Alteration in Lignin Biosynthesis Restricts Growth of <i>Fusarium</i> spp. in Brown Midrib Sorghum

Funnell-Harris, D. L., Pedersen, J. F., and Sattler, S. E. 2010. Alteration in lignin biosynthesis restricts growth of Fusarium spp. in brown midrib sorghum. Phytopathology 100:671-681. To improve sorghum for bioenergy and forage uses, brown midrib (bmr)6 and -12 near-isogenic genotypes were developed in different sorghum backgrounds. The bmr6 and bmr12 grain had significantly reduced colonization by members of the Gibberella fujikuroi species complex compared with the wild type, as detected on two semiselective media. Fusarium spp. were identified using sequence analysis of a portion of the translation elongation factor (TEF) 1-α gene. The pathogens Fusarium thapsinum, F. proliferatum, and F. verticillioides, G. fujikuroi members, were commonly recovered. Other frequently isolated Fusarium spp. likely colonize sorghum asymptomatically. The χ2 analyses showed that the ratios of Fusarium spp. colonizing bmr12 grain were significantly different from the wild type, indicating that bmr12 affects colonization by Fusarium spp. One F. incarnatum-F. equiseti species complex (FIESC) genotype, commonly isolated from wild-type and bmr6 grain, was not detected in bmr12 grain. Phylogenetic analysis suggested that this FIESC genotype represents a previously unreported TEF haplotype. When peduncles of wild-type and near-isogenic bmr plants were inoculated with F. thapsinum, F. verticillioides, or Alternaria alternata, the resulting mean lesion lengths were significantly reduced relative to the wild type in one or both bmr mutants. This indicates that impairing lignin biosynthesis results in reduced colonization by Fusarium spp. and A. alternata. Additional keywords: caffeic acid O-methyltransferase, cinnamyl alcohol dehydrogenase, Fusarium equiseti. Sorghum (Sorghum bicolor (L.) Moench) is a valuable crop due to its adaptability to drought-stressed conditions (16). Recently, interest in sorghum grain and biomass has been piqued because of their usability as feed stock for bioenergy industries (54). Genetic modification to reduce lignin content in sorghum and other potential bioenergy crops has promise for greater utilization by increasing biomass digestibility (32). However, lignin is important for structural support and water transport within the plant (7). Additionally, lignin is involved in defense against pathogens and insects by providing a physical barrier (18,62), and lignin biosynthesis is induced in response to biotic stresses (26). Two different mutated genes in the sorghum lignin biosynthesis pathway have allowed the development of forage and grain lines with significantly reduced lignin content (46). These mutations, brown midrib (bmr)6 and bmr12, result in the phenotype of reddish-brown leaf midveins which are associated with reduced lignin content and modified lignin composition (46,48). Bmr6 encodes cinnamyl alcohol dehydrogenase (CAD) (53,57) and Bmr12 encodes caffeic acid O-methyltransferase (COMT) (5). CAD converts the aldehyde group of monolignols to an alcohol for incorporation into the lignin polymer, while COMT methylates the 5-hydroxyl group of the hydroxyl coniferyl substrates forming sinapyl groups (24). Both bmr6 and bmr12 are caused by nonsense mutations that result in the absence of detectable gene products (5,48,57). When bmr6, bmr12, and near-isogenic wild-type lines were grown at two locations in field trials, density of bmr6 grain was reduced (46), which could be caused by infection by grainmolding pathogens (34). However, when some of this same grain was screened, results indicated that bmr genotypes could have significantly fewer colonies of Fusarium spp. per seed (20). Among species identified using morphological characters were Fusarium thapsinum (formerly known as F. moniliforme), F. proliferatum, and F. subglutinans, members of the species complex Gibberella fujikuroi (31,44). Greenhouse inoculation of peduncles of bmr6 and bmr12 plants with F. thapsinum resulted in mean lesion lengths significantly smaller than those on nearisogenic wild-type plants (20). When considering genetic background, one or both low-lignin lines had significantly smaller mean lesions as a result of inoculation with F. thapsinum in four of six genetic backgrounds. Therefore, this research indicated that bmr resulted in reduced colonization by Fusarium spp. but genetic background influenced the interaction with low-lignin plants (20). Several Fusarium spp. in the G. fujikuroi species complex are potentially pathogenic on sorghum (31). Infections of stalks or roots can reduce yield and grain quality while lodging can make harvest difficult (6). Infections of panicles can result in grain of reduced quality and usability (36). F. thapsinum and other Fusarium spp., including those considered nonpathogenic, are able to colonize sorghum without producing symptoms (20,21, 30). Many Fusarium spp. produce mycotoxins (31,41). Accumulation of these metabolites are often exacerbated when grain is stored or stover is ensiled (1,13). Deterioration by grain-molding fungi and contamination by mycotoxins may be a particular problem in cultures in which sorghum is used in foods and beverages as well as for livestock feed, and grain is stored in traditional containers on rural farms (14,55). Long-term exposure to mycotoxins Corresponding author: D. L. Funnell-Harris; E-mail address: [email protected] doi:10.1094 / PHYTO-100-7-0671 This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological