The transcription factor MYB110 regulates plant height, lodging resistance, and grain yield in rice.

The high-yielding Green Revolution varieties of cereal crops are characterized by a semidwarf architecture and lodging resistance. Plant height is tightly regulated by the availability of phosphate (Pi), yet the underlying mechanism remains obscure. Here, we report that rice (Oryza sativa) R2R3-type Myeloblastosis (MYB) transcription factor MYB110 is a Pi-dependent negative regulator of plant height. MYB110 is a direct target of PHOSPHATE STARVATION RESPONSE 2 (OsPHR2) and regulates OsPHR2-mediated inhibition of rice height. Inactivation of MYB110 increased culm diameter and bending resistance, leading to enhanced lodging resistance despite increased plant height. Strikingly, the grain yield of myb110 mutants was elevated under both high- and low-Pi regimes. Two divergent haplotypes based on single nucleotide polymorphisms in the putative promoter of MYB110 corresponded with its transcript levels and plant height in response to Pi availability. Thus, fine-tuning MYB110 expression may be a potent strategy for further increasing the yield of Green Revolution cereal crop varieties.

Potassium transporter OsHAK18 mediates potassium and sodium circulation and sugar translocation in rice.

High-affinity potassium (K+) transporter (HAK)/K+ uptake permease (KUP)/K+ transporter (KT) have been identified in all genome-sequenced terrestrial plants. They play an important role in K+ acquisition and translocation and in enhancing salt tolerance. Here, we report that plasma membrane-located OsHAK18 functions in K+ and sodium (Na+) circulation and sugar translocation in rice (Oryza sativa). OsHAK18 was expressed mainly, though not exclusively, in vascular tissues and particularly in the phloem. Knockout (KO) of OsHAK18 reduced K+ concentration in phloem sap and roots but increased K+ accumulation in the shoot of both 'Nipponbare' and 'Zhonghua11' cultivars, while overexpression (OX) of OsHAK18 driven by its endogenous promoter increased K+ concentration in phloem sap and roots and promoted Na+ retrieval from the shoot to the root under salt stress. Split-root experimental analysis of rubidium (Rb+) uptake and circulation indicated that OsHAK18-OX promoted Rb+ translocation from the shoot to the root. In addition, OsHAK18-KO increased while OsHAK18-OX reduced soluble sugar content in the shoot and oppositely affected the sugar concentration in the phloem and its content in the root. Moreover, OsHAK18-OX dramatically increased grain yield and physiological K+ utilization efficiency. Our results suggest that-unlike other OsHAKs analyzed heretofore-OsHAK18 is critical for K+ and Na+ recirculation from the shoot to the root and enhances the source-to-sink translocation of photo-assimilates.

Rice circadian clock regulator Nhd1 controls the expression of the sucrose transporter gene OsSUT1 and impacts carbon-nitrogen balance.

Interdependent metabolic and transport processes of carbon (C) and nitrogen (N) regulate plant growth and development, while the regulatory pathways remain poorly defined. We previously reported that rice circadian clock N-mediated heading date-1 (Nhd1) regulates growth duration-dependent N use efficiency. Here, we report that knockout of Nhd1 in rice reduced the rate of photosynthesis and the sucrose ratio of sheaths to blades, but increased the total C to N ratio and free amino acids. Leaf RNA-seq analysis indicated that mutation of Nhd1 dramatically altered expression of the genes linked to starch and sucrose metabolism, circadian rhythm, and amino acid metabolic pathways. We identified that Nhd1 can directly activate the transcriptional expression of sucrose transporter-1 (OsSUT1). Knockout of Nhd1 suppressed OsSUT1 expression, and both nhd1 and ossut1 mutants showed similar shorter height, and lower shoot biomass and sucrose concentration in comparison with the wild type, while overexpression of OsSUT1 can restore the defective sucrose transport and partially ameliorate the reduced growth of nhd1 mutants. The Nhd1-binding site of the OsSUT1 promoter is conserved in all known rice genomes. The positively related variation of Nhd1 and OsSUT1 expression among randomly selected indica and japonica varieties suggests a common regulatory module of Nhd1-OsSUT1-mediated C and N balance in rice.

