Mycorrhizal formation in disease resistant inbred lines of Zea mays correlated with root phosphorus concentrations
McReynolds, Manette J.
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Mycorrhizal associations commonly occur in many higher plants. This association is a symbiotic relationship between soil fungi and the plant's roots. Corn (Zea mays), a major agricultural crop, forms a type of mycorrhizal association known as endomycorrhiza. This endomycorrhiza is called vesicular-arbuscular: the fungal hyphae growing into the root cortical cells, either forming highly branched hyphae called arbuscules or by forming large, swollen hyphae called vesicles. These fungi belong to the family Endongonaceae. There are considerable similarities between infection of host cells by fungal pathogens and by mycorrhizal fungi. Since the 1950's, a large number of corn hybrids have been bred for resistance to a large number of fungal pathogens. Breeding for fungal resistance has resulted in a decreased ability of corn to form endomycorrhizas, suggesting increased general resistance to fungal pathogens has been bred into these hybrid corn varieties (inbred lines). It has generally been agreed upon that phosphorus has an inverse effect, on mycorrhizal formation. An increased supply of phosphorus reduces mycorrhizal formation. The resistant varieties of corn that have shown a decrease in mycorrhizal formation may also differ in their ability to remove phosphorus from the soil resulting in different root phosphorus concentrations. It was these concentrations that influenced mycorrhizal formation. Inbred lines of corn were planted in a DeKalb County, Illinois "typical" agricultural field. This field has been selected by DeKalb AgResearch, Inc., to represent average yield for this county. A random sampling was harvested after 2 and 8 weeks after planting, and the roots studied morphometrically for percentage of mycorrhizal infection. Correlations were made between the degree of mycorrhizal infection and resistance to fungal pathogens. The same varieties of corn were grown for 2 weeks in a phosphorus nutrient solution in an environmental chamber. Roots of these plants were analyzed for phosphorus content. This study supports the premise that an inverse correlation exists between percent mycorrhizal fungal infection and the degree of inbred resistance to fungal pathogens. Root phosphorus concentrations in 2 week old seedlings showed no significant difference among the inbred lines. These data suggest that genetic factors, not phosphorus, play the predominant role in the formation of mycorrhizal associations in corn.