Transactivation of SuSy1 by SbYABBY8-SbWRKY23 module promotes sucrose accumulation in sweet sorghum stalks

In addition to grains, sweet sorghum ( Sorghum bicolor L.) generally possesses juicy stalk with relatively higher sugar content, serving as important raw materials for bioethanol production. Sucrose synthase (SuSy) presents powerful biological functions in sucrose metabolism, starch biosynthesis, and cellulose and callose biosynthesis. Five genes of SbSuSy1 - SbSuSy5 are documented to encode SuSys in sorghum, among which SbSuSy1 exhibits relatively higher expression levels within the tissues of stalks. In the present study, a fragment affecting the promoter activity of SbSuSy1 was identified, and a transcription factor (TF) of SbYABBY8 was attained by screening the yeast library. SbYABBY8 encodes a protein of YABBY family that locates in the nucleus without self-activating activities in yeast. Electrophoretic Mobility Shift Assay (EMSA) confirmed that SbYABBY8 could bind to the fragment containing 5’-TAACTAA-3’ motif in promoter of SbSuSy1 , significantly enhancing the promoter activity. Furthermore, another WRKY family TF of SbWRKY23 was identified to interact with SbYABBY8, jointly promoting the promoter activity of SbSuSy1 with statistically increased efficiency. In summary, results of this study indicate that SbYABBY8-SbWRKY23 is a novel module in the transcriptional regulation of SbSuSy1 , playing potential and important roles in the sucrose accumulation in the stalk of sweet sorghum. Beyond grains, sweet sorghum generally possesses juicy and sugary stalk with relatively higher sugar content, serving as important raw materials for bioethanol production. Sucrose synthase (SuSy) presents powerful biological functions for the metabolism and accumulation of sucrose in plants. With the sweet characterization of sweet and non-sweet sorghum accessions, and the expression patter profiling of five SuSy genes in sorghum, i.e., SbSuSy1 to SbSuSy5 , SbSuSy1 was detected to highly express in the stalk, potential contributing to the higher sugar and sucrose contents in the stalk of sweet sorghum. A YABBY family transcription factor (TF) of SbYABBY8 was identified and verified to significantly activate the promoter activity and transcription of SbSuSy1 . Furthermore, a WRKY family TF of SbWRKY23 was screened out to interact with SbYABBY8 to enhance its transactivating efficiency. These findings integrate an unreported module of SbYABBY8-SbWRKY23 in transactivating of SbSuSy1 , implying novel molecular insights into the biosynthesis and accumulations of sucrose and fructose in the stalk of sweet sorghum. • SbSuSy1 was profiles to highly express in stalk, potential contributing to the sugary performance of sweet sorghum stalk. • SbYABBY8 was identified and verified to significantly activate the promoter activity and transcription of SbSuSy1 . • A module of SbYABBY8-SbWRKY23 was dissected to transactivate SbSuSy1 for the sugary juicy performance in sweet sorghum stalk.