Knockout of endoplasmic reticulum localized molecular chaperone HSP90.7 impairs seeding development and cellular auxin homeostasis in Arabidopsis

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 did not express any protein and showed seedling lethality. Microscopic analyses revealed its essential role in male and female fertility, trichomes and root hairs development, proper chloroplast function, and in apical meristem maintenance and differentiation. Comparative transcriptome and proteome analyses also revealed a role of the protein in a multitude of cellular processes. Particularly, the auxin responsive pathway was specifically down-regulated 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 demonstrated PIN1 and PIN5 expressions were dramatically reduced in the mutant, and the TAA-YUCCA primary auxin biosynthesis pathway was also down-regulated, thus revealing a critical new role of HSP90.7 in the regulation of auxin responses. 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 this molecular chaperone in regulating plant growth and development via modulating cellular auxin homeostasis.