A hot water treatment induces resistance to Penicillium digitatum and promotes the accumulation of heat shock and pathogenesis-related proteins in grapefruit flavedo

A new postharvest hot water brushing (HWB) treatment, which sprays hot wate r on fruit as they move along a bell of brush-rollers, induced resistance a gainst green mould decay caused by Penicillium digitatum (Pers, Fr.) Sacc i n 'Star Ruby' grapefruit. The HWB treatment (62 degreesC for 20 s) was most effective in inducing disease resistance when the fruit were inoculated af ter 1 and 3 days, but was less effective when the fruit were inoculated on the same day or 7 days later. The HWB treatment induced the accumulation of a 105-kDa protein that cross-reacted with an antibody raised against a bov ine heat shock protein 70 (HSP70) and 18- and 21-kDa proteins that cross-re acted with pea HSP18 and HSP21 antibodies. The accumulation of a grapefruit 17-kDa protein that cross-reacted with a pea HSP17 antibody was not affect ed by the HWB treatment, HWB also induced the accumulation of 21-, 22 and 2 5-kDa proteins that cross-reacted with citrus and tobacco chitinase antibod ies and 38-, 42- and 43-kDa proteins that cross-reacted with citrus and tob acco beta -1,3-glucanase antibodies. The induction of the 105-, 18- and 21- kDa HSPs by the HWB treatment indicates that it was sufficient to provide a heat stress and, thus, was able to induce biochemical changes in the fruit peel tissue. Nevertheless, the accumulation of these HSPs was probably not related to the induction of fruit resistance against P. digitatum, since t heir accumulation could not be induced neither by direct inoculation with t he pathogen nor following exposure to other treatments, such as UV irradiat ion, which also induce disease resistance, On the other hand, the increases in the accumulation of the 21-, 22- and 25-kDa chitinase proteins and of t he 38- and 43-kDa beta -1,3-glucanases proteins, which were observed 1 and 3 days after the HWB treatment when the fruit appeared to be more resistant to P, digitatum, may be part of the complex fruit disease resistance mecha nisms induced by the heat treatment.