The transcription factor OsWRKY10 inhibits phosphate uptake via suppressing OsPHT1;2 expression under phosphate-replete conditions in rice.

Plants have evolved delicate systems for stimulating or inhibiting inorganic phosphate (Pi) uptake in response to the fluctuating Pi availability in soil. However, the negative regulators inhibiting Pi uptake at the transcriptional level are largely unexplored. Here, we functionally characterized a transcription factor in rice (Oryza sativa), OsWRKY10. OsWRKY10 encodes a nucleus-localized protein and showed preferential tissue localization. Knockout of OsWRKY10 led to increased Pi uptake and accumulation under Pi-replete conditions. In accordance with this phenotype, OsWRKY10 was transcriptionally induced by Pi, and a subset of PHOSPHATE TRANSPORTER 1 (PHT1) genes were up-regulated upon its mutation, suggesting that OsWRKY10 is a transcriptional repressor of Pi uptake. Moreover, rice plants expressing the OsWRKY10-VP16 fusion protein (a dominant transcriptional activator) accumulated even more Pi than oswrky10. Several lines of biochemical evidence demonstrated that OsWRKY10 directly suppressed OsPHT1;2 expression. Genetic analysis showed that OsPHT1;2 was responsible for the increased Pi accumulation in oswrky10. Furthermore, during Pi starvation, OsWRKY10 protein was degraded through the 26S proteasome. Altogether, the OsWRKY10-OsPHT1;2 module represents a crucial loop in the Pi signaling network in rice, inhibiting Pi uptake when there is ample Pi in the environment.

The rice transcription factor Nhd1 regulates root growth and nitrogen uptake by activating nitrogen transporters.

Plants adjust root architecture and nitrogen (N) transporter activity to meet the variable N demand, but their integrated regulatory mechanism remains unclear. We have previously reported that a floral factor in rice (Oryza sativa), N-mediated heading date-1 (Nhd1), regulates flowering time. Here, we show that Nhd1 can directly activate the transcription of the high-affinity ammonium (NH4+) transporter 1;3 (OsAMT1;3) and the dual affinity nitrate (NO3-) transporter 2.4 (OsNRT2.4). Knockout of Nhd1 inhibited root growth in the presence of NO3- or a low concentration of NH4+. Compared to the wild-type (WT), nhd1 and osamt1;3 mutants showed a similar decrease in root growth and N uptake under low NH4+ supply, while nhd1 and osnrt2.4 mutants showed comparable root inhibition and altered NO3- translocation in shoots. The defects of nhd1 mutants in NH4+ uptake and root growth response to various N supplies were restored by overexpression of OsAMT1;3 or OsNRT2.4. However, when grown in a paddy field with low N availability, nhd1 mutants accumulated more N and achieved a higher N uptake efficiency (NUpE) due to the delayed flowering time and prolonged growth period. Our findings reveal a molecular mechanism underlying the growth duration-dependent NUpE.

A crucial role for a node-localized transporter, HvSPDT, in loading phosphorus into barley grains.

Grains are the major sink of phosphorus (P) in cereal crops, accounting for 60-85% of total plant P, but the mechanisms underlying P loading into the grains are poorly understood. We functionally characterized a transporter gene required for the distribution of P to the grains in barley (Hordeum vulgare), HvSPDT (SULTR-like phosphorus distribution transporter). HvSPDT encoded a plasma membrane-localized Pi/H+ cotransporter. It was mainly expressed in the nodes at both the vegetative and reproductive stages. Furthermore, its expression was induced by inorganic phosphate (Pi) deficiency. In the nodes, HvSPDT was expressed in both the xylem and phloem region of enlarged and diffuse vascular bundles. Knockout of HvSPDT decreased the distribution of P to new leaves, but increased the distribution to old leaves at the vegetative growth stage under low P supply. However, knockout of HvSPDT did not alter the redistribution of P from old to young organs. At the reproductive stage, knockout of HvSPDT significantly decreased P allocation to the grains, resulting in a considerable reduction in grain yield, especially under P-limited conditions. Our results indicate that node-based HvSPDT plays a crucial role in loading P into barley grains through preferentially distributing P from the xylem and further to the phloem.

