Histology of the Relation Between Minor and Major Genes for Resistance of Barley to Leaf Rust

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Abstract
Pa7 and Pa3, 2 major genes that confer hypersensitive resistance to leaf rust (caused by Puccinia hordei) in barley were introduced into 3 genetic backgrounds with different levels of partial resistance (PR) (the genetic complement controlling PR will be referred to collectively as PR genes) to study the interaction between both types of resistance. The growth rate and the degree of abortion of the colonies in the genotypes were determined by fluorescence microscopy. The degree of host cell necrosis was recorded. The PR genes affected the success of colony establishment in the host and reduced the growth and development rate of colonies after establishment. This effect was also apparent in the presence of Pa7, the effects of which were seen relatively late in the infection process. Apparently, PR genes and Pa7 acted independently and consecutively. Pa3 acted shortly after the establishment of the colonies and largely obscured the effect of the PR genes. Nevertheless, the level of PR can be assessed in the presence of Pa3 by determining the proportion of early aborted colonies not associated with host cell necrosis.