Impacts of lignin modification in sorghum resistance to aphids

Sorghum, a major cereal crop grown worldwide for food, feed, and/or fuel, is attacked by sugarcane aphid (SCA; Melanaphis sacchari), a devastating pest of sorghum in the United States. Lignin, a complex heterogenous polymer present in nearly all plant cell walls, plays a critical role in protecting plants from biotic stresses. However, little is known about how lignin modifications in sorghum will affect the plant resistance to aphids. The interaction between sorghum and SCA has revealed a role for the sorghum Brown midrib12 (Bmr12) gene, which encodes caffeic acid O-methyltransferase (COMT) enzyme responsible for catalysis of the penultimate step in the lignin biosynthesis pathway, in sorghum defense against aphids. In no-choice assays, we have found loss of Bmr12 function and overexpression of Bmr12 (Bmr12 OE) resulted in increased resistance and susceptibility to SCA, respectively, as compared with the wild-type (WT) plants. Choice bioassays revealed that significantly lower number of SCA settled on bmr12 plants compared to the WT plants, demonstrating antixenotic-mediated resistance to SCA. Monitoring of aphid feeding behavior using electrical penetration graph (EPG) technique revealed that the SCA spent a significantly longer time in reaching the phloem tissue on bmr12 plants compared to the WT plants. Thus, our results suggest that Bmr12 plays a major role in sorghum defense against SCA. Future efforts will focus on elucidating the underlying mechanisms of the Bmr12 gene and lignin modification in sorghum defenses against SCA.