Heat shock protein 70 and defence responses in plants: salicylic acid and programmed cell death.

Background: Heat-shock (HS) proteins (HSP) are induced or increasingly expressed to protect against lethal environmental stresses. Hsp70 in particular, provides protection against various stresses including oxidative stress, is implicated in thermotolerance and appears to have an anti-apoptotic function. Anti-inflammatory salicylates potentiate the induction of the 70 kDa HSP (Hsp70) in mammals in response to HS, enhance thermotolerance and induce apoptosis. In plants, salicylic acid (SA) is a natural signalling molecule, mediating resistance in response to avirulent pathogens. The effects of salicylic acid-mediated increases in Hsp70/Hsc70 expression and its relation to events associated with PCD/ apoptosis in plants are unknown. Hypothesis and Objectives: The hypothesis studied in this investigation was that SA influences Hsp70 expression similar to that found in mammalian cells and may influence the choice between survival or death, whether apoptosis or necrosis. In order to verify this hypothesis the effect of SA alone or in combination with HS on Hsp70/Hsc70 accumulation and events associated with apoptosis were investigated through three main objectives: 1) Determine whether SA in plants, as in mammalian cells, can potentiate heat-induced Hsp70/Hsc70 accumulation or induce Hsp70/Hsc70 by itself at elevated levels. This was done by investigating the effect of SA at various concentrations on Hsp70/Hsc70 expression at normal temperatures or following heat. 2) Establish flow cytometry as a rapid and quantitative alternative for the evaluation of Hsp70 accumulation in plant protoplasts to be evaluated in concert with various parameters indicative of cellular integrity. 3) Investigate whether Hsp70/Hsc70 expression modulated by SA influences cell death (apoptosis/necrosis) or associated events such as mitochondrial membrane permeability (MMP) or reactive oxygen species (ROS) in plant protoplasts using flow cytometry. Materials and Methods: The effect of SA alone or in combination with HS on Hsp70/Hsc70 levels in tomato cells was investigated using biometabolic labelling and Western blotting. A flow cytometric assay was developed to determine Hsp70/Hsc70 levels in tobacco protoplasts. MMP and ROS were monitored by the fluorescent probes DiIC1(5) and H2DCFDA respectively, phosphatidylserine externalisation by annexin V binding and DNA fragmentation by the TUNEL assay in protoplasts treated with SA and/or HS. Results: Results obtained in the attainment of the three main objectives were: 1) In plants, as in mammals, low concentrations of SA do not induce Hsp70/Hsc70 but significantly potentiate heat-induced Hsp70/Hsc70 levels while cytotoxic levels significantly induce Hsp70/Hsc70. In cell suspension cultures, this induction was preceded by increased membrane permeability. 2) Flow cytometry can be implemented as a rapid, quantitative alternative to detect intracellular Hsp70/Hsc70 accumulation in protoplasts. 3) In protoplasts exposed to low doses of SA at normal temperatures, PCD/apoptosis is increased as reflected by increased DNA fragmentation and phosphatidylserine externalisation, but not by increased MMP or ROS. High doses of SA were associated with increased levels of necrosis. Exposure of protoplasts to low doses of SA in combination with HS showed suppression of PCD/apoptosis (reflected by decreased DNA fragmentation and phosphatidylserine externalisation), accompanied by decreased levels of ROS and increased MMP. Discussion: These results suggest that SA-mediated increases in Hsp70/Hsc70 accumulation at normal temperatures are associated with cellular damage and protect cells against necrosis. On the other hand, low doses of SA that potentiate heat-induced Hsp70/Hsc70 accumulation abrogated the induction of apoptosis that was induced by low doses of SA at normal temperatures. The anti-apoptotic effects of Hsp70 could therefore influence plant resistance by interfering with the execution of PCD. These results could contribute to our understanding of heat-induced disease susceptibility, and the manipulation of SA-modulated Hsp70/Hsc70 should be carefully considered in the light of its ability to affect cell death, which may be advantageous or deleterious to the plant cell.Prof. L. Bornma