An efficient and improved method for virus-induced gene silencing in sorghum

Dharmendra Kumar Singh, Hee-Kyung Lee, Ismail Dweikat, Kirankumar S. Mysore

Although the draft genome of sorghum is available, the understanding of gene function is limited due to the lack of extensive mutant resources. Virus-induced gene silencing (VIGS) is an alternative to mutant resources to study gene function. This study reports an improved and efficient method for Brome mosaic virus (BMV)-based VIGS in sorghum. Sorghum plants were rub-inoculated with sap prepared by grinding 2 g of infected Nicotiana benthamiana leaf in 1 ml 10 mM potassium phosphate buffer (pH 6.8) and 100 mg of carborundum abrasive. The sap was rubbed on two to three top leaves of sorghum. Inoculated plants were covered with a dome to maintain high humidity and kept in the dark for two days at 18 °C. Inoculated plants were then transferred to 18 °C growth chamber with 12 h/12 h light/dark cycle. This study shows that BMV infection rate can be significantly increased in sorghum by incubating plants at 18 °C. A substantial variation in BMV infection rate in sorghum genotypes/varieties was observed and BTx623 was the most susceptible. Ubiquitin (Ubiq) silencing is a better visual marker for VIGS in sorghum compared to other markers such as Magnesium Chelatase subunit H (ChlH) and Phytoene desaturase (PDS). The use of antisense strand of a gene in BMV was found to significantly increase the efficiency and extent of VIGS in sorghum. In situ hybridization experiments showed that the non-uniform silencing in sorghum is due to the uneven spread of the virus. This study further demonstrates that genes could also be silenced in the inflorescence of sorghum. In general, sorghum plants are difficult to infect with BMV and therefore recalcitrant to VIGS studies. However, by using BMV as a vector, a BMV susceptible sorghum variety, 18 °C for incubating plants, and antisense strand of the target gene fragment, efficient VIGS can still be achieved in sorghum.