenced from 17 libraries that represent different tissues and stresses. Using a computational pipeline, we identified 116 miRNAs including not only conserved miRNAs that have not been reported in B. distachyon, but also non-conserved miRNAs that were not found in other plants. To investigate miRNA-mediated cleavage function, four PARE libraries were constructed from key tissues and sequenced to a total depth of approximately 70 million sequences. The roughly 5 million distinct genome-matched sequences that resulted represent an extensive dataset for analyzing small RNA-guided cleavage events. Analysis of the PARE and miRNA data provided experimental evidence for miRNA-mediated cleavage of 264 sites in predicted miRNA targets. In addition, PARE analysis revealed that differentially expressed miRNAs in the same family guide specific target RNA cleavage in a correspondingly tissue-preferential manner.

CONCLUSIONS

B. distachyon miRNAs and target RNAs were experimentally identified and analyzed. Knowledge gained from this study should provide insights into the roles of miRNAs and the regulation of their targets in B. distachyon and related plants.

Characterization of phosphorus-regulated miR399 and miR827 and their isomirs in barley under phosphorus-sufficient and phosphorus-deficient conditions

BACKGROUND

miR399 and miR827 are both involved in conserved phosphorus (P) deficiency signalling pathways. miR399 targets the PHO2 gene encoding E2 enzyme that negatively regulates phosphate uptake and root-to-shoot allocation, while miR827 targets SPX-domain-containing genes that negatively regulate other P-responsive genes. However, the response of miR399 and miR827 to P conditions in barley has not been investigated.

RESULTS

In this study, we investigated the expression profiles of miR399 and miR827 in barley (Hordeum vulagre L.) under P-deficient and P-sufficient conditions. We identified 10 members of the miR399 family and one miR827 gene in barley, all of which were significantly up-regulated under deficient P. In addition, we found many isomirs of the miR399 family and miR827, most of which were also significantly up-regulated under deficient P. Several isomirs of miR399 members were found to be able to cleave their predicted targets in vivo. Surprisingly, a few small RNAs (sRNAs) derived from the single-stranded loops of the hairpin structures of MIR399b and MIR399e-1 were also found to be able to cleave their predicted targets in vivo. Many antisense sRNAs of miR399 and a few for miR827 were also detected, but they did not seem to be regulated by P. Intriguingly, the lowest expressed member, hvu-miR399k, had four-fold more antisense sRNAs than sense sRNAs, and furthermore under P sufficiency, the antisense sRNAs are more frequent than the sense sRNAs. We identified a potential regulatory network among miR399, its target HvPHO2 and target mimics HvIPS1 and HvIPS2 in barley under P-deficient and P-sufficient conditions.

CONCLUSIONS

Our data provide an important insight into the mechanistic regulation and function of miR399, miR827 and their isomirs in barley under different P conditions.

Down-regulation of OsSPX1 causes high sensitivity to cold and oxidative stresses in rice seedlings

Rice SPX domain gene, OsSPX1, plays an important role in the phosphate (Pi) signaling network. Our previous work showed that constitutive overexpression of OsSPX1 in tobacco and Arabidopsis plants improved cold tolerance while also decreasing total leaf Pi. In the present study, we generated rice antisense and sense transgenic lines of OsSPX1 and found that down-regulation of OsSPX1 caused high sensitivity to cold and oxidative stresses in rice seedlings. Compared to wild-type and OsSPX1-sense transgenic lines, more hydrogen peroxide accumulated in seedling leaves of OsSPX1-antisense transgenic lines for controls, cold and methyl viologen (MV) treatments. Glutathione as a ROS scavenger could protect the antisense transgenic lines from cold and MV stress. Rice whole genome GeneChip analysis showed that some oxidative-stress marker genes (e.g. glutathione S-transferase and P450s) and Pi-signaling pathway related genes (e.g. OsPHO2) were significantly down-regulated by the antisense of OsSPX1. The microarray results were validated by real-time RT-PCR. Our study indicated that OsSPX1 may be involved in cross-talks between oxidative stress, cold stress and phosphate homeostasis in rice seedling leaves.