The rice phosphate transporter OsPHT1;7 plays a dual role in phosphorus redistribution and anther development.

Inorganic phosphate (Pi) is the predominant form of phosphorus (P) readily accessible to plants, and Pi Transporter 1 (PHT1) genes are the major contributors to root Pi uptake. However, the mechanisms underlying the transport and recycling of Pi within plants, which are vital for optimizing P use efficiency, remain elusive. Here, we characterized a functionally unknown rice (Oryza sativa) PHT1 member barely expressed in roots, OsPHT1;7. Yeast complementation and Xenopus laevis oocyte assay demonstrated that OsPHT1;7 could mediate Pi transport. Reverse-transcription quantitative polymerase chain reaction and histochemical analyses showed that OsPHT1;7 was preferentially expressed in source leaves and nodes. A further fine-localization analysis by immunostaining showed that OsPHT1;7 expression was restricted in the vascular bundle (VB) sheath and phloem of source leaves as well as in the phloem of regular/diffuse- and enlarged-VBs of nodes. In accordance with this expression pattern, mutation of OsPHT1;7 led to increased and decreased P distribution in source (old leaves) and sink organs (new leaves/panicles), respectively, indicating that OsPHT1;7 is involved in P redistribution. Furthermore, OsPHT1;7 showed an overwhelmingly higher transcript abundance in anthers than other PHT1 members, and ospht1;7 mutants were impaired in P accumulation in anthers but not in pistils or husks. Moreover, the germination of pollen grains was significantly inhibited upon OsPHT1;7 mutation, leading to a >80% decrease in seed-setting rate and grain yield. Taken together, our results provide evidence that OsPHT1;7 is a crucial Pi transporter for Pi transport and recycling within rice plants, stimulating both vegetative and reproductive growth.

OsWRKY108 is an integrative regulator of phosphorus homeostasis and leaf inclination in rice.

Phosphorus (P), which is taken up by plants as inorganic phosphate (Pi), is one of the most important macronutrients for plant growth and development. Meanwhile, it determines plant architecture in several ways, including leaf inclination. However, the molecular basis underlying the crosstalk between the signaling pathways of plant P homeostasis and architecture maintenance remains elusive. We recently characterized a WRKY transcription factor, OsWRKY108, in rice (Oryza sativa). It functions redundantly with OsWRKY21 to promote Pi uptake in response to Pi supply. Overexpression of either OsWRKY108 or OsWRKY21 led to up-regulation of Pi transporter genes and thus enhanced Pi accumulation. By contrast, transgenic rice plants expressing OsWRKY21-SRDX (a fusion protein transforming OsWRKY21 from an activator into a dominant repressor) but not the OsWRKY108-SRDX fusion showed decreased Pi accumulation under Pi-replete conditions. Here, we report that OsWRKY108 acts as a positive regulator of leaf inclination. OsWRKY108 overexpressors showed increased leaf inclination and OsWRKY108-SRDX plants showed an erect-leaf phenotype, irrespective of the Pi regimes. Nevertheless, the response of leaf inclination to Pi starvation was largely impaired upon OsWRKY108 overexpression. Moreover, in both OsWRKY108-SRDX plants and OsWRKY108 overexpressors, the 'percentage of leaf angle alteration relative to wild-type' under Pi-starvation condition was more significant than that under Pi-replete condition. These results suggest that the regulation of OsWRKY108 on leaf inclination is in part dependent on Pi availability. Altogether, our findings demonstrate that OsWRKY108 is an integrative regulator of P homeostasis and leaf inclination, serving as a link between plant nutrient signaling and developmental cues.

Modulation of plant root traits by nitrogen and phosphate: transporters, long-distance signaling proteins and peptides, and potential artificial traps.

