The sHSP22 heat-shock protein requires the ABI1 protein phosphatase to modulate polar auxin transport and downstream responses.

The phytohormones abscisic acid (ABA) and indole-3-acetic acid (IAA) response pathways interact synergistically or antagonistically to regulate plant development and environmental adaptation. Here, we show that ABI1, a key negative regulator of ABA signaling, is essential for auxin-modulated root development. We performed a microarray analysis using the loss-of-function mutant abi1-3 and Col-0 seedlings treated with IAA. For sHSP22, an endoplasmic reticulum (ER) small heat shock protein-encoding gene, the induction by IAA was dependent on ABI1. shsp22 displayed enhanced sensitivity to ABA in primary root growth. In contrast, overexpression of full-length, but not truncated sHSP22 lacking signal peptide and ER-retention sequences, resulted in decreased ABA sensitivity. Overexpressed (OX) sHSP22 partially rescued the ABA hypersensitivity of abi1-3. In addition, sHSP22 is involved in auxin-regulated hypocotyl elongation at high temperature treatment. sHSP22 also affected accumulation of auxin efflux carrier PIN proteins due to potentiated intracellular trafficking. And sHSP22 OX lines initiated more lateral roots after auxin application. Our results suggest that sHSP22 regulates auxin response through modulating auxin polar transport, and ABI1-sHSP22 provides a novel module orchestrating ABA and auxin signaling crosstalk in Arabidopsis (Arabidopsis thaliana).