Phosphorus effect on phosphatase activity in endomycorrhizal maize

Success of a mycorrhizal symbiosis is influenced by the availability of phosphorus (P) in the soil. Maize (Zea mays L. cv. Great Lakes 586) plants were grown under five different levels of soil P, either in the presence or absence of formononetin or the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus intraradices Schenck and Smith. We detected physiological differences in mycorrhizal roots very early in the development of symbiosis, before the onset of nutrient-dependent responses. Under low P levels, VAM roots accumulated a greater shoot dry weight (13%), root P concentration (15%) and protein concentration (30%) than non-VAM roots, although root growth was not statistically significantly different. At higher P levels, mycorrhizal roots weighed less than non-VAM roots (10%) without a concomitant host alteration of growth or root P concentration. Mycorrhizal colonization decreased as soil P increased. Formononetin-treatment enhanced colonization of the root by G. intraradices and partially overcame inhibition of VAM colonization by high soil P concentrations. This is the first report that formononetin improves root colonization under high levels of soil P. Acid phosphatase (ACP) and alkaline phosphatase (ALP) activities were closely related to the level of fungal colonization in corn roots. ACP activity in corn roots responded more to soil P availability than did ALP activity (38% more). These results suggest that ACP was involved in the increased uptake of P from the soil, while ALP may be linked to active phosphate assimilation or transport in mycorrhizal roots. Thus, soil P directly affected a number of enzymes essential in host-endophyte interplay, while formononetin enhanced fungal colonization.