Relation between fiber and pectic substances in root tissue and tolerance to fusarium root rot in asparagus plants infected with arbuscular mycorrhizal fungus

Abstract

Tolerance to fusarium root rot, caused by Fusarium oxysporum f. sp. asparagi (FOA; MAFF305556, SUF844, SUF1226, SUF1229), in asparagus (Asparagus officinalis L., cv. Mary Washington 500W) plants infected with arbuscular mycorrhizal (AM) fungus (Glomus sp. R10) was estimated, and the relationships between fiber (cellulose, hemicellulose and lignin) and pectic substances (water-soluble, hexametaphosphate-soluble and HCl-soluble pectin) in root tissue and the tolerance to fusarium root rot were investigated. Twelve weeks after FOA inoculation, the incidence and symptoms were significantly lower in AM plants than in non-AM ones, regardless of the FOA strains. Among non-AM plants, disease index reached 70-92, it ranged between 8-16 in the AM plants. Non-diseased and diseased AM plants produced more dry weight of feeder and storage roots than did diseased non-AM ones. Phosphorus concentration in feeder and storage roots differed little between non-AM and AM plots 10 weeks after AM fungus inoculation (just before FOA inoculation) and 12 weeks after FOA inoculation. Total fiber and each of its constituents in the feeder and storage roots did not differ between AM and non-AM plots before and after FOA treatment. In storage roots, total content of pectic substances was higher in AM plots than it was in non-AM ones at the end of the periods; especially, the increase in water-soluble pectin content in the AM plots. In feeder roots, the individual pectic substances in AM and non-AM plots were similar. These findings suggest that root rot tolerance in storage roots of AM fungus-infected plants is closely associated with the pectic substances, such as water-soluble pectin, rather than with the fiber content in root tissue.