On the left side, under HP conditions, PHR1 is bound by SPX proteins and thus remains inactive. Bacterial flg22 is perceived by the plant receptor FLS2, triggering the immune response and inhibiting bacterial growth. On the right side, under LP conditions, PHR1 is released from the SPX protein complex, binds to the promoters of RALF genes and activates RALF expression. RALF mRNA is translated and further processed into mature RALF peptides that interfere with flg22-induced immunity complex formation between FLS2 and BAK1 through FER. Suppression of plant immunity by RALF peptides enhances bacterial growth, which helps plants increase Pi uptake under Pi-limiting conditions and thus promotes plant growth. 