Characterization Of The Expression Pattern And Function Of Phosphate Deficiency-Responsive Transcription Factor Gene OsWRKY1in Rice

Phosphorus (P) is one of the essential macro-nutrients for plant growth and development, and plays important roles in energy transfer, signal transduction, and photosynthesis processes. It is also a structural component of many biologically important macro-molecules, such as nucleic acids, phospholipids and P-containing enzymes. However, since P is readily chelated by cations and precipitated in the soil and the only form of P available for plant uptake is inorganic ortho-phosphate (Pi), the mobility and availability of P is poor. Plants have evolved a suite of responses to adapt to P-deprived environment, including modification of root architecture, release of acid phosphatase, RNase and organic acids, as well as forming mutualistic symbiotic associations with arbuscular mycorrhizal (AM) fungi.In the past decades, a series of elaborate molecular mechanisms underlying these adaptive responses have been intensively studied, and a molecular regulatory network with regarding to Pi starvation and AM symbiosis has been generated. The nodes in this network, namely the genes involved, are closely related. The alteration in abundance, temporal or spatial expression of one gene may lead to a re-orchestration of the entire network. Although an increasing number of genes involved in Pi starvation and AM symbiosis signaling pathways have been and will be identified in diverse plant species, and the function of the conserved central regulator PHR that might connect the two signalings have been well elucidated, a lot of work should be done to unravel the complex regulatory mechanism of these signalings. In an attempt to get a better understanding of the Pi starvation signaling pathways, we isolated and/or functionally characterize a WRKY transcription factor encoding gene (designated as OsWRKY1) in rice in the present work. The main results acquired are listed as follows: 1. The homologous gene of Arabidopsis transcription factor encoding gene, WRKY75, was identified in rice, and designated as OsWRKY1. The transcriptional alteration in response to phosphate (Pi) starvation as well as subcellular localization of OsWRKY1were analyzed by RT-PCR and bombardment experiment. Contrary to AtWRKY75, OsWRKY1was down-regulated by Pi starvation in root, while its transcript abundance increased upon Pi starvation in leaf. In addition, OsWRKY1was also up-regulated by iron and nitrogen starvation in root and leaf, respectively. Time-course experiment showed that OsWRKY1was up-regulated since7day of Pi starvation in leaf, and an one day re-supply of Pi suppressed its expression, while OsWRKY1was dynamically expressed in root, indicating it is also regulated by potential developmental and/or other unknown factors. Moreover, the expression of OsWRKY1in response to Pi deprivation differed in two rice cultivars, Nipponbare and Dongjin. Furthermore, OsWRKYl localized to the nucleus in the onion epidermal cells.2. A homozygous T-DNA insertion mutant line was isolated for OsWRKY1(oswrky1). Both soluble Pi concentration and total P concentration were increased in oswrkyl as compared with that in the wild type (WT) plants under high phosphate (HP) condition. Accordingly, a subset of rice Phtl genes were up-regulated in the mutant, indicating the increased accumulation of Pi might be mediated by an increase in Pi uptake and translocation. Interestingly,OsPHO1;2which is responsible for Pi loading into xylem was down-regulated upon mutation of OsWRKY1, highlighting a complex regulation in the Pi starvation signaling by OsWRKYl.To summarize, OsWRKY1is the only identified member of the rice WRKY transcription factor family responding to Pi starvation. OsWRKYl modulates Pi homeostasis at least partially by regulating the transcriptional activation/repression of downstream phosphate starvation-induced genes.