Identification and expression analysis of Sorghum bicolor gibberellin oxidase genes with varied gibberellin levels involved in regulation of stem biomass

Sweet sorghum (Sorghum bicolor (L.) Moench) is globally evaluated as a key feedstock for bioethanol production due to its high biomass yield. In the present study, the stem biomass characteristics of the sweet sorghum Yajin 1 were revealed and compared with the grain sorghum Aikang 8 at different growth stages. Dynamic analysis of lignocellulosic composition in stems of Yajin 1 and Aikang 8 showed that cellulose content increased and lignin content decreased in the sweet sorghum over development and thus led to a higher saccharification efficiency, which was opposite in the grain sorghum. These varied biomass-associated traits were suggested to be related to changes of bioactive gibberellins GAl and GA4 levels in young leaves and stems at different growth stages. To investigate the role of gibberellin (GA) metabolism involved in the regulation of stem biomass accumulation and composition in sorghums, a large scale spatio-temporal expression analysis was carried out on the 9 sorghum gibberellin oxidase (SbGAox) genes that encode three classes of key enzymes in the GA biosynthesis. Most of the SbGAox displayed cultivar-, organ-, or stage-preferential expression patterns. Among them, SbGA20ox1 was more specifically expressed in the stem of the sweet sorghum, which was positively related to the stem biomass accumulation and composition, and bioactive GA levels. SbGA2ox1 was more specifically expressed in the grain sorghum than in the sweet sorghum, which was negatively related to the stem biomass accumulation and composition, and bioactive GA levels. As such, these two SbGAox genes may play opposite regulating roles in sorghum stem biomass accumulation and composition by controlling the bioactive GA levels. This study provides a comprehensive understanding of the SbGAox gene family in sorghums, offering a valuable resource to develop strategies for genetic improvement of sorghum biomass traits.