Reaction of Sorghum Lines Genetically Modified for Reduced Lignin Content to Infection by Fusarium and Alternaria spp

Funnell, D. L., and Pedersen, J. F. 2006. Reaction of sorghum lines genetically modified for reduced lignin content to infection by Fu.sarium and Alternariu spp. Plant Dis. 90:331-338. Two genes conferring the brown midrib (bmr) trait had been backcrossed into six elite sorghum lines, resulting in reduced lignin in the hrnr lines when compared with the wild-type parent. Seed and leaf tissue from field-grown plants, planted at two locations, were screened for Alternuriu spp. and Fusurium spp. on semi-selective media. The results suggest that hrnr lines do not have increased susceptibility to colonization by Alternuriu spp. However, significantly fewer colonies of Fusurium spp., including Fusarium moniliforme, were recovered from seed of reduced lignin lines from two genetic backgrounds. That the bmr trait in some genetic backgrounds might enable increased resistance to colonization by E moniliforme was further supported by greenhouse experiments in which peduncles of developing heads were inoculated with F moniliforme. Mean lesion measurements on hrnr lines were significantly lower than those resulting from inoculations on wild-type lines. Analysis of near-isogenic lines revealed that mean lesion lengths on hrnr lines were significantly less than those produced on their wild-type counterparts in four of the six genetic backgrounds. These results suggest that reduced lignin lines exhibit, in some cases, increased resistance to Fusurium spp., including E moniliforme. Additional keywords: hmr-6, hmr-12, Fu.surium verticillioide.~, grain mold The brown midrib (bmr) trait of grasses has been utilized in breeding programs of forage crops to increase digestibility for ruminant animals. First recognized as a spontaneous mutant of maize in 1924 (29), bmr is characterized by reddish-brown pigmentation, most noticeable in the midrib of young plants, and is associated with reduced lignin content (15). The bmr trait has been induced in sorghum using chemical mutagenesis (15). Genetic and molecular studies of bmr maize and sorghum have shown that the bmr phenotype results from impaired function of enzymes involved in the lignin biosynthetic pathway (58,73). Much focus has been placed on lignin and nutrient availability in forage crops (1 4,s 1); however, reduced lignin content Corresponding author: D. L. Funnell E-mail: dfunnell@ unlserve.unl.edu Mention of trade names or commercial products in this article is rolely for the purpose of providing specific information and does not imply recommendation or endorsement by the United States Department of Agriculture. Accepted for publication 21 October 2005. DOI: 10.1094lPD-90-0331 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, 2006. also may improve efficiency of ethanol production (33) or facilitate decomposition of residue in conservation tillage management systems (71). However, lignin is essential to integrity and fitness of plants, especially of those in natural (nonagronomic) systems (56). Reduced lignin can impact crop yields by alteration or impairment of development (72,78) or by increased lodging making harvesting and management difficult (78). Additionally, lignin likely is involved in defense against pathogens and insects ( 7 9 , first, by providing a physical barrier against initial ingress and infestation (9 , l l ) and, second, as an induced response. The induced response may be in the form of rapid deposition of lignin or lignin-like materials which may prevent further growth and confine the invading pest (4,8,24,41,65,68). With fungal pathogens, a correlation between resistance to an avirulent pathovar and accumulation of lignin or lignin-like products at the infection site has been observed (1 8,25). Precursors involved in the lignin biosynthetic pathway (74) also may play a role in disease defense (47). Metabolites of the lignin pathway have been shown to inhibit growth of pathogenic fungi or inhibit production of virulence factors, such as toxins, in vitro (6,17,23,39). Accumulation of lignin precursors following attempted infection by nonpathogens can occur concurrently with the induction of systemic resistance (66). Accumulation of ferulic acid, pcoumaric acid, and sinapic acid has been correlated with resistance to Fusarium spp. (39,67). On the other hand, cinnamic acid derivatives (70) or p-coumaric acid (34) can induce virulence during host interactions with Agrobacterium tumefaciens. Perturbations in the lignin biosynthetic pathway could modify interactions between plants and potential pathogens or insect pests on many levels: from the structural integrity of the whole plant to responses at the cellular level and at the interface between the plant and potential pest or beneficial microorganism. Nonetheless, increase of carbon accessibility has been balanced with possible losses due to reduced fitness (33,512). The bmr-6 and bmr-12 genes were backcrossed successfully into six elite grain sorghum lines (Wheatland, Redlan, RTx430, BTx623, BTx630, and BTx63 1 ; 53). Because grain sorghum previously was bred for reduced height (69), problems with lodging have been eliminated in these bmr lines (53). However, average grain yields in lines with bmr-6 or bmr-12 were significantly lower (83.5 and 80.5%, respectively) than in wild-type lines (53). Residue dry weight produced by bmr12 lines was significantly greater than that of wild-type and bmr-6 lines (by 10.5 and 23.1 %, respectively), whereas residue dry weights of bmr-6 lines were significantly less than those of wild-type lines (89.8%; 53). Lignin content was significantly less than wild-type for bmr-6 and bmr-12 lines, with lines containing bmr-12 having the greatest reduction (53). Two fungal genera that are prevalent in field-grown sorghum, even in asymptomatic tissue, are Alternaria and Fusarium (30,57,64,77). Under less than optimal conditions, such as plant wounding, colonization by microorganisms, or weather favorable for pathogenesis, apparent nonpathogenic growth by Fusarium or Alternaria spp. could become damaging (57,77). Both genera contain species that produce secondary metabolites toxic to humans and livestock (16,50,77). In particular. members of the Fusarium moniliforme complex can produce fumonisins that can cause inefficient growth and reduced production in livestock or immunosuppression or other serious and potenPlant Disease 1 March 2006 331