Plant Hormones Functions in Ontogenesis and Interactions with the Environment
Development of plants as well as their interactions with environment are regulated by plant hormones. Each phytohormone affects a range of physiological processes and vice versa each process is regulated (in positive or negative mode) by several hormones. Hormones mediate both fast responses (e.g. stomata closure during water deficit) and long-term adaptations, associated with modulation of growth and development. Positive regulators of cell division and growth are predominantly cytokinins and auxins. Negative growth regulator, which plays a decisive role during the seed development as well as in the response to stresses (especially to those associated with water deficit), is abscisic acid.
Regulation of pool of active cytokinins under drought stress.
Temporal and spatial regulation of the levels of physiologically active hormones is very strict (Dobrev et al., Plant Physiol. Biochem. 2002, Nováková et al., J. Exp. Bot. 2005), both at the level of tissues (e.g. developing meristems) and individual cell compartments. Our Laboratory has been involved in the study of hormone cross-talk (Vaňková et al. in Physiology and Biochemistry of Cytokinins 1992), mechanisms involved in the regulation of the levels of physiologically active forms (Veach et al., Plant Physiol. 2003, Blagoeva et al., Physiol. Plant. 2004, Mok et al., Plant Physiol. 2005), elucidation of their role during ontogenesis (Rodo et al., J. Exp. Bot. 2008) and abiotic stress response (Havlová et al., Plant Cell Environment 2008).
Using mass spectrometry we found that tobacco plants respond to water deficit by the establishment of bioactive cytokinin gradient in favour of upper leaves. Elevated cytokinin content enhanced the sink strength of the upper leaves, which was important for their preferential protection. Simultaneously, cytokinins and auxins were accumulated in drought-stressed roots, which seemed to affect positively the primary root growth, crucial for the access to underground water in deeper soil layers (Havlová et al., Plant Cell Environment 2008). Comparison of transgenic tobacco plants with constitutively enhanced cytokinin turn-over and plants with cytokinin elevation driven by senescence induced promoter revealed that enhanced cytokinin content (already before stress initiation) might delay stimulation of defence mechanisms, namely increase of abscisic acid and xanthophyl cycle pigments (Haisel et al., Biol. Plant. 2008). Taking into account that hormone levels are strongly affected not only by the stress strength and duration, but also by actual physiological state of plant, we characterized the stress dynamics (using quantitative RT PCR) via the transcription profiles of genes associated with the water stress (NtERD10B coding for dehydrine, CIG1, PDH1 and PDH1 for proline dehydrogenases and CSP41a coding the protein regulating turn-over of chloroplast transcriptome, Dobrá et al., submitted).
Comparison of the effect of modulation of cytokinin metabolism in cytoplasm on their content per whole leaf basis and in isolated chloroplasts revealed substantial autonomy of chloroplasts. These organelles, very important in photosynthesis and thus also in the stress response, have substantial capacity for maintenance of cytokinin homeostasis (Polanská et al., J. Exp. Bot. 2007).
The role of cytokinins in sex-specific floral development was followed in maize, using transformants over-expressing gene for trans-zeatin O-glucosyltransferase. Constitutive expression of this gene resulted in the suppression of male flower development coinciding with the delay of leaf senescence (Rodo et al., J. Exp. Bot. 2008). Positive correlation between enhanced cytokinin content and delay in the senescence of both leaves and flowers of petunia and chrysanthemum indicated potential utilization of hormonal regulations in practice (Khodakovskaya et al., J. Exp. Bot. 2005).
As abiotic stresses occur in nature usually in combination, we have focused at present on characterization of hormonal responses to heat stress and to the combination of heat and drought stress.