Asparagus decline is a serious and increasing threat in asparagus producing regions over the world (Hamel et al., 2005; Knaflewski et al., 2008; Reid et al., 2001; Wong and Jeffries, 2006). It is supposed to be caused by the contribution of both biotic factors (Knaflewski et al., 2008; Wong and Jeffries, 2006) and abiotic factors (Lake et al., 1993; Miller et al., 1991; Yong, 1984). As biotic factors, the most common phenomenon is fusarium crown and root rot caused by Foa, Fusarium proliferatum (Fp), and Fusarium redolens etc. (Knaflewski et al., 2008; Reid et al., 2002; Wong and Jeffries, 2006). In Japan, Nahiyan et al. (2011) demonstrated that Foa and Fp are dominant fusarium species in asparagus decline fields. However, the diseases are still difficult to control because no resistant cultivar or disinfesting method has been developed. On the other hand, biological control of fusarium disease was tried by the inoculation with non-pathogenic isolates of the fusarium species (Blok et al., 1997; Elmer, 2004). However, the method is not enough to control the diseases and has no growth-promoting effect.
Arbuscular mycorrhizal fungi are ubiquitous soil inhabitants and form a symbiotic relationship with roots of most of the terrestrial plants. AMF promotes host plant growth by enhancing phosphorus uptake through symbiosis (Marschner and Dell, 1994) and hence an alternative to high inputs of fertilizers and pesticides in sustainable crop production systems. Previously, we reported tolerance to fusarium root rot in mycorrhizal asparagus (cv. Mary Washington 500W) plants (Matsubara et al., 2003); however, many points remain unclear about the mechanisms of disease tolerance in mycorrhizal asparagus plants.
As for the changes in amino acid constituents related to disease tolerance in mycorrhizal plants, Baltruschat and Schonbeck (1975) demonstrated that the propagation of Thielaviopsis basicola was inhibited by the increase of arginine and citrulline in mycorrhizal tobacco plants. In addition, some reports mentioned that the free amino acid level in plants changes through AMF colonization. Sood (2003) and Fattah and Mohamedin (2000) reported that increases in the contents of free amino acids occurred in mycorrhizal tomato and sorghum plants, respectively. On the other hand, Rolin et al. (2001) reported that AMF colonization decreased total amino acid levels in mycorrhizal leek plants. However, it has been unclear how the contents of free amino acid change through symbiosis with AMF in asparagus plants and how the changes are associated with disease tolerance.
In this study, tolerance to fusarium root rot and the changes in free amino acid contents in mycorrhizal asparagus plants were investigated to clarify the relationship between free amino acid and disease tolerance.
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