Plants under Stress: Involvement of Auxin and Cytokinin

Abstract

Plant growth and development are critically influenced by unpredictable abiotic factors. To survive fluctuating changes in their environments, plants have had to develop robust adaptive mechanisms. The dynamic and complementary actions of the auxin and cytokinin pathways regulate a plethora of developmental processes, and their ability to crosstalk makes them ideal candidates for mediating stress-adaptation responses. Other crucial signaling molecules responsible for the tremendous plasticity observed in plant morphology and in response to abiotic stress are reactive oxygen species (ROS). Proper temporal and spatial distribution of ROS and hormone gradients is crucial for plant survival in response to unfavorable environments. In this regard, the convergence of ROS with phytohormone pathways acts as an integrator of external and developmental signals into systemic responses organized to adapt plants to their environments. Auxin and cytokinin signaling pathways have been studied extensively. Nevertheless, we do not yet understand the impact on plant stress tolerance of the sophisticated crosstalk between the two hormones. Here, we review current knowledge on the function of auxin and cytokinin in redirecting growth induced by abiotic stress in order to deduce their potential points of crosstalk.

Keywords: ROS; abiotic stress; adaptation; auxin; crosstalk; cytokinin; growth.

Figure 1

Auxin- and cytokinin-related genes differentially expressed by both hormones and various environmental cues. From 335 auxin- and 128 cytokinin-related genes selected from the literature and The Arabidopsis Information Resource (TAIR, Available online: www.arabidopsis.org), probes for 246 (auxin) and 128 (cytokinin) genes were found using the Genevestigator database and Affymetrix Arabidopsis ATH1 Genome Array (Available online: http://www.genevestigator.com/gv/index.jsp). We analyzed their expression upon 59 perturbations from 328 microarrays. Using perturbation tool analysis and applying fold change cut-offs of >1.5 and significance p-values of <0.05, we ended up with a total of 36 auxin-related genes and 19 cytokinin-related genes that responded to both hormones and to single or multiple stressors at the same time. The figure shows the selected experiments, which can be retrieved using the unique database ID for each. The genes’ expression responses are calculated as log₂-ratios between the signal intensities from different perturbations as compared with control or mock-treated samples. (A) The panel shows a hierarchical clustering heat map from genes downregulated or upregulated simultaneously by all selected stresses. The hierarchical clustering maps for the total number of selected genes. (B) The panel shows the expression profile of auxin-related genes divided into 5 groups according to their roles (transport, metabolism, biosynthesis, signaling, and response genes) and 3 groups for cytokinin-related genes (metabolism, perception and signaling, and response-related genes). Gene expression patterns are represented in a log2 ratio ranging from −2.5 (blue color, down-regulated) to +2.5 (yellow color, up-regulated). Abbreviations: d = days; min = minutes; h = hours; PEG = polyethylene glycol; MV = methyl viologen.