Abstract
Calcium ions (Ca(2+)) and Ca(2+)-related proteins mediate a wide array of downstream processes involved in plant responses to abiotic stresses. In Arabidopsis (Arabidopsis thaliana), disruption of the vacuolar Ca(2+)/H(+) transporters CAX1 and CAX3 causes notable alterations in the shoot ionome, including phosphate (P(i)) content. In this study, we showed that the cax1/cax3 double mutant displays an elevated P(i) level in shoots as a result of increased P(i) uptake in a miR399/PHO2-independent signaling pathway. Microarray analysis of the cax1/cax3 mutant suggests the regulatory function of CAX1 and CAX3 in suppressing the expression of a subset of shoot P(i) starvation-responsive genes, including genes encoding the PHT1;4 P(i) transporter and two SPX domain-containing proteins, SPX1 and SPX3. Moreover, although the expression of several PHT1 genes and PHT1;1/2/3 proteins is not up-regulated in the root of cax1/cax3, results from reciprocal grafting experiments indicate that the cax1/cax3 scion is responsible for high P(i) accumulation in grafted plants and that the pht1;1 rootstock is sufficient to moderately repress such P(i) accumulation. Based on these findings, we propose that CAX1 and CAX3 mediate a shoot-derived signal that modulates the activity of the root P(i) transporter system, likely in part via posttranslational regulation of PHT1;1 P(i) transporters.
Highlights
Calcium ions (Ca2+) and Ca2+-related proteins mediate a wide array of downstream processes involved in plant responses to abiotic stresses
Transient increases in cytoplasmic calcium concentrations namics of s([tCima2u+l]ucyst-)inodr uthceedspaalttiearlaatinodnsteminpo[Craal2+d]cyy-t constitute a signal that mediates a wide array of downstream processes involved in plant responses to many developmental cues and environmental stresses (Knight, 2000; McAinsh and Pittman, 2009)
Consistent with previous results showing that the growth defects of cax1/cax3 mutant were suppressed by supplemented exogenous Mg2+ (Cheng et al, 2005), we found that the exacerbated Pi resulting from accumulation exogenous Ca2+
Summary
Cax1/cax Mutant Accumulates High Levels of Pi in Shoot and Displays Increased Pi Transport Activity. When the expression of the PSR genes was examined in cax1/ cax, most of them, such as SPX1, SPX3, and PHT1;4, were less responsive to Pi starvation compared with wild-type plants (Fig. 6, A–C; Supplemental Table S1, subgroups 5 and 6) This may have resulted from their suppression by the high shoot Pi levels of cax1/cax (Fig. 1A). Our results here suggest that the cax1/cax scion genotype is responsible for high levels of Pi accumulation as a result of increased Pi uptake in the wild-type rootstocks and that a signal originating from cax1/cax shoots may move across grafting junctions to activate the Pi transport activity in roots, likely in part through posttranslational up-regulation of PHT1;1. CAX1 and CAX3 appear not to be involved in the generation of the putative systemic suppressor in +Pi roots and its long-distance movement
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