OsPHR2 is involved in phosphate-starvation signaling and excessive phosphate accumulation in shoots of plants

Previous research has demonstrated that AtPHR1 plays a central role in phosphate (Pi)-starvation signaling in Arabidopsis thaliana. In this work, two OsPHR genes from rice (Oryza sativa) were isolated and designated as OsPHR1 and OsPHR2 based on amino acid sequence homology to AtPHR1. Their functions in Pi signaling in rice were investigated using transgenic plants. Our results showed that both OsPHR1 and OsPHR2 are involved in Pi-starvation signaling pathway by regulation of the expression of Pi-starvation-induced genes, whereas only OsPHR2 overexpression results in the excessive accumulation of Pi in shoots under Pi-sufficient conditions. Under Pi-sufficient conditions, overexpression of OsPHR2 mimics Pi-starvation stress in rice with enhanced root elongation and proliferated root hair growth, suggesting the involvement of OsPHR2 in Pi-dependent root architecture alteration by both systematic and local pathways. In OsPHR2-overexpression plants, some Pi transporters were up-regulated under Pi-sufficient conditions, which correlates with the strongly increased content of Pi. The mechanism behind the OsPHR2 regulated Pi accumulation will provide useful approaches to develop smart plants with high Pi efficiency.     

Does OsPHR2, central Pi-signaling regulator,regulate some unknown factors crucial for plant growth?

OsPHR2, the homolog of AtPHR1, is a central Pi-signaling regulator. The Pi-signaling pathway downstream of AtPHR1, similarly of OsPHR2,^1,2 involves a noncoding RNA which targets mimicry of miR399. miRNA399 mediates cleavage of PHO2.^3,4 The regulating pathway downstream of OsPHR2 is negatively regulated by the Pi-signaling responsive gene OsSPX1.^5,6 Overexpression of AtPHR1 and OsPHR2 leads to an increased concentration of Pi in the shoot tissues with leaf toxic symptom and growth retardation similar as the phenotype of pho2 mutant, especially under Pi abundant conditions.^2,6,7 It has been known that the low affinity Pi transporter OsPT2 mainly contributes to the shoot Pi accumulation mediated by OsPHR2, and overexpression of OsPT2 results in shoot Pi accumulation and leaf toxic symptom and growth retardation under Pi abundant conditions.⁶ Two curious questions are emerging from the reported results: How Os SPX1 functions on the negative regulation of the pathway and what mechanism of the growth retardation mediated by OsPHR2. For the second question, our favored hypothesis is that the growth inhibition mediated by overexpression of OsPHR2 is caused by toxic physiological effects due to excessive Pi accumulation in shoots (Pi toxicity). In fact, the toxic symptoms become diminished with decreased Pi levels in growth medium. However, the plant growth retardation mediated by overexpression of OsPHR2 may be caused by some unknown genetic factor(s) regulated by OsPHR2.Key words: Oryza Sativa L, OsPHR2, OsSPX1, pi-signaling, plant growth     

SPX proteins regulate Pi homeostasis and signaling in different subcellular level

To cope with low phosphate (Pi) availability, plants have to adjust its gene expression profile to facilitate Pi acquisition and remobilization. Sensing the levels of Pi is essential for reprogramming the gene expression profile to adapt to the fluctuating Pi environment. AtPHR1 in Arabidopsis and OsPHR2 in rice are central regulators of Pi signaling, which regulates the expression of phosphate starvation-induced (PSI) genes by binding to the P1BS elements in the promoter of PSI genes. However, how the Pi level affects the central regulator to regulate the PSI genes have puzzled us for a decade. Recent progress in SPX proteins indicated that the SPX proteins play important role in regulating the activity of central regulator AtPHR1/OsPHR2 in a Pi dependent manner at different subcellular levels.     

