Does OsPHR2, central pi-signaling regulator, regulate some unknown factors crucial for plant growth?

Abstract

OsPHR2, the homolog of AtPHR1, is a central Pi-signaling regulator. The Pi-signaling pathway downstream of AtPHR1, similarly of OsPHR2,1,2 involves a noncoding RNA which targets mimicry of miR399. miRNA399 mediates cleavage of PHO2. (3,4) The regulating pathway downstream of OsPHR2 is negatively regulated by the Pi-signaling responsive gene OsSPX1. (5,6) Overexpression of AtPHR1 and OsPHR2 leads to an increased concentration of Pi in the shoot tissues with leaf toxic symptom and growth retardation similar as the phenotype of pho2 mutant, especially under Pi abundant conditions. (7,2,6) It has been known that the low affinity Pi transporter OsPT2 mainly contributes to the shoot Pi accumulation mediated by OsPHR2, and overexpression of OsPT2 results in shoot Pi accumulation and leaf toxic symptom and growth retardation under Pi abundant conditions. (6) Two curious questions are emerging from the reported results: How Os SPX1 functions on the negative regulation of the pathway and what mechanism of the growth retardation mediated by OsPHR2. For the second question, our favored hypothesis is that the growth inhibition mediated by overexpression of OsPHR2 is caused by toxic physiological effects due to excessive Pi accumulation in shoots (Pi toxicity). In fact, the toxic symptoms become diminished with decreased Pi levels in growth medium. However, the plant growth retardation mediated by overexpression of OsPHR2 may be caused by some unknown genetic factor(s) regulated by OsPHR2.

Figure 1

Shoot Pi concentration (A) and shoot biomass of 30 d-old plants of wild type (WT) and the transgenic plants (B) with overexpression of OsPHR2 under solution culture with abundant supplied Pi (10 mg Pi/L) and low Pi level (0.5 mg Pi/L). The solution pH is 5.5 and each plant occupied one liter of culture solution. The culture solution was replaced every two days.

Figure 2

Mutants showed rescue of plant growth under background of overexpression of OsPHR2. (A) Ninety days old plants under solution culture with Pi-supplied condition (10 mg Pi/L). PCR analysis using the primers flanking 3, 4, 5 introns of OsPHR2 and HYG (hygromycin gene) (below). (B) Relative expression levels of OsPHR2 and the PSI (Pi-starvation induced) genes downstream of OsPHR2. (C) Shoot Pi concentration in the wild type, transgenic plants with overexpression of OsPHR2 and the mutant.