Figure 9.

A Putative Model of FHY3 and FAR1 in Balancing Growth and Defense under Low R/FR Conditions.

In wild-type plants, shade (low R/FR ratios) induces stabilization of PIFs, FHY3, and JAZ proteins but destabilizes MYC2 protein. On the one hand, FHY3 and FAR1 activate the expression of PAR1/PAR2, which inhibits the expression of growth-related genes by forming non-DNA binding heterodimers with PIFs, thus preventing an exaggerated elongation growth. On the other hand, FHY3/FAR1, together with MYC2, activate the expression of JA-responsive defense genes, while JAZ proteins act to inhibit the activity of FHY3 and FAR1 to maintain a proper level of defense gene expression and defense response. In the fhy3 far1 mutant, loss of FHY3 and FAR1 leads to reduced expression level of PAR1/PAR2, resulting in overexpression of growth-related genes and exaggerated elongation growth. Meanwhile, the expression of JA-responsive defense genes is also reduced due to the absence of FHY3 and FAR1 proteins. As a result, the balance between growth and defense is severely disrupted in the fhy3 far1 mutant plants under shade conditions.