Analysis of Defense Genes in Response to White Mold Infection in Different Bean (Phaseolus vulgaris) Cultivars.

Understanding the function of quantitative resistance genes and their mode of expression is necessary to enable breeders to select which QTL to deploy in marker assisted selection. We investigated the role of PGIP, Glucanase, and PAL genes in the defense response of different genotypes (AN-37, P02630, Beryl and G122) following infection with Sclerotina sclerotiorum.  In general there was no glucanase activity detected in the cultivars AN-37 and P02630 at 0hpi while some low levels of transcription took place in the other two cultivars. Activity peaked at both 24 and 48 hpi in all the cultivars except in Beryl which had loss of expression at 48hrs. At 7 days all the cultivars had some level of glucanase activity with P02630 having the strongest signal relative to rest of the other lines. Overall phenylanine ammonia lyase (PAL) had low expression levels in response to Sclerotina sclerotiorum. There was no transcription in response to wounding in all the cultivars however Beryl and G122 peaked at 24 hrs while the most abundant transcripts were at 48 hrs in the two pinto lines. All the cultivars retained some enzyme activity at 7 days. There was also variable activity of PGIP1 in the different cultivars. In contrast to the other genes PGIP was induced with different signals in response to wounding at 0 hpi. The resistant line G122 showed the strongest signal at 0 and 48 hpi while the line AN-37 expressed the gene at 48 hpi only. At 7 dpi there was no signal except in P02630. These results suggest that the resistance reported in different cultivars of beans to white mold is due to different defense pathways. The induction of defense genes at wounding may have a confounding role in the interpretation of results from the commonly used greenhouse straw test.