The protein modifier SUMO is critical for Arabidopsis shoot meristem maintenance at warmer ambient temperatures

Short periods of heat (>37°C) are extremely damaging to non-acclimated plants and their capacity to acclimate to and recover from heat stress is a key parameter for their survival and longevity. To acclimate, the Heat Shock transcription Factor A1 (HSFA1) subfamily activates a transcriptional response that resolves the heat stress-induced protein damage. Importantly, HSFA1 activity is also critical for Arabidopsis to withstand sustained warmer periods of 28°C, a non-detrimental condition that triggers a thermomorphogenesis response. We find that SUMO, a protein modification whose adduct levels increase as a result of acute heat stress in eukaryotes, is also critical for plant longevity during warmer periods, in particular for shoot meristem development. The known E3 and E4 SUMO ligases (SIZ1, HPY1/MMS21, PIAL1/2) were not essential to endure these warmer periods, alone or in combination. Thermo-lethality was also not seen when plants lacked certain SUMO proteases (ESD4, OTS1/OTS2, SPF1/SPF2 combined) or when SUMO chain formation was blocked. Furthermore, SUMO thermo-resilience is not connected to the autoimmune phenotype found in the corresponding SUMO knockdown and a SIZ1 loss-of-function mutant. As acquired thermotolerance was normal in the SUMO knockdown mutant, we thus conclude that the role of SUMO in heat acclimation differs from that of HSFA1 and SIZ1. Combined, this study reveals that SUMO appears to be critical for shoot meristem integrity during warmer periods.

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