Glycine-Glomus-Rhizobium Symbiosis

Abstract

Soybean (Glycine max [L.] Menf.) plants grown in pot cultures were inoculated with the vesicular-arbuscular mycorrhizal (VAM) fungus Glo-mus mosseae (Nicol. & Gerd.) Gerd. and Trappe and Rhizobium japoni-cum strain 61A118 at planting (GIRI) or at 20 days (G2,R20), or with one of the endophytes after the other has colonized the host root (GIR2n, G2RJ). Nodulated (PRJ) and VAM (GIN) dipartite associations, or nonsymbiotic plants (PN) using nutrient solutions with N, P, or N + P concentrations providing endophyte-equivalent nutrient inputs were used as controls. The delayed tripartite associations received the appropriate N, P, or N + P amendment while one or both endophytes were absent during the first 20 days of growth. Prior inoculation with one endophyte significantly inhibited development of the other. Root hexose sugar concentrations were negatively correlated with VAM colonization (r =-0.89), nodule activity (r =-0.91), and root P content (r =-0.93). Nodule (r = 0.97) and root (r = 0.96) P content correlated positively with VAM colonization. Nodule weight or VAM-fungal biomass were significantly greater in associations grown with only one endophyte. Dry weights of the PN, GIN, PR,, and G2,R20 plants were significantly greater than those of tripartite plants inoculated at planting with either or both endophytes. Interendophyte inhibition is attributed to competition for root carbohydrates, and this effect apparently also affects overall plant productivity. The objective of the study was to determine if the timing of endophyte introduction and establishment affected the development of the other symbiotic partners. Since Asai's demonstration of the importance of VAM' fungi to N2 fixation in legumes (3) most ofthe interest in VAM fungus-nodule interactions has centered on the role of VAM fungi as a P source to legumes under P stress (5). This role is clearly important and may influence nodule activity either through the enhanced nutritional condition of the host plant (15) or may directly and preferentially stimulate nodule function (5). Whether the energy cost of this VAM fungus-mediated P uptake is significant to the tripartite association is not yet clear (25), but recent results show that VAM fungal colonization and nodula-tion are affected differently by carbohydrate stress (6). In plants not under carbohydrate stress, VAM fungi may inhibit growth depending on P availability (7) in spite of increased photosyn-thetic output (23) in response to the additional C demand by the endophytes (9). Little is known, however, about direct interactions between the endophytes except for those which may be deduced from source-sink effects within the association (15). When inoculated 'Abbreviation: VAM, vesicular-arbuscular mycorrhizal. at the same time, nodulation and VAM-fungal colonization proceed simultaneously, indicating that the endophytes do not compete for infection sites (27), but subsequent to infection, VAM fungi do not usually invade nodule cortical tissues (19). Since both Rhizobium and VAM fungi influence hormonal levels in plants (4, 5), the resulting changes may mutually affect the development of the endophytes. Stimulation or inhibition of endophyte growth will, in turn, affect the host plant (20) and also the biota of the rhizosphere (2). Since symbiotic associations between plants and microorganisms depend for their formation and function on interactions between their constituents (5), an elucidation of such mutual effects is needed for a more complete understanding of the symbiosis. The present study was made to determine the effect ofprevious establishment ofone endophyte on subsequent infection by the other, using controls not limited by nutrient deficiency. MATERIALS AND METHODS Experimental Design. The experiment consisted of eight treatments with six replications for a total of 48 plants. Plants were inoculated with VAM fungus (G) and a diazotrophic bacterium (R) simultaneously at planting (GIRI), at 20 d after planting (G20R20), or with one organism at planting and the other at 20 d (GIR20, G20RI). At 20 d after planting each symbiont introduced at 1 d had achieved a certain level ofinfection, so that inoculation by the second organism was superimposed on an already established symbiosis thus forming a tripartite association. The temporary absence of the VAM fungus (P source) was compensated for by the use of an N-free nutrient solution containing P which produced a plant growth response equivalent to that caused by the fungus. Phosphorus equivalency was established by'a method described elsewhere (21) in which nutrient solutions of different P concentrations were used to achieve a growth range in non-VAM plants which bracketed the size (dry weight) of the VAM plant. The 'VAM-equivalent' P concentration could then be determined from the growth curve based on the P concentration. Since P equivalency is affected by environmental conditions and the taxonomic identity of the symbionts, adjustments in P concentration must be made to achieve equivalent VAM and non-VAM plant growth when any of the factors are changed. The absence ofthe diazotroph (N source) was similarly accounted for by a P-free solution containing N (22). In addition, dipartite VAM and N2-fixing associations were grown using P and N complements for the entire time (GIN, PR,). For comparison, a N2-fixing association was also included without the provision of P or N (R,), as well as nonsymbiotic plants provided with both P and N (PN). The concentrations of nutrient complements (N, P, or N and P) were so chosen that dipartite or nonsymbiotic plants had the same dry weight at harvest. Biological Materials. Soybean (Glycine max [L.] Merr. cv 1054 https://plantphysiol.org Downloaded on May 23, 2021.-Published by