Chloroplasts are among the main targets of cytokinin action in the plant cell. We report here on the activation of transcription by cytokinin as detected by run-on assays with chloroplasts isolated from apical parts of first leaves detached from 9-d-old barley (Hordeum vulgare) seedlings and incubated for 3 h on a 2.2 x 10(-5) m solution of benzyladenine (BA). Northern-blot analysis also detected a BA-induced increase in the accumulation of chloroplast mRNAs. A prerequisite for BA activation of chloroplast transcription was preincubation of leaves for 24 h on water in the light, resulting in a decreased chloroplast transcription and a drastic accumulation of abscisic acid. Cytokinin enhanced the transcription of several chloroplast genes above the initial level measured before BA treatment, and in the case of rrn16 and petD even before preincubation. Cytokinin effects on basal (youngest), middle, and apical (oldest) segments of primary leaves detached from plants of different ages revealed an age dependence of chloroplast gene response to BA. BA-induced stimulation of transcription of rrn16, rrn23, rps4, rps16, rbcL, atpB, and ndhC required light during the period of preincubation and was further enhanced by light during the incubation on BA, whereas activation of transcription of trnEY, rps14, rpl16, matK, petD, and petLG depended on light during both periods. Our data reveal positive and differential effects of cytokinin on the transcription of chloroplast genes that were dependent on light and on the age (developmental stage) of cells and leaves.

The facts of both positive and negative influences of cytokinins on stress resistance of plants are known today. Without pretending to a final choice between these points of view, we have made an attempt to analyze the details of the experiments that gave rise to conclusions about the nature of the effect of cytokinins on the resistance to stress-causing influences with a focus on their intensity and duration. The review deals with the data concerning the influence of different adverse factors on the content of endogenous cytokinins and transduction of cytokinin signals, examines the influence on plant resistance of treatment with exogenous hormone, and the effects of genetic modifications causing changes in cytokinin content and signaling. Resistance is considered not only as a mean of plant survival under severe stress but also as an instrument of maintaining growth rate in plants exposed to moderate stress. Literature data and our own results make it possible to conclude that cytokinins play an important role in formation of plant resistance to adverse influences; however, the effect of these hormones depends on stress intensity. Under moderate stress, cytokinins ensure maintenance of plant growth, whereas a drop in cytokinins hampers growth under a strong influence of adverse factors, which is a prerequisite for mobilization of limited resources characteristic of severe stress and ensures preservation of plant viability.

Mild photooxidative stress combined with water deficit affected the expression of genes of cytokinin (CK) signal transduction. According to qRT-PCR data, these stress factors suppressed the expression of CK receptor genes AHK2 and AHK3 and the ARR5 response regulator gene, and slightly activated the expression of AHK4 gene. The absence of AHK2 and AHK3 receptors in Arabidopsis thaliana (L.) Heynh. mutants affected markedly the content of low-molecular-weight antioxidants (anthocyanins, carotenoids, proline) and resulted in the CK-dependent changes in the expression of genes, markers of oxidative stress, including RAB18, CHS, P5CS1, and PRODH1, under both normal and stressful conditions. At the same time, knockout of a single receptor gene or their combination did not induce CK-specific changes in the stress-activated expression of AOX1a, the marker gene of oxidative stress. CK receptor mutants lacked a statistically significant genotype influence on the photosystem II and photosystem I fluorescence yield under normal and stressful conditions. However, the system of CK perception was shown to be involved in the transcription regulation of the genes of the photosynthetic apparatus, ELIP2 and PSBS. Thus, CKs play an important role in plant responses to the photooxidative stress. The inactivation of the CK signaling system components proves to be one of the main strategies of Arabidopsis plant adaptation to the high light under conditions of water deficit.