Dottorato in BIOLOGIA CELLULARE E DELLO SVILUPPO

Titolo della tesi: Advances in understanding the role of pectic enzymes in plant metabolism and responses to biotic stresses

Ascorbic acid (AsA) participates in modulation of redox state and is involved in responses against biotic stresses. A previously generated Solanum pennellii in S. lycopersicum introgression sub-line of the region 12-4 (IL12-4-SL), harbouring a QTL on chromosome 12 that leads to an increase of AsA content in fruits, through the D-galacturonate alternative pathway, has been exploited. In this thesis the level of AsA in leaves of IL12-4-SL plants was evaluated in comparison with the M82 commercial variety and it was elucidated that specific pectin methylesterases (PMEs), polygalacturonases (PGs) and UDP-D-glucuronic-acid-4-epimerase (UGlcAE) isoforms, are involved in the AsA metabolism, mediated by the D-galacturonate pathway in tomato leaves. Pathogenicity assays were carried out in tomato M82 and IL12-4-SL leaves infected with B. cinerea to compare the susceptibility of these genotypes to the pathogen. The significant reduction of susceptibility to the pathogen was observed in IL 12-4-SL plants compared to leaves of M82 plants. Moreover the relationships among AsA content, pectic enzyme expression and CW modifications have been analysed to verify their possible contribution in plant resistance to the necrotrophic pathogen B.cinerea. Interestingly the expression of SolyProPME15 (Solyc01g091050) gene was induced during B. cinerea infection. AtProPME17, the Arabidopsis homologue of S.lycopersicum ProPME15, is a type 2 PME. The expression of the AtProPME17 isoform is strongly induced during infection of multiple pathogens and is considered a marker of pathogenesis. The aim of the second part of the thesis was to purify AtProPME17 as well as AtPME17 catalytic subunit, to contribute in the understanding of structure-function relationships of type 2 PME isoforms and in particular of this peculiar PME isoform. The long ProPME unprocessed form of the protein was purified after its expression in Escherichia coli and Pichia pastoris to accumulate the quantity of purified enzyme required to perform biochemical analyses. The intra-molecular inhibitory effect of the Pro-region on the enzymatic activity of the catalytic region was confirmed in the protein expressed in P. pastoris indicating that the glycosylation of the protein did not affect the intra-molecular inhibitory activity. The kinetic parameters of the catalytic region expressed in P. pastoris were determined to further characterize this interesting plant PME isoform.