SYSTEMIN INVOLVEMENT IN TOMATO DEFENSE PRIMING

As sessile organisms, plants cannot escape stress conditions so they are obliged to develop fine and elaborated defense strategies to protect themselves against different threats. Plants defend themselves against biotic and abiotic stresses via both constitutive and inducible defenses. Based on the mode of action, plant resistance traits can be distinguished in direct and indirect defenses. Volatile organic compounds (VOCs) that are released in large amount in response to herbivory and wounding, play an important role in indirect defense by the attractiveness towards natural enemies of insect pests. VOCs are also involved in a plant-to-plant communication mechanism called “priming” in which neighbouring plants (receivers) to infested plants (emitters), can activate their own defenses (primed state) (Conrath, 2011). In the Solanaceae, a family of defense-related peptide hormones called systemins are involved in the activation of defense genes in response to wounding and herbivore attacks (Ryan and Pearce 2003). Systemin (Sys) is a 18-amino-acid peptide hormone, which is initially released at wound sites, representing a primary wound signal in tomato. It activates defense genes involved in the octadecanoid signaling pathway, which leads to the production of jasmonates and C6 volatile compounds involved in direct and indirect defenses (Ryan, 2000; Corrado et al., 2007; El Oirdi et al., 2011). The aim of this research activity is to shed more light on the molecular and chemical basis of plant defense priming; therefore, the project evaluated the involvement of systemin in the defense priming of tomato plants. Firstly, the research focused on the evaluation of direct defenses activation following Sys peptide foliar applications through the study of gene expression profiles. Once established its effectiveness in defense response activation, its attitude to induce priming in neighboring plants was evaluated. To these aims, the expression analysis of defense related genes has been carried out in receiver plants exposed to volatiles released by three different kinds of sources: plant chewed by Spodoptera littoralis (S1), transgenic plants constitutively expressing ProSys (S2), plants treated with peptide Sys (S3). The results of these analyses underlined a modulation various and continuous of gene expression indicating that the different VOCs blends were perceived by receiver plants which resulted modified in their defense gene expression profiles. The exposed plants were found to be more tolerant to Spodoptera littoralis that resulted compromised in its growth and survival rates. Moreover, plants exposed to the different VOCs sources were more attractive towards the parasitoid of aphids, Aphidius ervi compared to the control. In order to get a wider overview on transcriptome reprogramming following exposure to S3 plants a RNAseq was performed. A total number of 1118 differentially expressed genes was identified. Among them, 537 genes were up-regulated while 581 were down-regulated. The resulting molecular functions and genes were found to be associated with variation in gene expression related to metabolism and stress response. All together the results indicate that Sys foliar application influences tomato defenses not only of the treated plants but also in the sourrounding undamaged plants via airborne signals. These are very interesting findings which suggest a possible use of a peptide to promote crop protection with natural molecules, according to the necessity of a sustainable agriculture.