As sessile organisms, plants rely on their roots for anchorage and uptake of water and nutrients. Plant root is an organ showing extensive morphological and metabolic plasticity in response to diverse environmental stimuli including nitrogen (N) and phosphorus (P) nutrition/stresses. N and P are two essential macronutrients serving as not only cell structural components but also local and systemic signals triggering root acclimatory responses. Here, we mainly focused on the current advances on root responses to N and P nutrition/stresses regarding transporters as well as long-distance mobile proteins and peptides, which largely represent local and systemic regulators, respectively. Moreover, we exemplified some of the potential pitfalls in experimental design, which has been routinely adopted for decades. These commonly accepted methods may help researchers gain fundamental mechanistic insights into plant intrinsic responses, yet the output might lack strong relevance to the real situation in the context of natural and agricultural ecosystems. On this basis, we further discuss the established-and yet to be validated-improvements in experimental design, aiming at interpreting the data obtained under laboratory conditions in a more practical view.

OsWRKY21 and OsWRKY108 function redundantly to promote phosphate accumulation through maintaining the constitutive expression of OsPHT1;1 under phosphate-replete conditions.

Plant Phosphate Transporter 1 (PHT1) proteins, probably the only influx transporters for phosphate (Pi) uptake, are partially degraded on sufficient Pi levels to prevent excessive Pi accumulation. Therefore, the basal/constitutive expression level of PHT1 genes is vital for maintaining Pi uptake under Pi-replete conditions. Rice (Oryza sativa) OsPHT1;1 is a unique gene as it is highly expressed and not responsive to Pi, however the mechanism for maintaining its basal/constitutive expression remains unknown. Using biochemical and genetic approaches, we identified and functionally characterised the transcription factors maintaining the basal/constitutive expression of OsPHT1;1. OsWRKY21 and OsWRKY108 interact within the nucleus and both bind to the W-box in the OsPHT1;1 promoter. Overexpression of OsWRKY21 or OsWRKY108 led to increased Pi accumulation, resulting from elevated expression of OsPHT1;1. By contrast, oswrky21 oswrky108 double mutants showed decreased Pi accumulation and OsPHT1;1 expression in a Pi-dependent manner. Moreover, similar to ospht1;1 mutants, plants expressing the OsWRKY21-SRDX fusion protein (a chimeric dominant suppressor) were impaired in Pi accumulation in Pi-replete roots, accompanied by downregulation of OsPHT1;1 expression. Our findings demonstrated that rice WRKY transcription factors function redundantly to promote Pi uptake by activating OsPHT1;1 expression under Pi-replete conditions, and represent a novel pathway independent of the central Pi signalling system.

Two ADP-glucose pyrophosphorylase subunits, OsAGPL1 and OsAGPS1, modulate phosphorus homeostasis in rice.

Plant acclimatory responses to phosphate (Pi) starvation stress include the accumulation of carbohydrates, namely sugar and starch. However, whether altered endogenous carbohydrate profile could in turn affect plant Pi starvation responses remains widely unexplored. Here, two genes encoding the large and small subunits of an ADP-glucose pyrophosphorylase (AGP) in rice (Oryza sativa), AGP Large Subunit 1 (AGPL1) and AGP Small Subunit 1 (AGPS1), were functionally characterized with regard to maintenance of phosphorus (P) homeostasis and regulation of Pi starvation signaling. AGPL1 and AGPS1 were both positively responsive to nitrogen (N) or Pi deprivation, and expressed in almost all the tissues except in the meristem and mature zones of root. AGPL1 and AGPS1 physically interacted in chloroplast, and catalyzed the rate-limiting step of starch biosynthesis. Low-N- (LN) and low-Pi (LP)-triggered starch accumulation in leaves was impaired in agpl1, agps1 and apgl1 agps1 mutants compared with the wild-type plants. By contrast, mutation of AGPL1 and/or AGPS1 led to an increase in the content of the major sugar, sucrose, in leaf sheath and root under control and LN conditions. Moreover, the Pi accumulation was enhanced in the mutants under control and LN conditions, but not LP conditions. Notably, the LN-induced suppression of Pi accumulation was compromised attributed to the mutation of AGPL1 and/or AGPS1. Furthermore, the increased Pi accumulation was accompanied by the specific suppression of OsSPX2 and activation of several Pi transporter genes. These results indicate that a balanced level of carbohydrates is vital for maintaining plant P homeostasis.

A nodule-localized phosphate transporter GmPT7 plays an important role in enhancing symbiotic N2 fixation and yield in soybean.

