Influence of phosphorus and formononetin on isozyme expression in the Zea mays‐Glomus intraradices symbiosis

Abstract

Maize (Zea mays L. cv. Great Lakes 586) plants were either inoculated with the vesicular‐arbuscular mycorrhizal (VAM) fungus Glomus intraradices Schenck and Smith, or grown in the presence of the isoflavone formononetin or were provided with both G. intraradices and formononetin. All plants were grown in soil containing one of five levels of inorganic P (between 8 and 110 µg g−1 soil). By 3 weeks there were significant differences in a number of enzyme activities and in the pattern of isoenzymes in roots colonized by the VAM fungus or treated with formononetin. One NAD‐malate dehydrogenase (MDH) isozyme was expressed only in mycorrhizal roots, whether treated or not with formononetin. Despite differences in the soil P level, the expression of this isozyme was not observed in non‐mycorrhizal roots, indicating specific expression in the mycorrhizae. We suggest that MDH isozyme could serve as a specific, early indicator of the Zea‐Glomus symbiosis. Differences in the esterase (EST) isozyme pattern were not detectable between VAM and non‐VAM roots, suggesting that this enzyme system is not a good parameter for the evaluation of mycorrhizal colonization. As available P in the soil increased, total EST activity appeared to increase as well. Interestingly, total peroxidase (POX) activity increased along with P suggesting that as plant P nutrition improved, both cell wall ramification and the quantity of defense peroxidases increased as well. Total POX activity from mycorrhizal roots was inversely correlated with root colonization, indicating that there was suppression of POX activity by the host under low soil P. Most interestingly, formononetin further decreased POX activity regardless of the level of P or mycorrhizal status. This may suggest one mechanism by which formononetin enhances root VAM colonization. The presence of this isoflavone suppressed POX activity in mycorrhizal roots allowing a rapid penetration and spread of the fungus in the root cortex. The interplay between host root, soil P levels, secondary metabolites and endogenous host enzyme activities and a particular VAM fungus has a profound effect on the efficiency, duration and functioning of an endomycorrhizal symbiosis.