Analysis of Cytokinin Mutants and Regulation of Cytokinin Metabolic Genes Reveals Important Regulatory Roles of Cytokinins in Drought, Salt and Abscisic Acid Responses, and Abscisic Acid Biosynthesis  

Abstract

Cytokinins (CKs) regulate plant growth and development via a complex network of CK signaling. Here, we perform functional analyses with CK-deficient plants to provide direct evidence that CKs negatively regulate salt and drought stress signaling. All CK-deficient plants with reduced levels of various CKs exhibited a strong stress-tolerant phenotype that was associated with increased cell membrane integrity and abscisic acid (ABA) hypersensitivity rather than stomatal density and ABA-mediated stomatal closure. Expression of the Arabidopsis thaliana ISOPENTENYL-TRANSFERASE genes involved in the biosynthesis of bioactive CKs and the majority of the Arabidopsis CYTOKININ OXIDASES/DEHYDROGENASES genes was repressed by stress and ABA treatments, leading to a decrease in biologically active CK contents. These results demonstrate a novel mechanism for survival under abiotic stress conditions via the homeostatic regulation of steady state CK levels. Additionally, under normal conditions, although CK deficiency increased the sensitivity of plants to exogenous ABA, it caused a downregulation of key ABA biosynthetic genes, leading to a significant reduction in endogenous ABA levels in CK-deficient plants relative to the wild type. Taken together, this study provides direct evidence that mutual regulation mechanisms exist between the CK and ABA metabolism and signals underlying different processes regulating plant adaptation to stressors as well as plant growth and development.

Positive regulatory role of strigolactone in plant responses to drought and salt stress

“…This viewpoint is supported by data indicating that abcg22 and abcg40 mutants are drought-sensitive as a result of higher transpirational water loss resulted from diminished stomatal closure compared with WT plants (25,26). Additionally, recent studies reported that the constitutive overexpression of CKX genes, namely CKX1, CKX2, CKX3, and CKX4, resulted in higher ABA sensitivity, enhanced ABA response and improved drought and salt tolerance in transgenic Arabidopsis and tobacco plants (17,18). The transcriptome analysis conducted in the present study revealed that, among seven Arabidopsis CKX genes, four (CKX1, CKX2, CKX3, and CKX5) were down-regulated in the max2 mutant with and/or without being subjected to dehydration (Dataset S7).…”

“…Whereas ABA and SL promote the senescence of leaves, CK delays leaf senescence (30,31). On the other hand, ABA (32) and SL (the present study) act as positive regulators of stomatal closure and thus stress response, whereas CK acts as a negative regulator of the same process (18,33). Importantly, results of the present study demonstrate the potential for the use of genetic engineering to improve the stress tolerance of crop plants by manipulating SL biosynthesis and/or SL signaling as suggested by the enhanced drought tolerance exhibited by WT plants sprayed with SL (Fig.…”

“…Genes involved in CK degradation, namely CKX1, CKX2, CKX3, and CKX5, were repressed in max2 mutant under normal and dehydrative conditions (Dataset S7). These genes have previously been correlated with stress and ABA responses (16)(17)(18). Thus, the down-regulation of a number of genes with known links to stress-, ABA-, and CK-signaling pathways is concomitant with reduced drought and salt tolerance of max mutant plants, clearly establishing the link of SL with plant stress responses.…”

“…Drought stress tolerance assay. The same tray method was used to evaluate drought tolerance as previously described (18). Seedlings were grown in soil by using Dio Propagation Mix no.…”

“…Germination and growth inhibition assays were performed on GM medium containing 1% sucrose and various concentrations of ABA as described previously (18).…”