Spatio-temporal transcript profiling of rice roots and shoots in response to phosphate starvation and recovery

Using rice (Oryza sativa) as a model crop species, we performed an in-depth temporal transcriptome analysis, covering the early and late stages of Pi deprivation as well as Pi recovery in roots and shoots, using next-generation sequencing. Analyses of 126 paired-end RNA sequencing libraries, spanning nine time points, provided a comprehensive overview of the dynamic responses of rice to Pi stress. Differentially expressed genes were grouped into eight sets based on their responses to Pi starvation and recovery, enabling the complex signaling pathways involved in Pi homeostasis to be untangled. A reference annotation-based transcript assembly was also generated, identifying 438 unannotated loci that were differentially expressed under Pi starvation. Several genes also showed induction of unannotated splice isoforms under Pi starvation. Among these, PHOSPHATE2 (PHO2), a key regulator of Pi homeostasis, displayed a Pi starvation-induced isoform, which was associated with increased translation activity. In addition, microRNA (miRNA) expression profiles after long-term Pi starvation in roots and shoots were assessed, identifying 20 miRNA families that were not previously associated with Pi starvation, such as miR6250. In this article, we present a comprehensive spatio-temporal transcriptome analysis of plant responses to Pi stress, revealing a large number of potential key regulators of Pi homeostasis in plants.

Adaptation of maize source leaf metabolism to stress related disturbances in carbon, nitrogen and phosphorus balance

Background

Abiotic stress causes disturbances in the cellular homeostasis. Re-adjustment of balance in carbon, nitrogen and phosphorus metabolism therefore plays a central role in stress adaptation. However, it is currently unknown which parts of the primary cell metabolism follow common patterns under different stress conditions and which represent specific responses.

Results

To address these questions, changes in transcriptome, metabolome and ionome were analyzed in maize source leaves from plants suffering low temperature, low nitrogen (N) and low phosphorus (P) stress. The selection of maize as study object provided data directly from an important crop species and the so far underexplored C₄ metabolism. Growth retardation was comparable under all tested stress conditions. The only primary metabolic pathway responding similar to all stresses was nitrate assimilation, which was down-regulated. The largest group of commonly regulated transcripts followed the expression pattern: down under low temperature and low N, but up under low P. Several members of this transcript cluster could be connected to P metabolism and correlated negatively to different phosphate concentration in the leaf tissue. Accumulation of starch under low temperature and low N stress, but decrease in starch levels under low P conditions indicated that only low P treated leaves suffered carbon starvation.

Conclusions

Maize employs very different strategies to manage N and P metabolism under stress. While nitrate assimilation was regulated depending on demand by growth processes, phosphate concentrations changed depending on availability, thus building up reserves under excess conditions. Carbon and energy metabolism of the C₄ maize leaves were particularly sensitive to P starvation.

Responses of root architecture development to low phosphorus availability: a review

BACKGROUND

Phosphorus (P) is an essential element for plant growth and development but it is often a limiting nutrient in soils. Hence, P acquisition from soil by plant roots is a subject of considerable interest in agriculture, ecology and plant root biology. Root architecture, with its shape and structured development, can be considered as an evolutionary response to scarcity of resources.

SCOPE

This review discusses the significance of root architecture development in response to low P availability and its beneficial effects on alleviation of P stress. It also focuses on recent progress in unravelling cellular, physiological and molecular mechanisms in root developmental adaptation to P starvation. The progress in a more detailed understanding of these mechanisms might be used for developing strategies that build upon the observed explorative behaviour of plant roots.

CONCLUSIONS

The role of root architecture in alleviation of P stress is well documented. However, this paper describes how plants adjust their root architecture to low-P conditions through inhibition of primary root growth, promotion of lateral root growth, enhancement of root hair development and cluster root formation, which all promote P acquisition by plants. The mechanisms for activating alterations in root architecture in response to P deprivation depend on changes in the localized P concentration, and transport of or sensitivity to growth regulators such as sugars, auxins, ethylene, cytokinins, nitric oxide (NO), reactive oxygen species (ROS) and abscisic acid (ABA). In the process, many genes are activated, which in turn trigger changes in molecular, physiological and cellular processes. As a result, root architecture is modified, allowing plants to adapt effectively to the low-P environment. This review provides a framework for understanding how P deficiency alters root architecture, with a focus on integrated physiological and molecular signalling.

