Biological role of phosphate stress responsive transcription factors in Arabidopsis thaliana
Phosphate (Pi) plays a major role in numerous metabolic processes in the plant cell. A better understanding of the processes that help plants adapt to Pi deficiency induced stress is thus vital. Transcription factors, often described as 'molecular switches', are believed to regulate adaptive mechanisms during Pi stress. Therefore, the current study was conducted to characterize three Pi stress responsive transcription factors (TFs) viz. WRKY75, ZAT6 and MYB62 in the model plant Arabidopsis thaliana. GFP-gene translational fusions in transgenic plants confirmed the nuclear localization of these TFs. Suppression of the WRKY75 expression through RNAi silencing caused the decreased expression of several Pi Starvation Induced (PSI) genes. Consequently, the Pi uptake of the mutant plant was also decreased during Pi-starvation. In addition, the lateral root length and number, as well as the root hair number, were significantly increased in the mutant plants. Thus, WRKY75 is a positive regulator of Pi acquisition while possibly functioning as a negative regulator of root development. The over-expression of ZAT6 retarded the growth of young seedlings due to lower Pi content in the plant, caused by decreased Pi uptake. In addition, the suppression of PSI genes and changes in root architecture of the mutant suggests that ZAT6 is a multifunctional TF synchronizing root development and Pi homeostasis. The over-expression of MYB62 caused stunted growth with delayed flowering and senescence which could be partially reversed by applying gibberellic acid (GA) exogenously. Several PSI genes were suppressed in this transgenic plant. A decrease in the expression of GA biosynthetic genes in the mutant suggests that MYB62 regulates GA biosynthesis as well as Pi homeostasis. These results suggest a hitherto unknown link between Pi stress responses and GA. Based on the above results, a hypothetical model can be proposed where WRKY75 and MYB62 directly regulate the expression of PSI genes and modulate various Pi stress responses via phytohormones in mutually exclusive pathways. On the other hand, ZAT6 affects root development, and thus controls the ability of the plant to acquire Pi and grow. Thus, this study characterizes three distinct transcriptional pathways that help in better understanding the role of transcription factors in regulating Pi stress responses.
Raghothama, Purdue University.
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