Differential impact of carbon assimilation on the expression of quantitative and qualitative resistance in alfalfa (Medicago sativa)

Quantitative resistance is desirable in agronomic crops because of its durability in contrast to the highly heritable but more volatile qualitative resistance. Qualitative resistance has a gene-for-gene interaction with the pathogen; quantitative resistance lacks that specificity. Despite the genetic differences, both types of resistance utilize products of the phenylpropanoid pathway to respond to infection. However, they differ dramatically when carbon assimilation is reduced by low photosynthetic photon flux density. Expression of quantitative resistance fails under low light. Qualitative resistance is insensitive to low light level. Carbon flux appears to explain the differences in sensitivity, with quantitative resistance requiring a greater carbon flux through the phenylpropanoid pathway for its expression than qualitative resistance. In the absence of sufficient carbon assimilation and/or carbohydrate reserves, plants with the genes for quantitative resistance fail to express that resistance. Quantitative resistance is sensitive to environmental factors, a sensitivity often attributed to micro-climatic effects on the pathogen. The sensitivity of quantitative resistance to carbon assimilation suggests that part of the environmental sensitivity of quantitative resistance is due to the effect of environmental factors on carbon assimilation.