Arabidopsis inositol phosphate kinases IPK 1 and ITPK 1 constitute a metabolic pathway in maintaining phosphate homeostasis

• Published in: The Plant journal : for cell and molecular biology Vol. 95; no. 4; pp. 613 - 630
• Main Authors: Kuo, Hui‐Fen, Hsu, Yu‐Ying, Lin, Wei‐Chi, Chen, Kai‐Yu, Munnik, Teun, Brearley, Charles A., Chiou, Tzyy‐Jen
• Format: Journal Article
• Language: English
• Published: 01.08.2018
• ISSN: 0960-7412; 1365-313X
• DOI: 10.1111/tpj.13974

Summary Emerging studies have suggested that there is a close link between inositol phosphate (InsP) metabolism and cellular phosphate (P i ) homeostasis in eukaryotes; however, whether a common InsP species is deployed as an evolutionarily conserved metabolic messenger to mediate P i signaling remains unknown. Here, using genetics and InsP profiling combined with P i ‐starvation response ( PSR ) analysis in Arabidopsis thaliana , we showed that the kinase activity of inositol pentakisphosphate 2‐kinase ( IPK 1), an enzyme required for phytate (inositol hexakisphosphate; InsP 6 ) synthesis, is indispensable for maintaining P i homeostasis under P i ‐replete conditions, and inositol 1,3,4‐trisphosphate 5/6‐kinase 1 ( ITPK 1) plays an equivalent role. Although both ipk1‐1 and itpk1 mutants exhibited decreased levels of InsP 6 and diphosphoinositol pentakisphosphate ( PP ‐InsP 5 ; InsP 7 ), disruption of another ITPK family enzyme, ITPK 4, which correspondingly caused depletion of InsP 6 and InsP 7 , did not display similar P i ‐related phenotypes, which precludes these InsP species from being effectors. Notably, the level of d / l ‐Ins(3,4,5,6)P 4 was concurrently elevated in both ipk1‐1 and itpk1 mutants, which showed a specific correlation with the misregulated P i phenotypes. However, the level of d / l ‐Ins(3,4,5,6)P 4 is not responsive to P i starvation that instead manifests a shoot‐specific increase in the InsP 7 level. This study demonstrates a more nuanced picture of the intersection of InsP metabolism and P i homeostasis and PSRs than has previously been elaborated, and additionally establishes intermediate steps to phytate biosynthesis in plant vegetative tissues. Significance Statement Regulation of phosphate homeostasis and adaptive responses to phosphate limitation are critical for plant growth and crop yield. Studies have implicated inositol phosphates as regulators of phosphate homeostasis in eukaryotes; however, the relationship between inositol phosphate metabolism and phosphate signaling in plants remains elusive. This study dissected the step in which inositol phosphate metabolism intersects with regulation of phosphate homeostasis and phosphate starvation responses.