Dissecting plant iron homeostasis under short and long-term iron fluctuations

A wealth of information on the different aspects of iron homeostasis in plants has been obtained during the last decade. However, there is no clear road-map integrating the relationships between the various components. The principal aim of the current review is to fill this gap. In this context we discuss the lack of low affinity iron uptake mechanisms in plants, the utilization of a different uptake mechanism by graminaceous plants compared to the others, as well as the roles of riboflavin, ferritin isoforms, nitric oxide, nitrosylation, heme, aconitase, and vacuolar pH. Cross-homeostasis between elements is also considered, with a specific emphasis on the relationship between iron homeostasis and phosphorus and copper deficiencies. As the environment is a crucial parameter for modulating plant responses, we also highlight how diurnal fluctuations govern iron metabolism. Evolutionary aspects of iron homeostasis have so far attracted little attention. Looking into the past can inform us on how long-term oxygen and iron-availability fluctuations have influenced the evolution of iron uptake mechanisms. Finally, we evaluate to what extent this homeostastic road map can be used for the development of novel biofortification strategies in order to alleviate iron deficiency in human.

Improvement of phosphorus efficiency in rice on the basis of understanding phosphate signaling and homeostasis

Rice is one of the most important cereal crops feeding a large segment of the world's population. Inefficient utilization of phosphate (Pi) fertilizer by the plant in rice production increases cost and pollution. Developing cultivars with improved Pi use efficiency is essential for the sustainability of agriculture. Pi uptake, translocation and remobilization are regulated by complex molecular mechanisms through the functions of Pi transporters (PTs) and other downstream Pi Starvation Induced (PSI) genes. Expressions of these PSI genes are regulated by the Pi Starvation Response Regulator (OsPHR2)-mediated transcriptional control and/or PHO2-mediated ubiquitination. SPX-domain containing proteins and the type I H(+)-PPase AVP1 involved in the maintenance and utilization of the internal phosphate. The potential application of posttranscriptional regulation of PT1 through OsPHF1 for Pi efficiency is proposed.

Systemic regulation of mineral homeostasis by micro RNAs

Plants frequently have to cope with environments with sub-optimal mineral nutrient availability. Therefore they need to constantly sense changes of ion concentrations in their environment. Nutrient availabilities and needs have to be tightly coordinated between organs to ensure a balance between uptake and demand for metabolism, growth, reproduction, and defense reactions. To this end information about the nutrient status has to flow from cell-to-cell, but also between distant organs via the long-distance transport tubes to trigger adaptive responses. This systemic signaling between roots and shoots is required to maintain mineral nutrient homeostasis in the different organs under varying environmental conditions. Recent results begin to shed light on the molecular components of the complex long-distance signaling pathways and it has been proposed that systemic signals can be transported through the xylem as well as via the phloem. Several molecules, including nutrients, hormones, sugars, and small RNAs have been suggested to be involved in systemic communication over long distance (Liu et al., 2009). Recent research has shown that in the case of mineral nutrients, the nutrients themselves, but also macromolecules like micro RNAs (miRNAs) can act as important information transmitters. The following review will summarize the current knowledge about phloem-mediated systemic signaling by miRNAs during ion nutrient allocation and adaptation to mineral nutrient deprivation, concentrating on the well-analyzed responses to a lack of potassium, sulfur, and copper.

Effort and contribution of T-DNA Insertion mutant library for rice functional genomics research in China: review and perspective

With the completion of the rice (Oryza sativa L.) genome-sequencing project, the rice research community proposed to characterize the function of every predicted gene in rice by 2020. One of the most effective and high-throughput strategies for studying gene function is to employ genetic mutations induced by insertion elements such as T-DNA or transposons. Since 1999, with support from the Ministry of Science and Technology of China for Rice Functional Genomics Programs, large-scale T-DNA insertion mutant populations have been generated in Huazhong Agricultural University, the Chinese Academy of Sciences and the Chinese Academy of Agricultural Sciences. Currently, a total of 372,346 mutant lines have been generated, and 58,226 T-DNA or Tos17 flanking sequence tags have been isolated. Using these mutant resources, more than 40 genes with potential applications in rice breeding have already been identified. These include genes involved in biotic or abiotic stress responses, nutrient metabolism, pollen development, and plant architecture. The functional analysis of these genes will not only deepen our understanding of the fundamental biological questions in rice, but will also offer valuable gene resources for developing Green Super Rice that is high-yielding with few inputs even under the poor growth conditions of many regions of Africa and Asia.

