POLYSACCHARIDES FROM ALOE BARBADENSIS REDUCE THE SEVERITY OF BACTERIAL SPOT AND ACTIVATE DISEASERELATED PROTEINS IN TOMATO
C. Luiz, R.B. Felipini, M.E.B. Costa, R.M. Di Piero
Bacterial spot is an important disease of tomato that causes a reduction of the leaf area and affects plant yield. The objective of this study was to evaluate the effect of polysaccharides from aloe vera (Aloe barbadensis Miller) parenchyma (PAV) on the severity of bacterial spot and to detect the defense mechanisms activated by them. Tomato plants at the five true leaf stage were treated with PAV, and inoculated with Xanthomonas gardneri four days later. The effect of heating PAV suspension on its effectiveness for disease control was also evaluated. The ability of the polysaccharides to induce the synthesis of pathogenesis-related proteins in plants was determined spectrophotometrically. Finally, the antimicrobial effect of PAV on X. gardneri cultures was checked. PAV heated at 100°C for 30 min reduced bacterial spot severity by up to 85.1%, but control level was decreased when the PAV heating time was doubled. Concerning peroxidase, polyphenoloxidase, and glucanase activities in tomato plants treated with PAV, a significant increase of these enzymes was found, even before challenging plants with X. gardneri. Although PAV showed little direct effect on the bacteria it seems to constitute a good alternative for the control of bacterial spot of tomato through induced resistance.