Protein kinase and ribonuclease domains of IRE1 confer stress tolerance, vegetative growth, and reproductive development in Arabidopsis

The unfolded protein response (UPR) endows plants with the capacity to perceive, respond, and protect themselves from adverse environmental conditions. The UPR signaling pathway in Arabidopsis has two "arms," one arm involving the bifunctional protein kinase (PK)/ribonuclease, IRE1, a RNA splicing enzyme, and another involving membrane-associated transcription factors, such as basic leucine zipper transcription factor 28 (bZIP28). Because of functional redundancies, single gene mutations in the plant UPR signaling pathway generally have not resulted in prominent phenotypes. In this study we generated multiple mutations in the UPR signaling pathway, such as an ire1a ire1b double mutant, which showed defects in stress tolerance and vegetative growth and development. Complementation of ire1a ire1b with constructs containing site-specific mutations in the PK or RNase domains of IRE1b demonstrated that a functional RNase domain is required for endoplasmic reticulum stress tolerance, and that both the PK and RNase domains are required for normal vegetative growth under unstressed conditions. Root growth under stress conditions was dependent on the splicing target of IRE1b, bZIP60 mRNA, and on regulated IRE1-dependent decay of target genes. However, root and shoot growth in the absence of stress was independent of bZIP60. Blocking both arms of the UPR signaling pathway in a triple ire1a ire1b bzip28 mutant was lethal, impacting pollen viability under unstressed conditions. Complementation with IRE1b constructs showed that both the PK and RNase domains are required for normal gametophyte development, but bZIP60 is not. Hence, the UPR plays a critical role in stress tolerance, and in normal vegetative growth and reproductive development in plants.