Submitted to: mBio
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/9/2021
Publication Date: 2/16/2021
Citation: Plant, S.R.; Irigoyen, S.; Liu, J.; Bedre, R.H.; Christensen, S.A.; Schmelz, E.A.; Sedbrook, J.; Scholthof, K.G.; Mandadi, K.K. 2021. Brachypodium phenylalanine ammonia lyase (PAL) promotes antiviral defenses against Panicum mosaic virus and its satellites. mBio. 12:e03518-20. https//doi.org/10.1128/mBio.03518-20
DOI: https://doi.org/10.1128/mBio.03518-20

Interpretive Summary: More than 50% of the world’s calories come from monocot crop grasses such as corn, rice, wheat, and sugar cane. Brachypodium distachyon (Brachypodium) is an increasingly important model plant for the study and understanding of monocot biology including how grass crops can better protect themselves from devastating diseases. In this study, USDA-ARS Scientists at the Center for Medical, Agricultural and Veterinary Entomology in Gainesville, FL, in collaboration with Texas A&M University discovered that a Brachypodium defense gene (Phenylalanine Ammonia Lyase – PAL) provides resistance to Panicum mosaic virus (PMV) and its satellite virus (SPMV), important pathogens of bioenergy and food related grasses. The knowledge generated from this study will contribute to molecular breeding strategies that will benefit farmers of grass crops by promoting natural resistance to virus pathogens.

Technical Abstract: Brachypodium distachyon (Brachypodium) has recently emerged as a premier model plant for monocot biology. We previously reported genome-wide transcriptomic and alternative splicing changes occurring in Brachypodium during compatible infection with Panicum mosaic virus (PMV) and its satellite virus (SPMV), an important pathogen of bioenergy and food related grasses. Here, we dissected the role of Brachypodium PHENYLALANINE AMMONIA LYASE1 (PAL1), a key enzyme in phenylpropanoid and salicylic acid (SA) biosynthesis, and in induced plant defenses. Metabolic profiling of PMV and SPMV infected Brachypodium using GC-MS revealed enhanced levels of multiple defense-related hormones and metabolites such as lignin, cinnamic acid, salicylic acid, and fatty-acid precursors during disease progression. The virus-induced accumulation of SA and lignin were significantly suppressed upon knock down of BdPAL1 using RNA-interference (RNAi). The compromised SA level in PMV and SPMV infected BdPAL1 RNAi plants also resulted in weaker induction of multiple SA-related defense gene markers (PATHOGENESIS RELATED [PR]-1, PR-3, PR-5, and WRKY75) and enhanced susceptibility to PMV and SPMV, when compared to wild-type (WT) plants. Furthermore, exogenous application of SA alleviated the PMV and SPMV necrotic disease phenotypes and delayed plant death caused by PMV and SPMV. Together, these results support an antiviral role for BdPAL1 and the various defense metabolites triggered during compatible interaction, perhaps as a last resort attempt to rescue the infected plant.