The PHD finger of Arabidopsis SIZ1 recognizes trimethylated histone H3K4 mediating SIZ1 function and abiotic stress response

Kenji Miura et al. investigate the role of the plant homeodomain (PHD) finger of the Arabidopsis SIZ1 protein. They show that the PHD finger is involved in hormone response and temperature sensitivity, and plays an important role in H3K4 methylation, thereby affecting recognition of histone code and transcriptional suppression. Arabidopsis SIZ1 encodes a SUMO E3 ligase to regulate abiotic and biotic stress responses. Among SIZ1 or mammalian PIAS orthologs, plant SIZ1 proteins contain the plant homeodomain (PHD) finger, a C4HC3 zinc finger. Here, we investigated the importance of PHD of Arabidopsis SIZ1. The Pro(SIZ1)::SIZ1(Delta PHD):GFP was unable to complement growth retardation, ABA hypersensitivity, and the cold-sensitive phenotype of the siz1 mutant, but Pro(SIZ1)::SIZ1:GFP could. Substitution of C162S in the PHD finger was unable to complement the siz1 mutation. Tri-methylated histone H3K4 (H3K4me3) was recognized by PHD, not by PHD(C162S). WRKY70 was up-regulated in the siz1-2 mutant and H3K4me3 accumulated at high levels in the WRKY70 promoter. PHD interacts with ATX, which mediates methylation of histone, probably leading to suppression of ATX's function. These results suggest that the PHD finger of SIZ1 is important for recognition of the histone code and is required for SIZ1 function and transcriptional suppression.