Eiji Nambara

SUMMARY The Arabidopsis endoplasmic reticulum‐localized heat shock protein HSP90.7 modulates tissue differentiation and stress responses; however, complete knockout lines have not been previously reported. In this study, we identified and analyzed a mutant allele, hsp90.7‐1 , which was unable to accumulate the HSP90.7 full‐length protein and showed seedling lethality. Microscopic analyses revealed its essential role in male and female fertility, trichomes and root hair development, proper chloroplast function, and apical meristem maintenance and differentiation. Comparative transcriptome and proteome analyses also revealed the role of the protein in a multitude of cellular processes. Particularly, the auxin‐responsive pathway was specifically downregulated in the hsp90.7‐1 mutant seedlings. We measured a much‐reduced auxin content in both root and shoot tissues. Through comprehensive histological and molecular analyses, we confirmed PIN1 and PIN5 accumulations were dependent on the HSP90 function, and the TAA‐YUCCA primary auxin biosynthesis pathway was also downregulated in the mutant seedlings. This study therefore not only fulfilled a gap in understanding the essential role of HSP90 paralogs in eukaryotes but also provided a mechanistic insight on the ER‐localized chaperone in regulating plant growth and development via modulating cellular auxin homeostasis.