Plant hormones through signaling networks mutually regulate several signaling and metabolic systems essential for both plant development and plant responses to different environmental stresses. Extensive research has enabled the main effects of all known phytohormones classes to be identified. Therefore, it is now possible to investigate the interesting topic of plant hormonal crosstalk more fully. In this review, we focus on the role of brassinosteroids and ethylene during plant growth and development especially flowering, ripening of fruits, apical hook development, and root and shoot growth. As well as it summarizes their interaction during various abiotic stress conditions.
Plant response to osmotic stress is a complex issue and includes a wide range of physiological and biochemical processes. Extensive studies of known cultivars and their reaction to drought or salinity stress are very important for future breeding of new and tolerant cultivars. Our study focused on the antioxidant activity, accumulations of osmotica, and the content of abscisic acid in apple (cv. “Malinové holovouské”, “Fragrance”, “Rubinstep”, “Idared”, “Car Alexander”) and cherry (cv. “Regina”, “Napoleonova”, “Kaštánka”, “Sunburst”, “P-HL-C”) cultivated in vitro on media containing different levels of polyethylene glycol PEG-6000. Our results indicated that the studied genotypes responded differently to osmotic stress manifested as reduction in the leaf relative water content (RWC) and increment in the activities of antioxidant enzymes, proline, sugars, and abscisic acid content. Overall, cherry cultivars showed a smaller decrease in percentage RWC and enzymatic activities, but enhanced proline content compared to the apple plants cultivars. Cultivars “Rubinstep”, “Napoleonova”, and “Kaštánka” exhibited higher antioxidant capacity and accumulation of osmoprotectants like proline and sorbitol that can be associated with the drought-tolerance system.
We have recently discovered that brassinosteroids (BRs) can inhibit the growth of etiolated pea seedlings dose-dependently in a similar manner to the ‘triple response’ induced by ethylene. We demonstrate here that the growth inhibition of etiolated pea shoots strongly correlates with increases in ethylene production, which also responds dose-dependently to applied BRs. We assessed the biological activities of two natural BRs on pea seedlings, which are excellent material as they grow rapidly, and respond both linearly and uni-phasically to applied BRs. We then compared the BRs’ inhibitory effects on growth, and induction of ethylene and ACC (1-aminocyclopropane-1-carboxylic acid) production, to those of representatives of other phytohormone classes (cytokinins, auxins, and gibberellins). Auxin induced ca. 50-fold weaker responses in etiolated pea seedlings than brassinolide, and the other phytohormones induced much weaker (or opposite) responses. Following the optimization of conditions for determining ethylene production after BR treatment, we found a positive correlation between BR bioactivity and ethylene production. Finally, we optimized conditions for pea growth responses and developed a new, highly sensitive, and convenient bioassay for BR activity.
Brassinosteroid hormones are indispensable for root growth and they control both cell division and cell elongation through the establishment of an increasing signaling gradient along the longitudinal root axis. Because of their limited mobility, the importance of brassinosteroid distribution for achieving the signaling maximum is largely overlooked. Expression pattern analysis of all known brassinosteroid biosynthetic enzymes revealed that not all cells in the Arabidopsis thaliana root possess full biosynthetic machinery and completion of biosynthesis relies on cell-to-cell movement of the hormone precursors. We demonstrate that brassinosteroid biosynthesis is largely restricted to the root elongation zone where it overlaps with brassinosteroid signaling maxima. Moreover, optimal root growth requires hormone concentrations, low in the meristem and high in the root elongation zone attributable to an increased biosynthesis. Our finding that spatiotemporal regulation of hormone synthesis results in a local hormone accumulation provides a paradigm for hormone-driven organ growth in the absence of long-distance hormone transport in plants.
Drought stress is a serious threat. Therefore, improvements in crop productivity under conditions of limited water availability are vital to keep global food security. Apples and cherries belong to the most produced fruit worldwide. Thus, searching for their tolerant or resistant cultivars is beneficial for crop breeders to produce more resistant plants. We studied five apple (“Malinové holovouské”, “Fragrance”, “Rubinstep”, “Idared”, “Car Alexander”) and five cherry (“Regina”, “Napoleonova”, “Kaštánka”, “Sunburst”, “P-HL-C”) cultivars for their adaptation in response to progressive drought stress. The reaction of an in vitro culture to osmotic stress simulated by increasing polyethylene glycol (PEG) concentration in medium was evaluated through the morphological (fresh and dry weight, water content, leaf area), physiological (chlorophyll and carotenoids content), and biochemical (reactive oxygen species and malondialdehyde content) parameters. Drought-like stress negatively affected the water content, leaf areas, and chlorophyll content in both fruit species. Oxidative status and membrane damage of plants under water deficiency conditions occurred to be important indicators of stress tolerance mechanism. Cherries exhibited higher hydrogen peroxide levels compared to apples, whereas their malondialdehyde values were generally lower. The overall results indicated wide tolerance range to water deficit among apple and cherry in vitro culture as well as among cultivars within single plant species.
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