Biological role of phosphate starvation induced genes in the maintenance of phosphate homeostasis in Arabidopsis thaliana
Phosphate (Pi) plays a central role in several metabolic processes and is an indispensable building block for the biosynthesis of nucleic acids and phospholipids. As a response to Pi deficiency the plant undergoes an array of morphophysiological, biochemical and molecular adaptations. In order to better understand these adaptive mechanisms, three candidate genes selected from a microarray analysis were characterized for their role during Pi stress. These three genes — an organic phosphate transporter, a transcription factor and a RING E3 ligase are induced during Pi starvation. Both forward and reverse genetics approach were used to define their biological function. Glycerol-3-phosphate permease (G3Pp) is a Pi/anion antiporter represented by a gene family of five members in Arabidopsis. The spatio-temporal expression pattern of members of this gene family during Pi stress was characterized by transcript analysis and generation of transgenic plants carrying promoter-GUS fusion constructs. All the five members were induced by Pi starvation but varied in their tissue specific expression. Also, the distinct expression of the G3Pps in the early stages of germination suggests a role in nutrient mobilization during this metabolically active stage. Characterization of a knock-down mutant for one of these transporters revealed a developmentally regulated functional compensation by other members of the gene family Transcription factors like AtERF070 form a part of regulatory networks as they can modulate the expression of target genes. AtERF070 belongs to the plant specific AP2 domain containing Ethylene Response Factor (ERF) family. RNAi mediated suppression of AtERF070 resulted in an increase in the number of lateral roots and root hairs which could have contributed to higher Pi content in the transgenic plants. Over-expression of this transcription factor caused a gibberellic acid (GA)-deficient phenotype. They exhibited delayed senescence, late flowering, increased accumulation of anthocyanin and reduction in the number of lateral roots which could be partially rescued by the exogenous application of the phytohormone GA. The expression of several GA biosynthetic genes was reduced, further corroborating the possibility of altered phytohormone levels in the over-expressor. Several Pi-starvation induced (PSI) genes especially microRNAs and RNAses were found to be upregulated by the over-expression of AtERF070 . These results suggest the involvement of AtERF070 in maintenance of Pi homeostasis and GA-mediated growth modulation during Pi starvation. The role of ATL8 , a RING E3 ligase induced during Pi deficiency was also investigated as a part of this study. Modulation of this gene by Pi starvation indicates a plausible post translational regulation of PSI responses. A T-DNA insertion mutant of ATL8 exhibited a reduction in lateral root number and a subsequent decrease in accumulation of Pi. In contrast, over-expression led to an abscissic acid resistant phenotype and more number of lateral roots leading to increased accumulation of Pi. Based on the above results we can conclude that modulation of the root system by PSI genes is an important biological adaptation during Pi deficiency. The modulation of plant growth is mediated by phytohormones like GA, cytokinin, auxin and involves genes like AtERF070 and ATL8 . These observations point towards a complex transcriptional and post translational regulation of the adaptations during Pi starvation.
Raghothama, Purdue University.
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