Role of Fungal Avirulant Pathogens in Defense Response of Strawberry

Abstract

The development of alternative strategies for the biocontrol of diseases requires the understanding of mechanisms induced in plants during the interaction with pathogens. In this paper we review the advances obtained by our group regarding the induction of the defense response and systemic acquired resistance (SAR) in strawberry plants mediated by two avirulent fungal isolates of Colletotrichum fragariae and Acremonium strictum. Evaluation of plant susceptibility showed that plants treated with both isolates acquired a strong resistance against the virulent isolate M11 of Colletotrichum acutatum the casual agent of anthracnose disease. Plants treated with the avirulent strains exhibited an early oxidative burst detected as the accumulation of H2O2 and O2. -, that is followed by an accumulation of salicylic acid (SA), callose and lignin. The up-regulation of two genes related to SAR: i) FaPR1, a SA-regulated Pathogenesis Related Protein (PR protein), and ii) FaPAL3, an enzyme involved in the phenylpropanoid pathway and the synthesis of many defense-related secondary metabolites including SA was observed. The up-regulation of FaPR1 gene during the first 48 hpt (hours post treatment) was correlated to the increase of salicylic acid in the phloematic sap of infected plants. At 48 hpt the upregulation of the transcripts of two Calmodulin-like genes (FaCML1 and FaCML2) and a glutathione transferase gene (FaGST), which are critical regulators of defense responses, were also observed. Finaly, the characterization of the fungal elicitor AsES obtained from cultures of A. strictum is reported. Plants treated with the elicitor exhibited a similar behaviour as those treated with the avirulent isolates.