SUMO modification of LBD30 by SIZ1 regulates secondary cell wall formation in Arabidopsis thaliana

A wide range of biological processes are regulated by sumoylation, a post-translational modification involving the conjugation of SUMO (Small Ubiquitin-Like Modifier) to protein. In Arabidopsis thaliana, AtSIZ1 encodes a SUMO E3 ligase for SUMO modification. siz1 mutants displayed defective secondary cell walls (SCWs) in inflorescence fiber cells. Such defects were caused by repression of SND1/NST1-mediated transcriptional networks. Yeast two-hybrid assay indicated that SIZ1 interacts with the LBD30 C-terminal domain, which was further confirmed using bimolecular fluorescence complementation and immunoprecipitation. Mass spectrometry and co-immunoprecipitation indicated that SIZ1 mediates SUMO conjugation to LBD30 at the K226 residue. Genes controlling SCW formation were activated by the overexpression of LBD30, but not in the LBD30((K226R)) mutant. LBD30 enhancement of SCW formation resulted from upregulation of SND1/NST1-mediated transcriptional networks. This study presents a mechanism by which sumoylation of LBD30, mediated by SIZ1, regulates SCW formation in A. thaliana. Author summary Secondary cell wall (SCW) is essential for upright plant growth and long-distance transport of water and solutes. Regulation of SCW formation at the transcriptional level has been much studied. Here we show that AtSIZ1, a small ubiquitin-related modifier (SUMO) E3 ligase, mediates the sumoylation of transcription factor LBD30, a process that regulates SCW formation. Elucidating a role of sumoylation in the post-translational regulation of SCW formation, complementing the body of research on regulation at the transcriptional level, provides additional insight into a key process that is essential for upright growth and the long-distance transport of water and solutes in plants.