Response of Fusarium thapsinum to Sorghum brown midrib Lines and to Phenolic Metabolites

Funnell-Harris, D. L. , Sattler, S. E. , and Pedersen, J. F. 2014. Response of Fusarium thapsinum to sorghum brown midrib lines and to phenolic metabolites. Plant Dis. 98:1300-1308. Sorghum lines were bred for reduced lignin for cellulosic bioenergy uses, through the incorporation of brown midrib (bmr)6 or -12 into two backgrounds (RTx430 and Wheatland) as either single or doublemutant lines. When these lines were assessed for resistance to Fusarium thapsinum stalk rot, a cause of lodging, they were as resistant to F. thapsinum as the near-isogenic wild type. Peduncles of newly identified bmr lines from an ethyl-methanesulfonate-mutagenized population, inoculated with F. thapsinum, were as resistant as the wild-type line, BTx623. One bmr line (1107) had significantly smaller mean lesion lengths than BTx623, suggesting that a mutation is associated with reduced susceptibility. Growing F. thapsinum on medium with ferulic, vanillic, sinapic, syringic, and caffeic acids (phenolic compounds derived from the lignin pathway and elevated in different bmr lines) indicated that F. thapsinum was tolerant to these compounds. When eight other sorghum fungi were tested for response to the presence of these compounds, ferulic acid inhibited these fungi. Most of the phenolics inhibited F. verticillioides and F. proliferatum. Accumulation of phenolic metabolites in bmr plants may inhibit growth of some sorghum pathogens, while other factors such as aromatic phytoalexins or salicylic acid may be involved in resistance to F. thapsinum. Sorghum (Sorghum bicolor (L.) Moench) is a valuable crop worldwide, in part due to its wide range of genetic diversity and phenotypic traits. Diversified sorghum types are available for biomass, grain, or sugar production (24,34), which makes sorghum a versatile bioenergy crop (8,41,51). Starch from grain sorghum and sugars from sweet sorghum are being used for ethanol production (38,41,52). Sorghum biomass is expected to be included as feedstock for cellulosic biofuels in the near future (11,49). Pathogens pose a serious threat to the economic sustainability of grain or biomass production for bioenergy (23,54). Panicle pathogens can negatively impact grain yield, quality, and viability (10,17,32). Root, stalk, and leaf pathogens can reduce biomass yield and cause lodging, which impairs harvest (12,47). The primary components of plant cell walls are the polysaccharides cellulose and hemicellulose, which can be broken down into sugars and converted to ethanol. The third major component of plant cell walls is lignin, which is a polymer made up of aromatic subunits that are cross-linked to the hemicellulose moiety. This complex construction provides strength and rigidity to prevent degradation and, in the case of vascular cell walls, allows them to conduct water under negative pressure (1,9). Lignin may also play a defensive role against pathogens and insects, as a barrier to initial invasion or as an induced response to prevent spread (2,4,48). Recently, however, by using lines with reduced lignin accumulation, this plant defense role has been challenged (43). Near-isogenic brown midrib (bmr)6 and bmr12 sorghum lines (in grain and forage) have been developed (35,36). The bmr mutations are described as “brown midrib” due to the reddish-brown discoloration of the midveins in maize, sorghum, and pearl millet associated with reduced lignin (44). Incorporation of bmr6 or bmr12 into elite sorghum lines resulted in significantly reduced lignin content and increased digestibility of the lignocellulose tissue for ruminant animal feed or for bioenergy uses (31,41). Bmr6 encodes a cinnamyl alcohol dehydrogenase (39,45) and Bmr12 encodes caffeic acid O-methyltransferase (5). Both bmr6 and bmr12 alleles used in this study resulted from nonsense mutations within their respective genes, and are likely null alleles (5,33,39,45). Near-isogenic double-mutant lines having both bmr6 and bmr12 alleles were developed that are significantly reduced in lignin content as compared with either single bmr mutant lines and have increased efficiency in ethanol conversion of the biomass (13,37). Further studies using bmr6, bmr12, and bmr6 bmr12 plants showed that concentrations of both soluble (micrograms per gram dry weight) and cell-wall-bound (micrograms per gram of cell walls) phenolic compounds were altered in the bmr lines compared with their wild-type counterparts (33), with bmr6 plants having the highest concentrations of free phenolic compounds overall. However, interactions were apparent between genetic background and bmr mutation that resulted in significant differences in the free phenolic profiles within the same mutation in different genetic backgrounds. In addition to reduced lignin content, both bmr mutants also resulted in altered monomeric composition of the lignin subunits (G, S, and H) relative to wild-type (33). Using bmr6 and bmr12 lines in five genetic backgrounds, it was previously shown that bmr lines were not more susceptible (e.g., no increased colonization) than the wild type, counter to previously held concepts (16,19). Fusarium spp. infected grain of bmr6 and bmr12 lines less frequently than wild-type lines. Two Fusarium spp. that commonly infect wild-type grain, Fusarium proliferatum and an F. incarnatum–F. equiseti species complex genotype, were significantly reduced or absent, respectively, in bmr12 grain (19). When peduncles (the upper part of the stalk that supports the head) were inoculated with Fusarium thapsinum Klittich, J.F. Leslie, P.E. Corresponding author: D. Funnell-Harris, E-mail: Deanna.Funnell-Harris@ars.usda. gov Mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the United States Department of Agriculture (USDA). *The e-Xtra logo stands for “electronic extra” and indicates that a supplementary table is available online. Accepted for publication 3 April 2014. http://dx.doi. org/10.1094 / PDIS-09-13-0980-RE This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 2014. e-Xtra*