OsSPX1 suppresses the function of OsPHR2 in the regulation of expression of OsPT2 and phosphate homeostasis in shoots of rice

Phosphate (Pi) homeostasis in plants is required for plant growth and development, and is achieved by the coordination of Pi acquisition, translocation from roots to shoots, and remobilization within plants. Previous reports have demonstrated that over‐expression of OsPHR2 (the homolog of AtPHR1) and knockdown of OsSPX1 result in accumulation of excessive shoot Pi in rice. Here we report that OsPHR2 positively regulates the low‐affinity Pi transporter gene OsPT2 by physical interaction and upstream regulation of OsPHO2 in roots. OsPT2 is responsible for most of the OsPHR2‐mediated accumulation of excess shoot Pi. OsSPX1 suppresses the regulation on expression of OsPT2 by OsPHR2 and the accumulation of excess shoot Pi, but it does not suppress induction of OsPT2 or the accumulation of excessive shoot Pi in the Ospho2 mutant. Our data also show that OsSPX1 is a negative regulator of OsPHR2 and is involved in the feedback of Pi‐signaling network in roots that is defined by OsPHR2 and OsPHO2. This finding provides new insight into the regulatory mechanism of Pi uptake, translocation, allocation and homeostasis in plants.     

Oxygen deficit alleviates phosphate overaccumulation toxicity in OsPHR2 overexpression plants

Overexpression of OsPHR2 increases phosphate (Pi) uptake and causes overaccumulation of Pi in rice plants, which is toxic to rice plants when they are grown in media with a sufficient Pi supply. The toxicity that results from OsPHR2 overexpression can be significantly relieved by growing the plants in a waterlogged paddy field. A comparison of the Pi uptake and growth status of OsPHR2-overexpression plants (PHR2-Oe plants) grown in paddy fields or in a laboratory setting in aerated or stagnant hydroponic conditions indicated that the oxygen limitation that is present in paddy fields and in stagnant rice culture solutions inhibits the Pi overaccumulation toxicity of PHR2-Oe plants by reducing their Pi uptake. Quantitative RT-PCR demonstrated that the expression of Pi-starvation-induced (PSI) genes was induced by oxygen limitation in both wild-type and PHR2-Oe plants. The induction of PSI genes is the consequence of reducing the Pi concentration in stagnant plants. Thus, when evaluating the efficiency of Pi use in rice germplasm or transgenic materials under hydroponic conditions, the impact of the low oxygen condition that exists in waterlogged paddies should be considered.     

Phosphate starvation induced OsPHR4 mediates Pi-signaling and homeostasis in rice

Key message

OsPHR4 mediates the regulation of Pi-starvation signaling and Pi-homeostasis in a PHR1-subfamily dependent manner in rice.

Abstract

Phosphate (Pi) starvation response is a sophisticated process for plant in the natural environment. In this process, PHOSPHATE STARVATION RESPONSE 1 (PHR1) subfamily genes play a central role in regulating Pi-starvation signaling and Pi-homeostasis. Besides the three PHR1 orthologs in Oryza sativa L. (Os) [(Os) PHR1, (Os) PHR2, and (Os) PHR3], which were reported to regulated Pi-starvation signaling and Pi-homeostasis redundantly, a close related PHR1 ortholog [designated as (Os) PHR4] is presented in rice genome with unknown function. In this study, we found that OsPHR4 is a Pi-starvation induced gene and mainly expresses in vascular tissues through all growth and development periods. The expression of OsPHR4 is positively regulated by OsPHR1, OsPHR2 and OsPHR3. The nuclear located OsPHR4 can respectively interact with other three PHR1 subfamily members to regulate downstream Pi-starvation induced genes. Consistent with the positive role of PHR4 in regulating Pi-starvation signaling, the OsPHR4 overexpressors display higher Pi accumulation in the shoot and elevated expression of Pi-starvation induced genes under Pi-sufficient condition. Besides, moderate growth retardation and repression of the Pi-starvation signaling in the OsPHR4 RNA interfering (RNAi) transgenic lines can be observed under Pi-deficient condition. Together, we propose that OsPHR4 mediates the regulation of Pi-starvation signaling and Pi-homeostasis in a PHR1-subfamily dependent manner in rice.