Symbiotic nitrogen (N2 ) fixation plays a vital role in sustainable agriculture. Efficient N2 fixation requires various materials, including phosphate (Pi); however, the molecular mechanism underlying the transport of Pi into nodules and bacteroids remains largely unknown. A nodule-localized Pi transporter, GmPT7, was functionally characterized in soybean (Glycine max) and its role in N2 fixation and yield was investigated via composite and whole transgenic plants. GmPT7 protein was localized to the plasma membrane and showed transport activity for Pi in yeast. Altered expression of GmPT7 changed 33 Pi uptake from rhizosphere and translocation to bacteroids. GmPT7 was mainly localized to the outer cortex and fixation zones of the nodules. Overexpression of GmPT7 promoted nodulation, and increased plant biomass, shoot nitrogen and phosphorus content, resulting in improved soybean yield by up to 36%. Double suppression of GmPT5 and GmPT7 led to nearly complete elimination of nodulation and over 50% reduction in plant biomass, shoot nitrogen and phosphorus content, indicating that both GmPT7 and GmPT5 contribute to Pi transport for N2 fixation. Taken together, our results indicate that GmPT7 is a transporter responsible for direct Pi entry to nodules and further to fixation zones, which is required for enhancing symbiotic N2 fixation and grain yield of soybean.

OsPHT1;3 Mediates Uptake, Translocation, and Remobilization of Phosphate under Extremely Low Phosphate Regimes.

Plant roots rely on inorganic orthophosphate (Pi) transporters to acquire soluble Pi from soil solutions that exists at micromolar levels in natural ecosystems. Here, we functionally characterized a rice (Oryza sativa) Pi transporter, Os Phosphate Transporter-1;3 (OsPHT1;3), that mediates Pi uptake, translocation, and remobilization. OsPHT1;3 was directly regulated by Os Phosphate Starvation Response-2 and, in response to Pi starvation, showed enhanced expression in young leaf blades and shoot basal regions and even more so in roots and old leaf blades. OsPHT1;3 was able to complement a yeast mutant strain defective in five Pi transporters and mediate Pi influx in Xenopus laevis oocytes. Overexpression of OsPHT1;3 led to increased Pi concentration both in roots and shoots. However, unlike that reported for other known OsPHT1 members that facilitate Pi uptake at relatively higher Pi levels, mutation of OsPHT1;3 impaired Pi uptake and root-to-shoot Pi translocation only when external Pi concentration was below 5 µm Moreover, in basal nodes, the expression of OsPHT1;3 was restricted to the phloem of regular vascular bundles and enlarged vascular bundles. An isotope labeling experiment with 32P showed that ospht1;3 mutant lines were impaired in remobilization of Pi from source to sink leaves. Furthermore, overexpression and mutation of OsPHT1;3 led to reciprocal alteration in the expression of OsPHT1;2 and several other OsPHT1 genes. Yeast-two-hybrid, bimolecular fluorescence complementation, and coimmunoprecipitation assays all demonstrated a physical interaction between OsPHT1;3 and OsPHT1;2. Taken together, our results indicate that OsPHT1;3 acts as a crucial factor for Pi acquisition, root-to-shoot Pi translocation, and redistribution of phosphorus in plants growing in environments with extremely low Pi levels.

Transport properties and regulatory roles of nitrogen in arbuscular mycorrhizal symbiosis.

Many terrestrial plants can form root symbiosis with beneficial microorganisms for enhancing uptake of mineral nutrients or increasing fitness to adverse environmental challenges. Arbuscular mycorrhizal (AM) symbiosis that is formed by AM fungi and the roots of vascular flowering plants is the most widespread mutualistic associations in nature. As a typical endosymbiosis, AM interactions involves the differentiation of both symbionts to create novel symbiotic interfaces within the root cells, and requires a continuous nutrient exchange between the two partners. AM plants have two pathways for nutrient uptake, either direct uptake via the root hairs and root epidermis at the plant-soil interface, or indirectly through the AM fungal hyphae at the plant-fungus interface. Over the last few years, great progress has been made in deciphering the mechanisms underlying the AM-mediated modulation of nutrient uptake processes, and an increasing number of plant and fungal genes responsible for transporting nutrients from the soil or across the intraradical symbiotic interfaces have been identified and functionally characterized. Here, we summarize the recent advances in the nitrogen uptake, assimilation and translocation in the AM symbiosis, and also explore the current understanding of how the N status and interplay with C and P in modulating the development of AM associations.

