Potential role for salicylic acid in induced resistance of asparagus roots to Fusarium oxysporum f.sp. asparagi

Pre‐inoculation of asparagus ( Asparagus officinalis ) roots with selected nonpathogenic isolates of Fusarium oxysporum (np Fo ) has previously been shown to induce systemic resistance against infection by F. oxysporum f.sp. asparagi ( Foa ) through activation of plant‐defence mechanisms. To elucidate the putative np Fo ‐mediated defence pathways, the effect of salicylic acid (SA) was examined in a split‐root system of asparagus where one half of the seedling root system was drenched with SA and the activation of defence responses was measured subsequently on the remaining roots. SA‐treated plants exhibited enhanced systemic resistance, with a significant reduction in disease severity of the roots inoculated with Foa , compared with untreated plants. SA activated peroxidase and phenylalanine ammonia‐lyase, as well as lignification, upon Foa attack, in a manner similar to that observed with np Fo pretreatment. In addition, application of diphenyleneiodonium, an SA biosynthesis inhibitor, led to failure of np Fo to induce lignin deposition and systemic resistance. Treatment of fungal spores with SA did not affect germination and growth of either np Fo or Foa in in vitro antifungal assays. Production of SA at the site of np Fo infection may be involved in the induction of Foa resistance in asparagus roots.