Abstract
Brassinosteroids (BRs) are a class of steroid phytohormones, which regulate various processes of morphogenesis and physiology—from seed development to regulation of flowering and senescence. An accumulating body of evidence indicates that BRs take part in regulation of physiological reactions to various stress conditions, including drought. Many of the physiological functions of BRs are regulated by a complicated, and not fully elucidated network of interactions with metabolic pathways of other phytohormones. Therefore, the aim of this study was to characterize phytohormonal homeostasis in barley (Hordeum vulgare) in reaction to drought and validate role of BRs in regulation of this process. Material of this study included the barley cultivar “Bowman” and five Near-Isogenic Lines (NILs) representing characterized semi-dwarf mutants of several genes encoding enzymes participating in BR biosynthesis and signaling. Analysis of endogenous BRs concentrations in these NILs confirmed that their phenotypes result from abnormalities in BR metabolism. In general, concentrations of 18 compounds, representing various classes of phytohormones, including brassinosteroids, auxins, cytokinins, gibberellins, abscisic acid, salicylic acid and jasmonic acid were analyzed under control and drought conditions in the “Bowman” cultivar and the BR-deficient NILs. Drought induced a significant increase in accumulation of the biologically active form of BRs—castasterone in all analyzed genotypes. Another biologically active form of BRs—24-epi-brassinolide—was identified in one, BR-insensitive NIL under normal condition, but its accumulation was drought-induced in all analyzed genotypes. Analysis of concentration profiles of several compounds representing gibberellins allowed an insight into the BR-dependent regulation of gibberellin biosynthesis. The concentration of the gibberellic acid GA7 was significantly lower in all NILs when compared with the “Bowman” cultivar, indicating that GA7 biosynthesis represents an enzymatic step at which the stimulating effect of BRs on gibberellin biosynthesis occurs. Moreover, the accumulation of GA7 is significantly induced by drought in all the genotypes. Biosynthesis of jasmonic acid is also a BR-dependent process, as all the NILs accumulated much lower concentrations of this hormone when compared with the “Bowman” cultivar under normal condition, however the accumulation of jasmonic acid, abscisic acid and salicylic acid were significantly stimulated by drought.
Highlights
Brassinosteroids (BRs) are a class of polyhydroxylated steroid phytohormones, which regulate various processes of plant morphogenesis and physiology—from seed development and germination up to regulation of flowering and senescence (Choudhary et al, 2012; Vriet et al, 2012)
The endogenous concentrations of abscisic acid (ABA) increased at least seven times in reaction to drought in all analyzed genotypes, no tendency was observed between the BR-deficient, BR-insensitive Near-Isogenic Lines (NILs) and the “Bowman” cultivar, when a relative increase in the ABA content with regard to the control value was calculated for each genotype. These results indicate that perturbations in BR biosynthesis or BR signaling do not affect the endogenous ABA accumulation under the control condition, and do not influence the capacity of the barley BR mutants to react to drought with such a significant increase in ABA accumulation
It should be kept in mind that CS is suggested as an end product of the BR biosynthesis pathway in monocots, representing the highest biological activity (Kim et al, 2008)
Summary
Brassinosteroids (BRs) are a class of polyhydroxylated steroid phytohormones, which regulate various processes of plant morphogenesis and physiology—from seed development and germination up to regulation of flowering and senescence (Choudhary et al, 2012; Vriet et al, 2012). An accumulating body of evidence indicates that the regulation of such vast aspects of plant physiology is mediated through a network of molecular interactions between BR metabolism and signalosomes of other phytohormones (Nemhauser et al, 2006; Hansen et al, 2009; Domagalska et al, 2010; Chung et al, 2011; Gruszka, 2013). Studies on BR metabolism, which have been conducted for the last 30 years mainly in the model plant species Arabidopsis thaliana, led to identification of numerous components of BR biosynthesis and signaling pathways (Vriet et al, 2012), the regulation and the course of these processes are far less understood in crop species, including barley. Research on physiological reaction of the barley semi-dwarf BR mutants to drought stress has been conducted only recently. Mutants deficient in BR metabolism are very valuable tools to study the mechanism of action of these hormones in response to stress conditions (Janeczko et al, 2016)
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