Stomatal ontogenesis is a key element of plant adaptation aiming to control photosynthetic efficiency and water management under fluctuating environments 1,2,3 . Development of stomata is guided by endogenous and environmental cues and is tightly coupled to overall plant growth 1,2,3 .
Heat shock proteins 90 (HSP90) are essential and play critical roles in the adaptation of organisms to diverse stimuli. In plants, HSP90 are involved in auxin, jasmonate and brassinosteroid (BR) signalling pathways. The BR‐promoted activation of the BES1 transcription factor regulates BR‐responsive genes.
Using genetic, physiological, fluorescence live cell imaging, molecular and biochemical approaches, such as phenotypic analysis, co‐immunoprecipitation assay, yeast‐two hybrid and Bimolecular fluorescence complementation (BiFC), we studied complex formation between BES1 and HSP90 under control conditions and active BR signalling. Further, we determined the effect of the pharmacological inhibition of HSP90 ATPase activity on hypocotyl elongation of bes1-D mutant.
We determined that HSP90 interact with BES1 in the nucleus and in the cytoplasm. During active BR signalling, nuclear complexes were absent while cytoplasmic HSP90/BES1 complexes were prominent. Our results showed that the hypocotyl length of bes1‐D mutants was highly reduced when HSP90 was challenged by the geldanamycin (GDA) inhibitor of the ATPase activity of HSP90. Active BR signalling could not rescue the GDA effect on the hypocotyl elongation of bes1-D.
Our results reveal that the constitutively active BES1 in the bes1‐D mutant is hypersensitive to GDA. The interaction of HSP90 with BES1 argues that HSP90 facilitate the nuclear metastable conformation of BES1 to regulate BR‐dependent gene expression, and our data show that HSP90 assist in the compartmentalised cycle of BES1 during active BR signalling.
Summary
Auxin homeostasis and signaling affect a broad range of developmental processes in plants. The interplay between HSP90 and auxin signaling is channeled through the chaperoning capacity of the HSP90 on the TIR1 auxin receptor. The sophisticated buffering capacity of the HSP90 system through the interaction with diverse signaling protein components drastically shapes genetic circuitries regulating various developmental aspects. However, the elegant networking capacity of HSP90 in the global regulation of auxin response and homeostasis has not been appreciated.
Arabidopsis hsp90 mutants were screened for gravity response. Phenotypic analysis of root meristems and cotyledon veins was performed. PIN1 localization in hsp90 mutants was determined.
Our results showed that HSP90 affected the asymmetrical distribution of PIN1 in plasma membranes and influenced its expression in prompt cell niches. Depletion of HSP90 distorted polar distribution of auxin, as the acropetal auxin transport was highly affected, leading to impaired root gravitropism and lateral root formation. The essential role of the HSP90 in auxin homeostasis was profoundly evident from early development, as HSP90 depletion affected embryo development and the pattern formation of veins in cotyledons.
Our data suggest that the HSP90‐mediated distribution of PIN1 modulates auxin distribution and thereby auxin signaling to properly promote plant development.
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