Functional analysis of AHK1/ATHK1 and cytokinin receptor histidine kinases in response to abscisic acid, drought, and salt stress in <i>Arabidopsis</i>

TLDR

Multistep component systems mediate signal transduction to environmental stimuli and growth regulators, and in Arabidopsis the six nonethylene receptor histidine kinases—including AHK1/ATHK1, AHK2, AHK3, and CRE1—are stress‑responsive, indicating they regulate abiotic stress responses. Gain‑ and loss‑of‑function studies in Arabidopsis showed that AHK1 positively regulates drought, salt stress, and ABA signaling. The ahk1 mutant displays down‑regulation of numerous stress‑ and ABA‑inducible genes, indicating that AHK1 acts upstream of AREB1, ANAC, and DREB2A to positively control stress responses through both ABA‑dependent and independent pathways, while loss of AHK2, AHK3, and CRE1 releases negative regulation of ABA signaling and osmotic stress, leading to enhanced drought and salt tolerance and highlighting cytokinin’s role via CRE1.

Abstract

In plants, multistep component systems play important roles in signal transduction in response to environmental stimuli and plant growth regulators. Arabidopsis contains six nonethylene receptor histidine kinases, and, among them, AHK1/ATHK1 , AHK2 , AHK3 , and CRE1 were shown to be stress-responsive, suggesting their roles in the regulation of plant response to abiotic stress. Gain- and loss-of-function studies in Arabidopsis indicated that AHK1 is a positive regulator of drought and salt stress responses and abscisic acid (ABA) signaling. Microarray analysis of the ahk1 mutant revealed a down-regulation of many stress- and/or ABA-inducible genes, including AREB1, ANAC, and DREB2A transcription factors and their downstream genes. These data suggest that AHK1 functions upstream of AREB1, ANAC, and DREB2A and positively controls stress responses through both ABA-dependent and ABA-independent signaling pathways. In addition, AHK1 plays important roles in plant growth because the ahk1 ahk2 ahk3 triple mutant showed further reduced growth. Unlike AHK1, loss-of-function analysis of ahk2 , ahk3 , and cre1 implied that the stress-responsive AHK2, AHK3, and CRE1 act as negative regulators in ABA signaling. AHK2 and AHK3 also negatively control osmotic stress responses in Arabidopsis because ahk2 , ahk3 , and ahk2 ahk3 mutants were strongly tolerant to drought and salt stress due to up-regulation of many stress- and/or ABA-inducible genes. Last, cytokinin clearly mediates stress responses because it was required for CRE1 to function as a negative regulator of osmotic stress.