Streptomycin reduces plant response to mycorrhizal infection

Abstract

Additions of streptomycin to lettuce plants reduced their response to infection by a vesicular-arbuscular mycorrhixal (VAM) fungus as assessed by shoot dry weight. This reduction in response to mycorrhizal infection was not associated with either a decrease in mycorrhixal infection or a decrease in host plant phosphorus or nitrogen concentration. We offer testable alternative hypotheses to account for this phenomenon. Numerous experiments have demonstrated that VAM fungi can increase plant growth and reproduction (Koide, 1991). The degree to which the plant responds to infection, however, may be strongly affected by many environmental factors including the soil fertility (Hayman, 1983), temperature (Smith and Roncadori, 1986), the availability of light (Bethlenfalvay and Pacovsky, 1983) and the presence of other microorganisms in the rhizosphere. These root-associated microorganisms may influence the extent to which a plant responds to mycorrhixal infection in, a number of ways. One mechanism may involve an effect on the rate of infection development. The germination of VAM fungal spores (Daniels and Trappe, 1980; Axcon-Aguilar et al., 1986; Axcon, 1989), the extension of germ tubes (AzconAguilar and Barea, 1985), the formation of appressoria and penetration into the cortex (Mosse, 1962), the fractional root colonization (Meyer and Lindennan, 1986; Ames, 1989) and the density of external hyphae (Ames, 1989) may all be increased by the presence of other root-associated microorganisms. These microorganisms may also increase plant response to mycorrhizal infection in the absence of an effect on the rate of infection development (Meyer and Linderman, 1986; Axcon, 1989), by acting in concert with mycorrhixal fungi to increase P availability (Bagyaraj, 1984; Meyer and Linderman, 1986; Linderman, 1988). We investigated the effects of an antibiotic (streptomycin) on the response of lettuce to vesicular-arbuscular mycorrhixal infection. Seeds of lettuce (Lactucu suriua L. cv. Salinas) were sown on 12 April 1989 in vermiculite. Eight days later, seedlings were transplanted into pots (150 ml) containing a mixture of autoclaved sand and soil (Hagerstown silty-clay loam) at a ratio of 1: 1. The bicarbonate-extractable P concentration of the soil was approx. 3 pg g-r. The plants were placed in a greenhouse. There were four treatment combinations (myc-