Identification of brassinosteroid genes in Brachypodium distachyon

Brassinosteroids (BRs) are steroidal phytohormones that are involved in diverse physiological processes and affect many important traits, such as plant stature, stress tolerance, leaf angle, fertility, and grain filling. BR signaling and biosynthetic pathways have been studied in various plants, such as the model dicot Arabidopsis thaliana; however, relatively little is known about these pathways in monocots.

To characterize BR-related processes in the model grass Brachypodium distachyon, we studied the response of these plants to the specific BR biosynthesis inhibitor, propiconazole (Pcz). We found that treatments with Pcz produced a dwarf phenotype in B. distachyon seedlings, similar to that observed in Pcz-treated Arabidopsis plants and in characterized BR-deficient mutants. Through bioinformatics analysis, we identified a list of putative homologs of genes known to be involved in BR biosynthesis and signaling in Arabidopsis, such as DWF4, BR6OX2, CPD, BRI1, and BIN2. Evaluating the response of these genes to Pcz treatments revealed that candidates for BdDWF4, BR6OX2 and, CPD were under feedback regulation. In addition, Arabidopsis plants heterologously expressing BdDWF4 displayed tall statures and elongated petioles, as would be expected in plants with elevated levels of BRs. Moreover, heterologous expression of BdBIN2 in Arabidopsis resulted in dwarfism, suggesting that BdBIN2 functions as a negative regulator of BR signaling. However, the dwarf phenotypes of Arabidopsis bri1-5, a weak BRI1 mutant allele, were not complemented by overexpression of BdBRI1, indicating that BdBRI1 and BRI1 are not functionally equivalent.

We identified components of the BR biosynthetic and signaling pathways in Brachypodium, and provided examples of both similarities and differences in the BR biology of these two plants. Our results suggest a framework for understanding BR biology in monocot crop plants such as Zea mays (maize) and Oryza sativa (rice).