ROMERO Fernando Matias
congresos y reuniones científicas
Botrydial causes host cell death by inducing the hypersensitive response and by interacting with host signaling pathways that mediate defense responses against pathogens
Necrotrophic plant pathogens such as B. cinerea kill host tissues and feed on the remains. B. cinerea secretes a variety of nonspecific phytotoxic metabolites that provoke host cell death. Among them, the tricyclic sesquiterpene botrydial is known to be toxic to several plant hosts and seems to act as a strain-specific virulence factor. The hypersensitive response (HR), almost universally accepted as a form of programmed cell death, is typically triggered upon recognition of a pathogen-encoded avirulence protein by a resistance protein of the host plant. This response provides resistance to biotrophic pathogens by restricting their access to living tissues. However, the HR enhances virulence of necrotrophic pathogens, by increasing tissue damage. In fact, B. cinerea is able to promote the host HR and thus takes advantage of this cell death process. Moreover, it has been proposed that HR induction by B. cinerea is mediated by elicitors that act as virulence factors. However, the identity of such HR elicitors has not been clearly established. The aim of this work was to determine if the well-known sesquiterpene botrydial produced by B. cinerea merely acts as a "plain toxin" or if, on the contrary, is able to induce the host HR. For this purpose, biochemical and molecular indicators of the HR were analysed on leaves of the model plant Arabidopsis thaliana treated with botrydial. Aniline-blue staining and epifluorescense microscopy revealed that botrydial induced callose deposition as early as 3 h after treatment (a.t.), this effect being more noticeable 24 h a.t. Autofluorescence analysis of leaves treated with botrydial revealed that this toxin also induced the accumulation of phenolic compounds in host tissues, as evaluated both 3 and 24 h a.t.. Moreover, rapid accumulation of reactive oxygen species in botrydial–treated leaves was evidenced by the redox-sensitive dye dihydrofluorescein diacetate. In addition to the above-mentioned biochemical markers of the HR, botrydial was found to cause a rapid and transient increase in mRNA levels of Athsr3 This gene is known to be induced early during the HR of A. thaliana elicited by pathogen attack and is considered as a molecular marker of the HR . In this way, it can be concluded that botrydial is able to induce the HR in plant tissues, this tricyclic sesquiterpene probably being one of the effectors involved in HR elicitation by B. cinerea on its hosts. Additional evidence that botrydial is not a "plain toxin" and that depends on host cell machinery to exert its toxicity was obtained by analyzing the effects of botrydial on genetically modified plants affected in signaling pathways mediated by salicylic acid and jasmonic acid. Botrydial caused an increase in mRNA levels of PR1 and PDF1.2, two pathogenesis related proteins whose expression is respectively mediated by salicylic acid and jasmonic acid signaling. In addition, A. thaliana and tobacco (Nicotiana tabacum) plants defective in salicylic acid signaling were more resistant to botrydial than wild type plants. On the contrary, and interestingly, A.thaliana plants defective in jasmonic acid signaling were more sensitive than wild type plants to botrydial toxicity. Thus, botrydial toxicity was found to depend on host signaling pathways mediated by salicylic acid and jasmonic acid, both hormones seeming to play opposed roles in the development of host cell death induced by this toxin.