Maintenance of phosphate homeostasis and root development are coordinately regulated by MYB1, an R2R3-type MYB transcription factor in rice.

The adaptive responses of plants to phosphate (Pi) starvation stress are fine-tuned by an elaborate regulatory network. In this study, we identified and characterized a novel Pi starvation-responsive gene, MYB1, encoding an R2R3-type transcription factor in rice. MYB1 was transcriptionally induced in leaf sheaths and old leaf blades. It was localized to the nucleus and expressed mainly in vascular tissues. Mutation of MYB1 led to an increase in Pi uptake and accumulation, accompanied by altered expression of a subset of Pi transporters and several genes involved in Pi starvation signaling. Furthermore, MYB1 affected the elongation of the primary root in a Pi-dependent manner and lateral roots in a Pi-independent manner. Moreover, gibberellic acid (GA)-triggered lateral root elongation was largely suppressed in wild-type plants under Pi starvation conditions, whereas this suppression was partially rescued in myb1 mutant lines, correlating with the up-regulation of a GA biosynthetic gene upon MYB1 mutation. Taken together, the findings of this study highlight the role of MYB1 as a regulator involved in both Pi starvation signaling and GA biosynthesis. Such a co-regulator might have broad implications for the study of cross-talk between nutrient stress and other signaling pathways.

Three cis-Regulatory Motifs, AuxRE, MYCRS1 and MYCRS2, are Required for Modulating the Auxin- and Mycorrhiza-Responsive Expression of a Tomato GH3 Gene.

Auxin is well known to be a key regulator that acts in almost all physiological processes during plant growth, and in interactions between plants and microbes. However, to date, the regulatory mechanisms underlying auxin-mediated plant-arbuscular mycorrhizal (AM) fungi symbiosis have not been well deciphered. Previously we identified a GH3 gene, SlGH3.4, strongly responsive to both auxin induction and mycorrhizal symbiosis. Here, we reported a refined dissection of the SlGH3.4 promoter activity using the ß-glucuronidase (GUS) reporter. The SlGH3.4 promoter could drive GUS expression strongly in mycorrhizal roots of soybean and rice plants, and in IAA-treated soybean roots, but not in IAA-treated rice roots. A promoter deletion assay revealed three cis-acting motifs, i.e. the auxin-responsive element, AuxRE, and two newly identified motifs named MYCRS1 and MYCRS2, involved in the activation of auxin- and AM-mediated expression of SlGH3.4. Deletion of the AuxRE from the SlGH3.4 promoter caused almost complete abolition of GUS staining in response to external IAA induction. Seven repeats of AuxRE fused to the Cauliflower mosaic virus (CaMV) 35S minimal promoter could direct GUS expression in both IAA-treated and AM fungal-colonized roots of tobacco plants. Four repeats of MYCRS1 or MYCRS2 fused to the CaMV35S minimal promoter was sufficient to drive GUS expression in arbuscule-containing cells, but not in IAA-treated tobacco roots. In summary, our results offer new insights into the molecular mechanisms underlying the potential cross-talk between the auxin and the AM regulatory pathways in modulating the expression of AM-responsive GH3 genes in diverse mycorrhizal plants.

Nitrate increases ethylene production and aerenchyma formation in roots of lowland rice plants under water stress.