Approaches towards nitrogen- and phosphorus-efficient rice

BACKGROUND AND AIMS

Food production has to increase to meet the demand of a growing population. In light of the high energy costs and increasingly scarce resources, future agricultural systems have to be more productive and more efficient in terms of inputs such as fertilizer and water. The development of rice varieties with high yield under low-nutrient conditions has therefore become a breeding priority. The rapid progress made in sequencing and molecular-marker technology is now beginning to change the way breeding is done, providing new opportunities.

SCOPE

Nitrogen (N) and phosphorus (P) are applied to agricultural systems in large quantities and a deficiency of either nutrient leads to yield losses and triggers complex molecular and physiological responses. The underlying genes are now being identified and studied in detail, and an increasing number of quantitative trait loci (QTLs) related to N and P uptake and utilization are being reported. Here, we provide an overview of the different aspects related to N and P in rice production systems, and apply a breeder's perspective on the potential of relevant genes and pathways for breeding applications.

MAIN POINTS

For the development of nutrient-efficient rice, a holistic approach should be followed combining optimized fertilizer management with enhanced nutrient uptake via a vigorous root system, leading to increased grain filling and yield. Despite an increasing number of N- and P-related genes and QTLs being reported, very few are actively used in molecular breeding programmes. The complex regulation of N- and P-related pathways challenges breeders and the research community to identify large-effect genes/QTLs. For this it will be important to focus more on the analysis of tolerant genotypes rather than model plants, since tolerance pathways may employ a different set of genes.

Nonredundant regulation of rice arbuscular mycorrhizal symbiosis by two members of the phosphate transporter1 gene family

Pi acquisition of crops via arbuscular mycorrhizal (AM) symbiosis is becoming increasingly important due to limited high-grade rock Pi reserves and a demand for environmentally sustainable agriculture. Here, we show that 70% of the overall Pi acquired by rice (Oryza sativa) is delivered via the symbiotic route. To better understand this pathway, we combined genetic, molecular, and physiological approaches to determine the specific functions of two symbiosis-specific members of the PHOSPHATE TRANSPORTER1 (PHT1) gene family from rice, ORYsa;PHT1;11 (PT11) and ORYsa;PHT1;13 (PT13). The PT11 lineage of proteins from mono- and dicotyledons is most closely related to homologs from the ancient moss, indicating an early evolutionary origin. By contrast, PT13 arose in the Poaceae, suggesting that grasses acquired a particular strategy for the acquisition of symbiotic Pi. Surprisingly, mutations in either PT11 or PT13 affected the development of the symbiosis, demonstrating that both genes are important for AM symbiosis. For symbiotic Pi uptake, however, only PT11 is necessary and sufficient. Consequently, our results demonstrate that mycorrhizal rice depends on the AM symbiosis to satisfy its Pi demands, which is mediated by a single functional Pi transporter, PT11.

A constitutive expressed phosphate transporter, OsPht1;1, modulates phosphate uptake and translocation in phosphate-replete rice

A number of phosphate (Pi) starvation- or mycorrhiza-regulated Pi transporters belonging to the Pht1 family have been functionally characterized in several plant species, whereas functions of the Pi transporters that are not regulated by changes in Pi supply are lacking. In this study, we show that rice (Oryza sativa) Pht1;1 (OsPT1), one of the 13 Pht1 Pi transporters in rice, was expressed abundantly and constitutively in various cell types of both roots and shoots. OsPT1 was able to complement the proton-coupled Pi transporter activities in a yeast mutant defective in Pi uptake. Transgenic plants of OsPT1 overexpression lines and RNA interference knockdown lines contained significantly higher and lower phosphorus concentrations, respectively, compared with the wild-type control in Pi-sufficient shoots. These responses of the transgenic plants to Pi supply were further confirmed by the changes in depolarization of root cell membrane potential, root hair occurrence, (33)P uptake rate and transportation, as well as phosphorus accumulation in young leaves at Pi-sufficient levels. Furthermore, OsPT1 expression was strongly enhanced by the mutation of Phosphate Overaccumulator2 (OsPHO2) but not by Phosphate Starvation Response2, indicating that OsPT1 is involved in the OsPHO2-regulated Pi pathway. These results indicate that OsPT1 is a key member of the Pht1 family involved in Pi uptake and translocation in rice under Pi-replete conditions.

