Molecular and Physiological Mechanisms Underlying Sorghum-Sugarcane Aphid Interactions

Sorghum (Sorghum bicolor) is one of the most important monocot crops cultivated worldwide and known for its versatility as a food, forage, and bioenergy crop. Sugarcane aphid (SCA; Melanaphis sacchari Zehntner) is considered a major threat to sorghum production, which severely damages the plant by sucking sap from leaves, thereby reducing its photosynthetic ability. Although several studies have been focused on identification of sorghum resistant/tolerant varieties to SCA, very little is known about how sorghum mounts direct defenses against SCA. In the current study, we screened a panel of sorghum nested association mapping (NAM) founder lines to identify and characterize sorghum resistance/tolerance mechanisms to SCA. We identified sorghum genotype SC35 as the aphid-tolerant line among the different sorghum genotypes that displayed minimal plant biomass loss and a robust photosynthetic machinery, despite supporting higher aphid population. Phytohormone analysis revealed significantly higher basal levels of 12-oxo-phytodienoic acid, a precursor in the jasmonic acid biosynthesis pathway, and increased levels of cytokinins in the sorghum SCA-tolerant SC35 plants. Next, we found SC265 genotype demonstrates a combination of antibiosis- and antixenosis-mediated resistance mechanisms to SCA. Feeding behavior studies using electrical penetration graph (EPG) technique revealed that SCA spent less time in the sieve element phase and more time in the pathway phase of SC265, compared to reference line, RTx430 and susceptible line, SC1345. Furthermore, phytohormonal analysis suggests that the higher constitutive and SCA feeding-induced levels of salicylic acid in SC265 can provide defenses against aphids. Proteome profiling of SC265 after SCA infestation at days 1 and 7 revealed the suppression of plant defense-related proteins and upregulation of plant defense and signaling-related proteins, respectively. In addition, we utilized the sorghum brown midrib (bmr) mutants impaired in lignin biosynthesis to understand the defense mechanisms against SCA. We found that bmr12 impaired in caffeic acid O‐methyltransferase (COMT) activity, involved in the synthesis of S-lignin subunits, provides resistance to SCA. Results from this work will help us to better understand the sorghum defense signaling mechanisms against phloem-feeding insects