Chitinase and β-1,3-glucanase enzymatic activities in response to infection by Alternaria alternata evaluated in two stages of development in different tomato fruit varieties

Chitinase (ChA) and β-1,3-glucanase (GA) activity had been related with plant defense mechanism against pathogen attack in vegetative tissues. Scarce information is available about the behavior of these enzymes in response to different stages of development and fungi infection in fruits. The changes in ChA and GA activities in response to Alternaria alternata infection were evaluated in mature green (MG) and red ripe (RR) developmental stages of Sunpride, Geronimo and Charleston varieties of tomato fruit. Tomato fruits were inoculated with a conidial suspension of A. alternata and stored for 10 days at 25 °C and 90–92 H.R.%. The degree of fruit infection was measured by a hedonic scale every 2 days. ChA activity was determined fluorometrically by quantifying the release of 4-methylumbelliferyll (4-MU) from 4-methylumbelliferyll β-d-N,N′N″-triacetylchitotrioside, and GA activity was measured quantifying the release of glucose from β-1,3-glucan (laminarin) by HPLC. Tomato fruit in RR stage was more susceptible to fungi infection than MG stages. Geronimo was the most resistant variety, whereas Sunpride was the most susceptible for both stages of development (MG and RR). Higher levels of ChA and GA activities were observed for mature green stage in Charleston variety at the end of the storage period. An induction in ChA and GA in response to infection by A. alternata was observed in all varieties. Particularly high levels of ChA were found for inoculated Geronimo in RR stage and inoculated Charleston in MG stage which correlated with low levels of fungi infection. Higher levels of GA induction in response to fungi infection were recorded for Sunpride variety in RR stage, whereas no substantial induction was observed for Geronimo and Charleston varieties at the same stage of development. This GA induction correlated negatively with the resistance showed by the different varieties to fungi infection. We concluded that chitinase and β-1,3-glucanase induction are part of the tomato fruit defense mechanism against A. alternata infection with a different behavior depending upon stage of development and variety.