     

Relationships between some transcription factors and concordantly expressed drought stress-related genes in bread wheat

The challenge of climate change makes it mandatory to improve tolerance to drought stress in bread wheat (Triticum aestivum) via biotechnological approaches. Drought stress experiment was conducted followed by RNA-Seq analysis for leaves of two wheat cultivars namely Giza 168 and Gemmiza 10 with contrasting genotypes. Expression patterns of the regulated stress-related genes and concordantly expressed TFs were detected, then, validated via qPCR for two loss-of-function mutants in Arabidopsis background harboring mutated genes analogue to those in wheat. Drought-stress related genes were searched for concordantly expressed TFs and a total of eight TFs were shown to coexpress with 14 stress-related genes. Among these genes, one TF belongs to the zinc finger protein CONSTANS family and proved via qPCR to drive expression of a gene encoding a speculative TF namely zinc transporter 3-like and two other stress related genes encoding tryptophan synthase alpha chain and asparagine synthetase. Known functions of the two TFs under drought stress complement those of the two concordantly expressed stress-related genes, thus, it is likely that they are related. This study highlights the possibility to utilize metabolic engineering approaches to decipher and incorporate existing regulatory frameworks under drought stress in future breeding programs of bread wheat.     

OsARF16, a transcription factor,is required for auxin and phosphate starvation response in rice (Oryza sativa L.)

Plant responses to auxin and phosphate (Pi) starvation are closely linked. However, the underlying mechanisms connecting auxin to phosphate starvation (?Pi) responses are largely unclear. Here, we show that OsARF16, an auxin response factor, functions in both auxin and ?Pi responses in rice (Oryza sativa L.). The knockout of OsARF16 led to primary roots (PR), lateral roots (LR) and root hair losing sensitivity to auxin and ?Pi response. OsARF16 expression and OsARF16::GUS staining in PR and LR of rice Nipponbare (NIP) were induced by indole acetic acid and ?Pi treatments. In ?Pi conditions, the shoot biomass of osarf16 was slightly reduced, and neither root growth nor iron content was induced, indicating that the knockout of OsARF16 led to loss of response to Pi deficiency in rice. Six phosphate starvation‐induced genes (PSIs) were less induced by ?Pi in osarf16 and these trends were similar to a knockdown mutant of OsPHR2 or AtPHR1, which was a key regulator under ?Pi. These data first reveal the biological function of OsARF16, provide novel evidence of a linkage between auxin and ?Pi responses and facilitate the development of new strategies for the efficient utilization of Pi in rice.     

Overexpression of maize mitogen-activated protein kinase gene, ZmSIMK1 in Arabidopsis increases tolerance to salt stress

Mitogen-activated protein kinase (MAPK) cascades play a remarkably crucial role in plants. It has been studied intensively in model plants Arabidopsis, tobacco and rice. However, the function of MAPKs in maize (Zea mays L.) has not been well documented. ZmSIMK1 (Zea mays salt-induced mitogen-activated protein kinase 1) is a previously identified MAPK gene in maize. In this research, we charactered ZmSIMK1 and showed that ZmSIMK1 was involved in Arabidopsis salt stress. The genomic organization of ZmSIMK1 gene and its expression in maize have been analyzed. In order to investigate the function of ZmSIMK1, we generated transgenic Arabidopsis constitutively overexpressing ZmSIMK1. Ectopic expression of ZmSIMK1 in Arabidopsis resulted in increased resistance against salt stress. Importantly, ZmSIMK1-overexpressing Arabidopsis exhibited constitutive expression of stress-responsive marker genes, RD29A and P5CS1. Furthermore, RD29A and P5CS1 were upregulated under salt stress. These results suggest that ZmSIMK1 may play an important role in plant salt stress.     

Molecular mechanisms regulating Pi-signaling and Pi homeostasis under OsPHR2, a central Pi-signaling regulator, in rice

Phosphorus (P) is one of the most important major mineral elements for plant growth and metabolism. Plants have evolved adaptive regulatory mechanisms to maintain phosphate (Pi) homeostasis by improving phosphorus uptake, translocation, remobilization and efficiency of use. Here we review recent advances in our understanding of the OsPHR2-mediated phosphate-signaling pathway in rice. OsPHR2 positively regulates the low-affinity Pi transporter OsPT2 through physical interaction and reciprocal regulation of OsPHO2 in roots. OsPT2 is responsible for most of the OsPHR2-mediated accumulation of excess Pi in shoots. OsSPX1 acts as a repressor in the OsPHR2-mediated phosphate-signaling pathway. Some mutants screened from ethyl methanesulfonate (EMS)-mutagenized M2 population of OsPHR2 overexpression transgenic line removed the growth inhibition, indicating that some unknown factors are crucial for Pi utilization or plant growth under the regulation of OsPHR2.