Sorghum brown midrib12 resistance to sugarcane aphids

Lignin is an important homogenous polymer present in the plant cell walls and its accumulation has been associated with pathogen attack, but this response has not been well studied in response to insect attack. Sugarcane aphid (SCA), a key pest of sorghum, feeds on plant sap, primarily from phloem, and impacts the yield adversely. We utilized the sorghum brown midrib (bmr) mutants impaired in lignin biosynthesis to understand sorghum defense mechanisms against SCA. The sorghum Bmr6 and Bmr12 genes encode for cinnamyl alcohol dehydrogenase (CAD) enzyme that catalyzes the conversion of hydroxycinnamoyl aldehydes to monolignols and caffeic acid O-methyltransferase (COMT) enzyme responsible for catalysis of penultimate step in lignin biosynthesis pathway, respectively. Aphid no-choice bioassays revealed that bmr12 plants provide enhanced resistance to SCA whereas no significant differences were observed in bmr6 plants. Monitoring of the aphid feeding behavior revealed that SCA spent more time in reaching the first sieve element phase on bmr12 plants compared with wild-type and Bmr12- overexpression (OE) plants. The transcriptomic and metabolomic analysis revealed that bmr12 resistance to SCA is independent of JA and SA pathways. Interestingly, bmr12 plants displayed altered auxin metabolism upon SCA infestation and specifically, elevated levels of IAA-Asp were observed in bmr12 plants.