1a. Objectives (from AD-416):
1. To develop transgenic lines overexpressing phenylpropanoid biosynthetic enzymes. 2. To determine the effects of modifications to lignin biosynthesis through bmr loci or transgenic overexpression on phloem-feeding and chewing insects of sorghum. 3. To determine the effects these lignin modifications have on fungi causing foliar or stalk diseases in sorghum. 4. To determine the alternations in metabolitic profiles of relevant lines with significantly increased resistance to either insects or fungi. 5. To determine the changes in global gene expression profiles of relevant lines with significantly increased resistance to either insects or fungi.
1b. Approach (from AD-416):
Altering lignin content and composition are targets to improve lignocellulosic biomass for bioenergy conversion processes, the affects of these alterations on plant-biotic interactions are unknown. We hypothesize that specific modification to the lignin biosynthetic pathway will alter the defense responses of host plants to insects and/or fungal pathogens. Our aim is to gain insight on the underlying causes that contribute to the altered defense responses. The sorghum genotypes we will use for the proposed experiments will carry three bmr mutants, which reduce lignin content and alter lignin composition. To increase phenolic compounds and lignin, we will utilize transgenic approaches whereby transgenic sorghum events will be generated that harbor cassettes for the over-expressing of PAL, C3H and CRR, three key enzymes monolignol biosynthesis along with a transcription factor strategy through expression of MYB68, a known regulator of lignin biosynthesis. These transgenic events should increase flux through the phenylpropanoid pathway thereby increasing phenolic subunits available for lignin biosynthesis. However, we are cognizant that these transgenic events may display altered phenylpropanoid metabolism and phenolic profiles, without a concomitant lignin content change. We will use the RTx430 background for these experiments, for which we currently have near-isogenic lines for bmr6 and bmr12.
3. Progress Report:
This research is aimed at determining responses of sorghum bmr lines to infestation by greenbugs, screening of sorghum bmr lines with altered lignin content in comparison to their counterparts (wild type). The experiments were conducted to assess the responses of the selected bmr lines to infestation by greenbug biotype E, a weak virulent biotype of greenbug. The counterparts of the bmr lines (i.e., non-bmr BTx623) and two greenbug-resistant lines were included as comparison. For evaluation of greenbug damage, a rating scale (1-10) was used, and scoring was conducted in short intervals. Based on the observation and data analysis, bmr mutants differentially responded to greenbug feeding. As a general trend, greenbugs caused more severe damage to sorghum seedlings of all bmr lines in comparison to those of the non-bmr lines, which led to a tentative conclusion that reduced lignin content in the plant cell wall of bmr sorghum has a negative effect on host plant defense against greenbug.