Recent Molecular and Genomic Studies on Stress Tolerance of Forage and Turf Grasses

Improvement in stress tolerance of forage and turf grasses is a major breeding goal. Most forage and some turf grasses are grown on marginal lands under stressful environments with minimal inputs. In contrast, current high-input turf grass production systems such as golf courses and lawns are expensive and often environmentally unfriendly. Cultivars with improved stress tolerance are necessary for the development of sustainable and environmentally friendly production systems. Until recently, decades of breeding and selection have resulted in limited improvements of stress tolerance of forage and turf grass species. Recent developments in molecular and genomic sciences suggest new methods to improve stress tolerance in many plants, but compared to major crop plants (e.g., rice [Oryza sativa L.], wheat [Triticum spp.], and maize [Zea mays L.]), the development of molecular and genomic resources for forage and turf grasses has been limited. In this review, we present an overview of recent molecular and genomic studies aimed at improving stress tolerance of forage and turf grasses, including endophyte grass interactions. Important molecular and genomic resources are now available for some forage and turf grasses, including ryegrasses (Lolium spp.) and fescues (Festuca spp.). Noteworthy progress is being made in improvements of both biotic and abiotic stress tolerances of these grasses, but the challenge is to simplify and streamline the molecular tools and new discoveries for cost-effective and efficient application in forage and turf grass breeding. Stress tolerances of many forage and turf grasses are influenced by their mutualistic association with Neotyphodium spp. endophytes, and this area of research is discussed. FORAGE AND TURF GRASSES occupy twice the land area of grain crops worldwide due largely to their adaptability (Jauhar, 1993). The cultivated grasses used as forage and turf provide tremendous benefits to humans. The forage grasses sustain millions of dairy and beef cattle, horses, sheep, other livestocks, and countless wild animals (Wang et al., 2001). Production of turf grasses in golf courses and lawns is a multibillion dollar industry in theUnites States. Apart from the direct economic benefits realized from forage and turf grasses, their contributions in soil conservation, environmental protection, recreation, and aesthetics are substantial. In the Poaceae, 40 species are currently used for forage and turf purposes (Moser and Hoveland, 1996). The species most intensively used for forage and turf include fescues, ryegrasses, bentgrasses (Agrostis avenacea J.F. Gmel.), bluegrasses (Panicum dichotomiflorum Michx.), bromegrasses (Bromus riparius Rehmann), orchardgrass (Dactylis glomerata L.), bermudagrass [Cynodon dactylon (L.) Pers.], and Panicum spp. In part because of the genetic complexity of forage and turf grasses, relatively little investment has been directed toward understanding and improving stress tolerance of these economically important species. Molecular technologies, such as genetic transformation and markerassisted breeding, have become effective and efficient procedures for improving stress tolerance of some major crop species {e.g., soybean [Glycinemax (L.)Merr.], rice, cotton [Gossypium hirsutum L.], and wheat}; however, conventional breeding through sexual hybridization is still the principal route for the development of stressresistant forage and turf varieties. Several major factors contribute to the slow advancements in molecular and genomics research for improvement of stress tolerance in forage and turf grass species. First, most grasses are genetically complex, being out-crossing polyploids, and thus present highly complicated research targets. Second, most stress-resistance traits are under complex control of a number of genes and map as quantitative trait loci (QTL) throughout the genome (Fujimori et al., 2003; Yamada et al., 2004). Third, the lack of wellcharacterized genetic materials and absence of repeatable and efficient phenotyping protocols for many target traits limit the application of genomics technologies. Finally, the forage and turf grass community in general has failed to attract major funding for molecular and genomics research from either government agencies or private industries. Nevertheless, appreciable progress has been made in developing molecular and genomics tools for improvement of these grasses. Biotic and Abiotic Stresses and Their Interactions At one time or another during their life cycle, most plants encounter biotic and abiotic environmental stresses, which are two major factors that determine the distribution and productivity of crop plants. Thus, improvement of stress tolerance is a major plant breeding goal (Tolmay, 2001; Araus et al., 2002; Lecoq et al., 2004). Biotic stresses are caused by organisms such as bacteria, fungi, insects, nematodes, and viruses (Dangl and Jones, 2001). In contrast, abiotic stresses are caused by an unfavorable physical or chemical environment surrounding the plant. The mechanisms and physiology of abiotic stress tolerance of plants have been studied Y. Zhang and J.H. Bouton, The Samuel Roberts Noble Foundation, Inc., 2510 Sam Noble Parkway, Ardmore, Oklahoma, 73401; M.A.R. Mian, USDA-ARS, 1680 Madison Avenue, Wooster, OH 44691. Received 27 Sept. 2004. *Corresponding author ([email protected]). Published in Crop Sci. 46:497–511 (2006). Review & Interpretation doi:10.2135/cropsci2004.0572 a Crop Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: ABA, abscisic acid; AFP, antifreeze protein; AFLP, amplified fragment length polymorphisms; cM, centimorgan; EST, expressed sequence tag; kb, kilobase; MAS, marker-assisted selection; ORF, open reading frame; PAP, pokeweed antiviral proteins; PCR, polymerase chain reaction; PSII, photosystem II; RAPD, random amplified polymorphic DNA; RFLP, restriction fragment length polymorphisms; RMV, Ryegrass mosaic virus; QTL, quantitative trait loci; SNP, single nucleotide polymorphism; SSH, suppression subtractive hybridization; SSR, simple sequence repeat; STS, sequencetagged sites. R e p ro d u c e d fr o m C ro p S c ie n c e . P u b lis h e d b y C ro p S c ie n c e S o c ie ty o f A m e ri c a . A ll c o p y ri g h ts re s e rv e d . 497 Published online February 1, 2006