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 abiotic stresses (especially to those associated with water deficit), is abscisic acid.

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 (Polanská et al., Journal of Experimental Botany 2007). 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 as well as biotic stress responses (Havlová et al., Plant Cell Environment 2008; Dobrá et al., Journal of Plant Physiology 2010; Kosová et al., Journal of Plant Physiology 2012; Vaňková et al., Environmental Experimental Botany 2014; Dobrá et al., Plant Science 2015; Gaudinova et al., Physiological and Molecular Plant Pathology 2009).

Using mass spectrometry we found that tobacco plants respond to water deficit by the establishment of active 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 (before stress initiation) might delay stimulation of defence mechanisms, namely increase of abscisic acid and xanthophyll cycle pigments (Haisel et al., Biologia Plantarum 2008). By determination of SAG12 expression we proved that recovery after drought stress is an active process and not only the return to control conditions (Vaňková et al., Plant Signaling and Behaviour 2012).

Regulation of pool of active cytokinins under drought stress

Regulation of pool of active cytokinins under drought stress.

Hormonal as well as transcriptome changes observed in the early phase of heat stress, i.e. transient elevation of cytokinins and simultaneous down-regulation of abscisic acid, indicated short-term stomata opening (Dobrá et al., J. Plant Physiol. 2010). As abiotic stresses occur in nature usually in combination, we compared heat stress response of well watered and drought stressed plants. High increase of the heat stress severity under the conditions of water deficit confirmed the importance of enhanced transpiration for cooling of the leaf surface, at least during the early stage of the response. Monitoring of stomata aperture upon transfer of tobacco plants into elevated temperature confirmed assumed dynamics of stomata movement (Macková et al., J. Exp. Bot. 2013). Targeting of the heat stress to Arabidopsis shoots, roots or to the whole plant revealed fast communication within the plant, associated with transient up-regulation of cytokinin signalling in the non-exposed tissue (Dobrá et al., Plant Science 2015). Transcription of individual antioxidant enzymes was followed during heat and/or drought stress (Lubovská et al., J. Plant Physiol. 2014). During adverse conditions, transcription of chloroplast antioxidant enzymes was suppressed (simultaneously with the decrease of photosynthesis), while the transcription of cytoplasmic ones was enhanced under stress conditions. Redox control of plant development and stress responses was summarized in Kocsy et al., Plant Sci. 2013.

Dynamics of hormonal changes under heat stress

Dynamics of hormonal changes under heat stress.

Proline represents an important osmolyte, which has protective role especially during the drought stress. When the drought and/or heat stress responses were compared in plants with constitutively elevated proline content and the corresponding wild-type, mild increase of stress tolerance was observed, indicating that abiotic stress tolerance is a complex trait (Dobrá et al., J. Plant Physiol. 2010). Determination of expression profiles of genes for proline biosynthesis and degradation revealed differential regulation of proline content in shoots and roots during the stress progression and subsequent recovery (Dobrá et al., Journal of Plant Physiology 2011).

In cooperation with RIKEN Plant Science Centre, Yokohama, Japan (Prof. Lam-Son Tran) we characterized drought and salt responses of Arabidopsis plants constitutively over-expressing cytokinin oxidase/dehydrogenase gene (Nishiyama et al., Plant Cell 2011). The data showed that down-regulation of cytokinin levels confers enhanced drought and salinity tolerance, being associated with lower basal ABA levels, but increased ABA sensitivity. The comparison of the impact of down-regulation of cytokinin levels by over-expression of cytokinin oxidase/dehydrogenase gene targeted either to roots (WRKY6 promoter) or constitutively (35S promoter) showed that changed morphology and slow shoot growth rate played a decisive role in the drought tolerance of the constitutive transformant (Macková et al., Journal of Experimental Botany 2013, in cooperation with Free University Berlin, Germany, Prof. Thomas Schmülling). Our experience in evaluation of cytokinin roles in abiotic stress responses was summarized in a review (Ha et al., Trends in Plant Science 2012).

