Plant foliar disease suppression mediated by composted forms of paper mill residuals exhibits molecular features of induced resistance

Abstract

Arabidopsis thaliana grown in soil from field plots amended with composted forms of paper mill residuals (PMR) exhibited reduced symptoms of bacterial speck caused by Pseudomonas syringae pv. tomato ( Pst) compared with plants grown in soil from field plots amended with a non-composted PMR or non-amended soils. Similar results were obtained with tomato ( Lycopersicon esculentum Mill.). No relationship between foliar disease suppression and plant nutrition or stature was observed. In Arabidopsis, the reduction of foliar disease symptoms ranged between 34 and 65%, depending on the type of composted PMR amendment, and was associated with reduced Pst titers in planta. An Arabidopsis npr1 defense mutant and a NahG transgenic line, both of which exhibit disrupted systemic acquired resistance, were also disrupted in their suppression of Pst disease symptoms in composted PMR treatments. Arabidopsis grown in soil amended with composted PMR also displayed an increased expression of pathogenesis-related defense genes prior to pathogen inoculation. We conclude that plants grown in soils with composted PMR-amendments were more resistant to disease caused by Pst due to the induction of plant defenses, similar to systemic acquired resistance. The identity of the PMR elicitor(s) is as yet unknown, but was shown to be heat labile.