OsMYB2P-1, an R2R3 MYB transcription factor, is involved in the regulation of phosphate-starvation responses and root architecture in rice

An R2R3 MYB transcription factor, OsMYB2P-1, was identified from microarray data by monitoring the expression profile of rice (Oryza sativa ssp. japonica) seedlings exposed to phosphate (Pi)-deficient medium. Expression of OsMYB2P-1 was induced by Pi starvation. OsMYB2P-1 was localized in the nuclei and exhibited transcriptional activation activity. Overexpression of OsMYB2P-1 in Arabidopsis (Arabidopsis thaliana) and rice enhanced tolerance to Pi starvation, while suppression of OsMYB2P-1 by RNA interference in rice rendered the transgenic rice more sensitive to Pi deficiency. Furthermore, primary roots of OsMYB2P-1-overexpressing plants were shorter than those in wild-type plants under Pi-sufficient conditions, while primary roots and adventitious roots of OsMYB2P-1-overexpressing plants were longer than those of wild-type plants under Pi-deficient conditions. These results suggest that OsMYB2P-1 may also be associated with the regulation of root system architecture. Overexpression of OsMYB2P-1 led to greater expression of Pi-responsive genes such as Oryza sativa UDP-sulfoquinovose synthase, OsIPS1, OsPAP10, OsmiR399a, and OsmiR399j. In contrast, overexpression of OsMYB2P-1 suppressed the expression of OsPHO2 under both Pi-sufficient and Pi-deficient conditions. Moreover, expression of OsPT2, which encodes a low-affinity Pi transporter, was up-regulated in OsMYB2P-1-overexpressing plants under Pi-sufficient conditions, whereas expression of the high-affinity Pi transporters OsPT6, OsPT8, and OsPT10 was up-regulated by overexpression of OsMYB2P-1 under Pi-deficient conditions, suggesting that OsMYB2P-1 may act as a Pi-dependent regulator in controlling the expression of Pi transporters. These findings demonstrate that OsMYB2P-1 is a novel R2R3 MYB transcriptional factor associated with Pi starvation signaling in rice.

Phosphate starvation signaling in rice

Phosphorus is one of the most essential and limiting macronutrients for plants. Phosphate (Pi) deficiency could affect crop productivity seriously in agriculture. How to cope with this problem? Unveiling the molecular mechanism behind the Pi starvation responses of plants will be helpful to solve this issue. Rice is one of the most important crops, which feeds over one-third of the people in the world. In this review, we summarize the recent progress on Pi starvation signaling in rice with the intention to provide a further insight into the molecular mechanism of Pi starvation responses in rice and to give a new research direction to design transgenic plants with high Pi efficiency.

The phosphate transporter gene OsPht1;8 is involved in phosphate homeostasis in rice

Plant phosphate transporters (PTs) are active in the uptake of inorganic phosphate (Pi) from the soil and its translocation within the plant. Here, we report on the biological properties and physiological roles of OsPht1;8 (OsPT8), one of the PTs belonging to the Pht1 family in rice (Oryza sativa). Expression of a β-glucuronidase and green fluorescent protein reporter gene driven by the OsPT8 promoter showed that OsPT8 is expressed in various tissue organs from roots to seeds independent of Pi supply. OsPT8 was able to complement a yeast Pi-uptake mutant and increase Pi accumulation of Xenopus laevis oocytes when supplied with micromolar (33)Pi concentrations at their external solution, indicating that it has a high affinity for Pi transport. Overexpression of OsPT8 resulted in excessive Pi in both roots and shoots and Pi toxic symptoms under the high-Pi supply condition. In contrast, knockdown of OsPT8 by RNA interference decreased Pi uptake and plant growth under both high- and low-Pi conditions. Moreover, OsPT8 suppression resulted in an increase of phosphorus content in the panicle axis and in a decrease of phosphorus content in unfilled grain hulls, accompanied by lower seed-setting rate. Altogether, our data suggest that OsPT8 is involved in Pi homeostasis in rice and is critical for plant growth and development.