The Role of Brassinosteroids in the Development of Sorghum

Current day seed sorghum stocks contain up to fourdwarf genes that minimize the size of vegetative partsand increase harvest index. To date, only the d3 dwarfgene has been characterized molecularly. The geneencodes a homolog of the Arabidopsis and maize ABCtransporter ABCB1, which is associated with polartransport of the plant growth hormone auxin. Themechanism behind the other dwarf mutations is currentlyunknown. Brassinosteroids (BRs) have a profound effect ondevelopment of monocot species, as they are involved in celldivision, cell elongation, and sex determination. Mutationsin the biosynthesis or the reception of BRs result in shorter,stunted plants (dwarf). This project aims to determine therole of BRs in growth and development of sorghum.We developed a strategy to phenocopy a BR mutantby treating sorghum plants (Tx623) with the potentbrassinosteroid inhibitor propiconazole (PCZ). PCZis a chemical inhibitor of the rate-limiting step of BRbiosynthesis in plants. The compound is also usedcommercially as fungicide to treat turf grass diseases.After analyzing the growth retardation effect of PCZ, weperformed a phenotypic screen in a mutagenized sorghumcollection provided by Professors Clifford Weil andMitchell Tuinstra (Department of Agronomy).The results showed that the PCZ-treated plants hadreduced height (61% and 33% of control for 100μM PCZand 500μM PCZ, respectively) and tiller number (control= 4.6, 100μM PCZ = 3.3, 500μM PCZ = 1.4). Surprisingly,yield (8.5g = control, 19.3g = 100μM PCZ, 20.7g = 500μMPCZ) and harvest indices (seed yield/total biomass)(control = 0.09, 100μM PCZ = 0.31, 500μM PCZ = 0.38)were significantly greater in PCZ-treated plants.