Gene expression profile of Arabidopsis thaliana wild type and siz1-2 mutant during in vitro shoot regeneration on callus inducing media (CIM) and shoot inducing media (SIM)

Plants form callus and regenerate new organs when incubated on phytohormone-containing media. While accumulating evidence suggests that these regenerative processes are governed by transcriptional networks orchestrating stress responses and developmental transitions, it remains unknown if post-translational regulatory mechanisms are involved in this process. Here, we find that SIZ1, which encodes an E3 ligase catalyzing attachment of the SMALL UBIQUITIN-LIKE MODIFIER (SUMO) to proteins, regulates wound-induced signal transduction and organ regeneration. We show that loss-of-function mutants for SIZ1 exhibit over-production of shoot meristems under in vitro tissue culture conditions, while this defect is rescued in a complementation line expressing pSIZ1::SIZ1. RNA-sequencing analysis revealed that siz1-2 mutant exhibits enhanced transcriptional responses to wound stress, resulting in the hyper-induction of over 500 genes immediately after wounding. Among them, we show that elevated level of WOUND INDUCED DEDIFFERENTIATION 1 (WIND1) and WIND2 contribute to enhanced shoot regeneration observed in siz1 mutants, as the dominant-negative WIND1-SRDX partly rescues this phenotype in siz1-3. Although compromised SIZ1 function does not modify transcription of genes implicated in auxin-induced callus formation and/or pluripotency acquisition, it does lead to enhanced induction of cytokinin-induced shoot meristem regulators like WUSCHEL (WUS), promoting the formation of WUS-expressing foci in explants. This study thus suggests that SIZ1 negatively regulates shoot regeneration in part by repressing wound-induced cellular reprogramming.