Unfolded protein-independent IRE1 activation contributes to multifaceted developmental processes in Arabidopsis

Abstract

In Arabidopsis, the <i>IRE1A</i> and <i>IRE1B</i> double mutant (<i>ire1a/b</i>) is unable to activate cytoplasmic splicing of <i>bZIP60</i> mRNA and regulated IRE1-dependent decay under ER stress, whereas the mutant does not exhibit severe developmental defects under normal conditions. In this study, we focused on the Arabidopsis <i>IRE1C</i> gene, whose product lacks a sensor domain. We found that the <i>ire1a/b/c</i> triple mutant is lethal, and heterozygous <i>IRE1C</i> (<i>ire1c</i>/+) mutation in the <i>ire1a/b</i> mutants resulted in growth defects and reduction of the number of pollen grains. Genetic analysis revealed that <i>IRE1C</i> is required for male gametophyte development in the <i>ire1a/b</i> mutant background. Expression of a mutant form of IRE1B that lacks the luminal sensor domain (ΔLD) complemented a developmental defect in the male gametophyte in <i>ire1a/b/c</i> haplotype. In vivo, the ΔLD protein was activated by glycerol treatment that increases the composition of saturated lipid and was able to activate regulated IRE1-dependent decay but not <i>bZIP60</i> splicing. These observations suggest that IRE1 contributes to plant development, especially male gametogenesis, using an alternative activation mechanism that bypasses the unfolded protein-sensing luminal domain.