Caracterització, millora i aplicació de Pèptids Plant Elicitor com a noves eines de control

The Rosaceae family comprise economically highly relevant species e.g., edible temperate zone fruits, ornamentals and medicinal species. Their cultivation systems are constrained by diverse phytopathogens, for example, Xanthomonas arboricola pv. pruni (Xap). Xap is the agent responsible of the bacterial spot of stone fruits, which compromises crop production and causes significant economic losses. Management of this disease is based on treatments with copper derivatives, which are related to toxicity to the host plants, copper-tolerant bacterial strains and environmental pollution. Thus, there is a critical need to develop new strategies for a more efficient and sustainable management of bacterial spot.

Plants respond to invading pathogens through signaling mechanisms, a component of which are of them being plant elicitor peptides (Peps). In the model species Arabidopsis, exogenous application of Peps activates defense mechanisms and reduces the symptoms of pathogen infection by a few pathogens. We previously identified the Rosaceae Peps and showed, in an ex vivo system, that their topical application onto peach leaves efficiently enhanced resistance to challenging doses of Xap. In this Thesis we demonstrate the effectiveness of Prunus persica peptides PpPep1 and PpPep2 in protecting peach plants in vivo at nanomolar doses, with up to 60% reduction of the symptoms following Xap massive infection. PpPep effect on plant protection was dose- and time-dependent; and application of either PpPep1 or PpPep2 in their corresponding optimal conditions resulted in comparable symptom reduction. Combined treatment with the two peptides did not confer extra protection.

We used deep sequencing to characterize the transcriptomic response of peach plants to preventive treatment with PpPep1 and PpPep2 and constructed a P. persica Pep background knowledge network to visualize the transcriptomics dynamics. The two peptides induced highly similar massive transcriptomic reprogramming in the plant. One hour, one day and two days after peptide application there were changes in the expression of up to 8% of peach genes, indicating a PAMP-triggered immunity (PTI) response. Minor variations in plant responses to PpPep1 and PpPep2 might explain why they have different optimal application conditions. Comparison with previously published immune response datasets showed that the plant response to Peps was similar but quicker than the response to pathogen attack. In addition, we observed that 15% of P. persica miRNAs modulate the PTI response one and 24 h after PpPep2 treatment. The protective effect of Peps was demonstrated as well in the Solanaceae, i.e., a different economically relevant plant family, characterized by a specific set of molecules comparable to Peps e.g., systemin. Topical application of SlPep6 on tomato plants reduced symptoms of fungal and bacterial infections.

The significant potential of Peps as natural molecules for protecting crops from pathogen diseases prompted us to address the issue of obtaining the molecules at an affordable cost. We optimized the production and purification of recombinant GST-PpPep1 and GST-PpPep2 in an E. coli system and proved the full protecting effect of these molecules. With these findings PpPeps are closer to emerge as a natural, targeted, safe and environmental-friendly tool to control Xap in peach orchards and possibly, more generally, enhance plant resistance to bacterial and fungal diseases.