Ethylene increases root cortical aerenchyma formation in maize (Zea mays L.), rice (Oryza sativa L.) and other species. To further investigate the effects of nitrate, ammonium and water stress on ethylene production and aerenchyma formation in roots, two lowland rice cultivars (Shanyou 63, hybrid indica, and Yangdao 6, inbred indica) were cultured hydroponically with 10% (w/v) polyethylene glycol to simulate water stress. Water stress decreased shoot biomass, stomatal conductivity and leaf water potential in cultivars fed with nitrate but not with ammonium. Water stress induced more aerenchyma formation in cultivars fed with nitrate rather than ammonium, and increased cortical aerenchyma was found in Yangdao 6. Endogenous ethylene production by roots increased significantly under water stress in plants fed with nitrate rather than ammonium. Exogenous ethylene stimulated root cortical aerenchyma formation. Expression of the ethylene biosynthesis gene 1-aminocyclo-propane-1-carboxylic acid (ACC) synthase (ACS5) was greater in roots fed with nitrate rather than ammonium in the presence and absence of water stress. The expression of ethylene signalling pathway genes involved in programmed cell death (lesion-simulating disease (L.S.D.)1.1 and L.S.D.2; enhanced disease susceptibility (EDS) and phytoalexin-deficient (PAD4)) were regulated by the N form and water stress. In plants of cultivars fed with ammonium, L.S.D.1.1 expression increased under water stress, whereas L.S.D.2, EDS and PAD4 expression decreased. In conclusion, nitrate increases ethylene production and cortical aerenchyma formation in roots of water-stressed lowland rice. However, ammonium increased L.S.D.1.1 expression in water-stressed roots, and decreased ACS5, EDS and PAD4 expression, which would inhibit ethylene production and aerenchyma formation.

Analysis of tomato plasma membrane H(+)-ATPase gene family suggests a mycorrhiza-mediated regulatory mechanism conserved in diverse plant species.

In plants, the plasma membrane H(+)-ATPase (HA) is considered to play a crucial role in regulating plant growth and respoding to environment stresses. Multiple paralogous genes encoding different isozymes of HA have been identified and characterized in several model plants, while limited information of the HA gene family is available to date for tomato. Here, we describe the molecular and expression features of eight HA-encoding genes (SlHA1-8) from tomato. All these genes are interrupted by multiple introns with conserved positions. SlHA1, 2, and 4 were widely expressed in all tissues, while SlHA5, 6, and 7 were almost only expressed in flowers. SlHA8, the transcripts of which were barely detectable under normal or nutrient-/salt-stress growth conditions, was strongly activated in arbuscular mycorrhizal (AM) fungal-colonized roots. Extreme lack of SlHA8 expression in M161, a mutant defective to AM fungal colonization, provided genetic evidence towards the dependence of its expression on AM symbiosis. A 1521-bp SlHA8 promoter could direct the GUS reporter expression specifically in colonized cells of transgenic tobacco, soybean, and rice mycorrhizal roots. Promoter deletion assay revealed a 223-bp promoter fragment of SlHA8 containing a variant of AM-specific cis-element MYCS (vMYCS) sufficient to confer the AM-induced activity. Targeted deletion of this motif in the corresponding promoter region causes complete abolishment of GUS staining in mycorrhizal roots. Together, these results lend cogent evidence towards the evolutionary conservation of a potential regulatory mechanism mediating the activation of AM-responsive HA genes in diverse mycorrhizal plant species.

Aquaporin plays an important role in mediating chloroplastic CO2 concentration under high-N supply in rice (Oryza sativa) plants.

Our previous studies demonstrated that chloroplastic CO2 concentration (Cc) is not sufficient under high-nitrogen (N) supply in rice plants. In this research, we studied how aquaporins- (AQPs) mediated Cc under different N-supply levels. A hydroponic experiment was conducted in a greenhouse with three different N levels (low N, 0.71 mM; intermediate N, 2.86 mM; and high N, 7.14 mM) in a rice cultivar (Oryza sativa cv. Shanyou 63) and with an ospip1;1 mutant (Oryza sativa cv. Nipponbare). The photosynthetic nitrogen-use efficiency (PNUE) decreased with increasing leaf-N content. Under high-N supply, the estimated Cc was significantly lower than the theoretical Cc and the specific Rubisco activity (carboxylation efficiency/Rubisco content, CE/Rubisco) decreased, because of a decrease of relative CO2 diffusion conductance (total CO2 diffusion conductance/leaf-N content, gt /N) in mesophyll cells. Real Time Quantitative PCR (Q-RT-PCR) showed that most OsPIP1s and OsPIP2s expression were downregulated under the high-N supply. Furthermore, Cc and gm decreased in the ospip1;1 mutant line compared with that of the wild-type plant. It was concluded that under high-N supply, the decreased PNUE was associated with non-sufficient Cc, mediated by AQP in mesophyll conductance.

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.