Bikinin induces genotype- and time-dependent changes in barley growth, leaf architecture, and phytohormone profiles

Abstract Brassinosteroids are major regulators of plant growth, architecture, and stress responses, but the consequences of perturbing BR signaling may depend strongly on genotype and developmental stage. We investigated the effects of bikinin, an inhibitor of plant Glycogen Synthase Kinase 3-like kinases, on early development of two barley ( Hordeum vulgare L.) cultivars, Golden Promise (GP) and Haruna Nijo (HN). Seedlings were treated with 100 µM bikinin, and shoot and root growth, biomass, successive leaf development, lamina joint inclination (LJI), transcript abundance of HvGSK2.1 and HvBZR1 , and endogenous phytohormone profiles were assessed 2, 4, 6, 8, and 10 days after treatment. Bikinin induced pronounced genotype-, organ-, and time-dependent responses. In GP, shoot elongation increased from day 6 onward, whereas HN showed an earlier stimulation followed by attenuation of the response. Root biomass increased transiently in both genotypes but declined at later stages in HN. Bikinin strongly increased first- and second-leaf LJI and differentially affected the lengths of successive leaf blades and sheaths. These morphological responses were accompanied by transient increases in abscisic acid, dynamic changes in jasmonic acid, an overall decrease in indole-3-acetic acid, and changes in selected cytokinin forms, including trans-zeatin, cis-zeatin riboside, and isopentenyladenine. No consistent bikinin-induced changes in HvGSK2.1 or HvBZR1 transcript abundance were detected. Endogenous BR profiles showed descriptive genotype- and time-dependent variation but were not subjected to inferential testing. Overall, inhibition of GSK3-like kinases altered barley growth and hormone profiles without uniformly promoting biomass accumulation. The contrasting temporal responses of GP and HN demonstrate that genetic background and developmental stage are critical determinants of bikinin responsiveness.