Potential Roles of the GRF Transcription Factors in Sorghum Internodes during Post-Reproductive Stages

Growth-regulating factor (GRF) is a plant-specific family of transcription factors crucial for meristem development and plant growth. Sorghum (<i>Sorghum bicolor</i> L. Moench) is a cereal species widely used for food, feed and fuel. While sorghum stems are important biomass components, the regulation of stem development and the carbohydrate composition of the stem tissues remain largely unknown. Here, we identified 11 SbGRF-encoding genes and found the <i>SbGRF</i> expansion driven by whole-genome duplication events. By comparative analyses of <i>GRF</i>s between rice and sorghum, we demonstrated the divergence of whole-genome duplication (WGD)-derived <i>OsGRF</i>s and <i>SbGRF</i>s. A comparison of <i>SbGRF</i>s' expression profiles supports that the WGD-duplicated <i>OsGRFs</i> and <i>SbGRF</i>s experienced distinct evolutionary trajectories, possibly leading to diverged functions. RNA-seq analysis of the internode tissues identified several <i>SbGRF</i>s involved in internode elongation, maturation and cell wall metabolism. We constructed co-expression networks with the RNA-seq data of sorghum internodes. Network analysis discovered that <i>SbGRF1</i>, <i>5</i> and <i>7</i> could be involved in the down-regulation of the biosynthesis of cell wall components, while <i>SbGRF4</i>, <i>6</i>, <i>8</i> and <i>9</i> could be associated with the regulation of cell wall loosening, reassembly and/or starch biosynthesis. In summary, our genome-wide analysis of <i>SbGRF</i>s reveals the distinct evolutionary trajectories of WGD-derived <i>SbGRF</i> pairs. Importantly, expression analyses highlight previously unknown functions of several <i>SbGRF</i>s in internode elongation, maturation and the potential involvement in the metabolism of the cell wall and starch during post-anthesis stages.