Together with Tran's group we followed cytokinin metabolism in various soybean tissues at different developmental stages under control and drought conditions. Cytokinin content as well as expression of cytokinin metabolic genes showed that reduction of cytokinin levels belonged to major events under abiotic stresses (Le et al., PLoS ONE 2012). In cooperation with AgroBioInstitute, Sofia, Bulgary (Dr. Dimitar Djilianov) we followed the hormonal dynamics during desiccation and recovery of resurrection species Haberlea rhodopensis and revealed important role of jasmonic acid during dehydration (Djilianov et al., J. Plant Growth Regul. 2013).

We intensively collaborate with Departments of Genetics and Plant Physiology, Agricultural Research Institute, HAS, Martonvásár, Hungary (Dr. Gábor Galiba and Dr. Tibor Janda and coworkers) on hormonal regulations of cold stress resistance of wheat cultivars and Arabidopsis (Majláth et al., J. Plant Physiol. 2011 and Physiol. Plant. 2012). Dynamics of hormonal levels and proteome were characterized during individual phases of cold stress in winter and spring wheat (Kosová et al., J. Plant Physiol. 2012 and J. Proteome Res. 2013). Hormonal changes during long-term cold response of two Triticum monococcum lines were elucidated by Vaňková et al., Environ. Exp. Bot. 2014. In the study of responses to extreme temperatures, we further collaborate with Faculty of Agronomy, Mendel University, Brno (Prof. Břetislav Brzobohatý and coworkers) and Institute of Crop Research, Prague (Dr. I. T. Prášil and coworkers).

In cooperation with Institute of Horticulture, Volcani Center, Bet Dagan, Israel (Prof. Etti Or) we studied functions of cytokinins in grapevine berry initiation, which includes stimulation of the expression of TFL1 (homologue of terminal flower 1), extending the branching period by delaying acquisition of floral meristem (Crane et al., Planta 2012).

We were involved in the characterization of SE7 somaclonal line of finger millet (Eleusine coracana). More branched phenotype and increased grain yield of this line resulted from attenuated expression of cytokinin oxidase/dehydrogenase genes and subsequent elevation of active cytokinins that stimulated meristematic activity (Radchuk et al., J. Exp. Bot. 2012).

The effects of abscisic acid on the transcription of chloroplast genes were described in barley (Yamburenko et al., J. Exp. Bot. 2013), in cooperation with Humboldt University, Berlin, Germany, Prof. Thomas Bőrner).

In cooperation with Bar-Ilan University, Ramat-Gan, Israel (Prof. Gad Miller) we characterized the role of ascorbate peroxidase 6 in protection of Arabidopsis desiccating seeds and in the cross-talk between ROS, abscisic acid and auxin (Chen et al., Plant Physiol. 2014).

Apart from the study of the role of plant hormones during abiotic stress responses, we are interested also in mechanisms of defence to biotic stresses. We found that Blackcurrant reversion virus (BRV) infection in flowers of white and red currant was associated with flower malformation (full blossom) caused by elevation of active cytokinins at the early flower stage (Gaudinová et al., Physiol. Mol. Plant Pathol. 2009). In cooperation with University of Natural Resources and Life Sciences, Tulln, Austria (Prof. Krzysztof Wieczorek and Dr. Nina Kammerhofer) we characterized hormonal changes accompanying infection of Arabidopsis roots with nematode (Kammerhofer et al., New Phytol. 2015). The importance of cytokinins in the response to wounding and herbivore attack was evaluated (Schäfer et al., J. Integrative Plant Biol., 2015) in cooperation with Max Planck Institute for Chemical Ecology, Jena, Germany (Prof. Martin Schäfer and Dr. Stefan Meldau).

The aim of our work has been characterization of the role of individual plant hormones (especially of cytokinins, auxin and abscisic acid) in the response to abiotic and biotic stresses, evaluation of their intensive cross-talk and elucidation of the underlying mechanisms as well as elucidation of cytokinin roles in plant development.