Fig. 2.

Simplified scheme of PSR. Although the presence of P sensors was suggested, their exact position remains unclear. P responses include regulatory components controlling expression of target genes. These regulatory components exhibit either negative (e.g. ubiquitin conjugase PHO2, target gene for miRNA399) or positive (e.g. transcription factor WRKY75 and SPX1 proteins) effects. The MYB transcription factor PHR1 is required for Pi starvation-dependent responses, with SUMO E3 ligase SIZ1 acting upstream of PHR1. SIZ1 may also control the auxin receptor TIR1, thereby modifying the root system architecture. Lines and arrows are as in Fig. 1 legend.