Endophytic Niche and Grass Defense
Recent advances have extended our understanding of the basic biology, genetics, and molecular biology of many fungal endophytes. Molecular biology in particular has made it possible for identi cation and phylogeny of the clavicipitalean fungi (Schardl and Siegel, 1992; Glenn et al., 1996; Schardl et al., 1997; Wilkinson and Schardl, 1997; Leuchtmann, 1999; Moon et al., 1999; Clay and Schardl, 2002; Schardl and Schardl and Moon, 2003; Haarmann et al., 2005; Spiering et al., 2005). Additionally, recent advances have extended our knowledge of host-fungal interactions that result in both quantitative and qualitative measures of mutualistic responses to most clavicipitalean-infected grasses and sedges (Clay, 1988; Wilkinson and Schardl, 1997; Schardl, 2001; Clay and Schardl, 2002; Spiering et al., 2005). Fungal endophytes do have common sites of colonization within grass plants, although the distribution of the endophytic habit may not be entirely distributed intercellularly throughout the plant axis. For example, some endophytes form reproductive structures on the surface of the plant, i.e., epibiotic, although part of the thallus is either subcuticular or endophytic, and their perennial nature is due to being localized to the plant’s apical meristematic tissues. These epibiotic endophytes also occupy areas that are components of the apoplasm, indicating a need to better de ne the endophytic niche beyond the boundaries of the “intercellular spaces” of plants as it is commonly de ned by others (Yoshihara et al., 1988; Prat et al., 1997; Jarvis, 1998). The apoplast is the plant’s internal milieu from which internal homeostasis is maintained (Sakurai, 1998) and through which environmental stimuli are received including growth
responses to auxins and other functions. This underlines the importance of apoplasm as the habitat for endophytic fungi. In this review we will use the term “apoplast” as a synonym for “intercellular spaces.” However, there are distinctions and the pertinent ones are mentioned in the discussions that follow.