Molecular and physiological responses of Arabidopsis thaliana plants deficient in the genes responsible for ABA and cytokinin reception and metabolism to heat shock

Using mutant plants of Arabidopsis thaliana, participation of the genes involved in abscisic acid (ABA) and cytokinins (CKs) metabolism and signaling in plant defense responses to heat shock (HS) was investigated. The magnitude of the stress action was assessed with biochemical indicators, such as accumulation of proline and malonic dialdehyde (MDA) and changes in the content of gene transcripts of heat shock proteins (HSPs) (HSP90.1 and HSP90.5) and transcription factor of HS (HSFA2) as well as stress-inducible genes, markers of oxidative stress (AOX1a, RD29, PRODH1, and P5CS1). Mutants with inactivated genes of ABA synthesis and, especially, signal perception exhibited lower thermo-resistance and accumulated elevated amounts of CK metabolism and signal transduction genes. In contrast, plants with inactivated components of CK synthesis and signal perception displayed increased tolerance to high temperatures and reduced levels of mRNA of oxidative stress genes as compared to wild type (WT) plants. However, enhancement of HS contributed to decrease of thermo-resistance of CKs receptor mutants up to the level of WT plants. Under hyperthermia, the stimulation of the plant defense mechanisms was accompanied by downregulation of the expression of CK metabolism and signal transduction genes (IPT3, CKX1, ARR5, AHK2, and AHK4) and ABA catabolism gene (CYP707A1) and upregulation of the expression of ABA synthesis and signal perception genes (ABA3 and ABI2). In the mutants insensitive to ABA, CYP707A1 gene was upregulated under the HS, while the expression of CK receptor gene expression did not reliably change. The results indicate that the response of the plants to elevated temperatures was determined not only by strength and the duration of the stress but the state of their ABA and CK metabolic and signaling systems as well.