Department of Bioscience, Tokyo University of Agriculture, Tokyo, 156-8502 Japan.
RIKEN BioResource Research Center, Ibaraki, 305-0074 Japan.
Plant Cell Physiol. 2021 May 11;62(2):272-279. doi: 10.1093/pcp/pcaa157.
Plants are often exposed not only to short-term (S-) heat stress but also to diurnal long-term (L-) heat stress over several consecutive days. To reveal the mechanisms underlying L-heat stress tolerance, we here used a forward genetic screen for sensitive to long-term heat (sloh) mutants and isolated sloh4. The mutant was hypersensitive to L-heat stress but not to S-heat stress. The causal gene of sloh4 was identical to MIP3 encoding a member of the MAIGO2 (MAG2) tethering complex, which is composed of the MAG2, MIP1, MIP2 and MIP3 subunits and is localized at the endoplasmic reticulum (ER) membrane. Although sloh4/mip3 was hypersensitive to L-heat stress, the sensitivity of the mag2-3 and mip1-1 mutants was similar to that of the wild type (WT). Under L-heat stress, the ER stress and the following unfolded protein response (UPR) were more pronounced in sloh4 than in the WT. Transcript levels of bZIP60-regulated UPR genes were strongly increased in sloh4 under L-heat stress. Two processes known to be mediated by INOSITOL REQUIRING ENZYME1 (IRE1) - accumulation of the spliced bZIP60 transcript and a decrease in the transcript levels of PR4 and PRX34, encoding secretory proteins - were observed in sloh4 in response to L-heat stress. These findings suggest that misfolded proteins generated in sloh4 under L-heat stress may be recognized by IRE1 but not by bZIP28, resulting in the initiation of the UPR via activated bZIP60. Therefore, it would be possible that only MIP3 in the MAG2 complex has an additional function in L-heat tolerance, which is not related to the ER-Golgi vesicle tethering.
植物不仅经常暴露于短期（S-）热应激中，而且还会连续几天受到日间长期（L-）热应激的影响。为了揭示 L-热应激耐受的机制，我们在此使用了一个针对长期热敏感（sloh）突变体的正向遗传筛选，并分离出 sloh4。该突变体对 L-热应激敏感，但对 S-热应激不敏感。sloh4 的致病基因与编码 MAIGO2（MAG2）连接复合物成员的 MIP3 相同，该复合物由 MAG2、MIP1、MIP2 和 MIP3 亚基组成，位于内质网（ER）膜上。尽管 sloh4/mip3 对 L-热应激敏感，但 mag2-3 和 mip1-1 突变体的敏感性与野生型（WT）相似。在 L-热应激下，sloh4 中的 ER 应激和随后的未折叠蛋白反应（UPR）比 WT 更为明显。在 L-热应激下，bZIP60 调节的 UPR 基因的转录水平在 sloh4 中强烈增加。在 sloh4 中观察到两种已知由 INOSITOL REQUIRING ENZYME1（IRE1）介导的过程 - 剪接的 bZIP60 转录物的积累和编码分泌蛋白的 PR4 和 PRX34 转录物水平的降低 - 对 L-热应激的反应。这些发现表明，在 sloh4 下产生的错误折叠蛋白可能被 IRE1 识别，但不能被 bZIP28 识别，从而通过激活的 bZIP60 启动 UPR。因此，MAG2 复合物中的 MIP3 可能具有额外的 L-热耐受功能，这与 ER-Golgi 囊泡连接无关。
