<i>ToxA</i>–<i>Tsn1</i>Interaction for Spot Blotch Susceptibility in Indian Wheat: An Example of Inverse Gene-for-Gene Relationship

Abstract

The <i>ToxA</i>-<i>Tsn1</i> system is an example of an inverse gene-for-gene relationship. The gene <i>ToxA</i> encodes a host-selective toxin (HST) which functions as a necrotrophic effector and is often responsible for the virulence of the pathogen. The genomes of several fungal pathogens (e.g., <i>Pyrenophora tritici-repentis, Parastagonospora nodorum,</i> and <i>Bipolaris sorokiniana</i>) have been shown to carry the <i>ToxA</i> gene. <i>Tsn1</i> is a sensitivity gene in the host, whose presence generally helps a <i>ToxA</i>-positive pathogen to cause spot blotch in wheat. Cultivars lacking <i>Tsn1</i> are generally resistant to spot blotch; this resistance is attributed to a number of other known genes which impart resistance in the absence of <i>Tsn1</i>. In the present study, 110 isolates of <i>B. sorokiniana</i> strains, collected from the ME₅A and ME₄C megaenvironments of India, were screened for the presence of the <i>ToxA</i> gene; 77 (70%) were found to be <i>ToxA</i> positive. Similarly, 220 Indian wheat cultivars were screened for the presence of the <i>Tsn1</i> gene; 81 (36.8%) were found to be <i>Tsn1</i> positive. When 20 wheat cultivars (11 with <i>Tsn1</i> and 9 with <i>tsn1</i>) were inoculated with <i>ToxA</i>-positive isolates, seedlings of only those carrying the <i>Tsn1</i> allele (not <i>tsn1</i>) developed necrotic spots surrounded by a chlorotic halo. No such distinction between <i>Tsn1</i> and <i>tsn1</i> carriers was observed when adult plants were inoculated. This study suggests that the absence of <i>Tsn1</i> facilitated resistance against spot blotch of wheat. Therefore, the selection of wheat genotypes for the absence of the <i>Tsn1</i> allele can improve resistance to spot blotch.