Reduction of chilling injury and transcript accumulation of heat shock proteins in tomato fruit by methyl jasmonate and methyl salicylate

Abstract

Changes in heat shock protein (HSP) gene expression induced by vapor application of methyl jasmonate (MeJA) and methyl salicylate (MeSA) in tomato fruit were investigated and compared to the well-described heat shock response. Northern hybridization experiments involving six cDNAs, encoding class I and II tomato small HSPs (sHSPs) and three members of HSP 70 family, showed that accumulation of class I and II sHSP mRNAs was increased significantly by MeJA and MeSA. When the treated fruits were transferred to low temperature, class I and II mRNA levels initially decreased, but then subsequently increased. Accumulation of HSP transcripts was also observed in non-treated fruit between 7 and 14 days at low-temperature storage, but all decreased to undetectable levels after 21 days. Following MeJA and MeSA treatments, the transcripts of HSP 70 family accumulated to higher levels than following the heat treatment. MeJA- and MeSA-treatments were clearly shown to alleviate chilling injury (CI), whereas tomato fruit stored at 5 °C without pretreatment developed typical symptoms and severe decay. These results demonstrated that MeJA and MeSA induced the accumulation of sHSP transcripts in tomato. The increased transcript abundance of HSPs, especially class II sHSPs, was correlated with protection against CI.

Introduction

One of the main postharvest problems affecting tropical and subtropical commodities is their sensitivity to low temperature, resulting in chilling injury (CI). This phenomenon limits storage life and leads to significant degradation of produce quality. Apart from the visible symptoms, several biochemical and physiological processes are altered as a consequence of the direct effects of low temperature on cellular constituents [1]. Therefore, various methods have been developed to alleviate CI symptoms [2].

High-temperature stress has been found to protect a number of fruits and vegetables, including tomatoes, against CI. The protection afforded by heat shock against CI in tomato was found to persist up to 21 days at 2 °C [3]. All organisms respond to high temperatures by inducing the synthesis of a small group of evolutionarily conserved polypeptides known as heat shock proteins (HSPs). Plants synthesize numerous small HSPs (sHSPs), ranging from 15 to 40 kDa and have been divided into six classes according to their subcellular localization, cross-reactivity and amino acid sequence homology [4]. Stress 70 molecular chaperones include most HSPs and their homologues—heat shock cognate (HSC). In most organisms, the HSPs and HSCs are encoded by a multigene family and found in all of the major subcellular compartments of plant cells [5]. The expression of the stress 70 molecular chaperones in response to heat shock is well known and it appears that low-temperature exposure can also stimulate their expression. Overall, the HSPs and the heat shock responses of plants, animals and prokaryotes are highly conserved [6], suggesting that they perform a universally basic and essential function during high-temperature stress. Recent studies with heat-stressed tomato fruits have shown a correlation between the accumulation of sHSP and the acquisition of chilling tolerance [7], [8]. The results suggest that HSPs could be involved to some extent in the ability of heat shock to increase chilling tolerance.

Jasmonic acid (JA) and its methyl ester, methyl jasmonate (MeJA), have been found to occur naturally in a wide range of higher plants. This compound, defined as a natural plant growth regulator, was found to be active in many physiological systems. In many cases, it has an action similar to that of abscisic acid (ABA). Recent studies demonstrated that jasmonate could induce a response similar to that of ABA in plants for alleviating CI [9], [10]. In addition, some specific proteins induced by JA are similar to those induced by heat shock [11]. However, there are no reports of the expression of HSP genes triggered by JA or MeJA in association with the chilling tolerance in plants.

Salicylic acid (SA) is a natural signaling molecule, mediating resistance in response to avirulent pathogens. In mammals, prior treatment with moderate levels of SA potentiates the induction of HSP 70 in response to heat stress [12]. In plants, SA is endogenously synthesized, playing an essential role in thermogenesis and in the defense against pathogen attack [13]. Despite a proposed evolutionary relationship between HSP and pathogenesis-related (PR) proteins, little attention has been paid to the induction of HSP by SA in plants. If SA potentiates the heat shock response in plants, as in mammals, SA pretreatment could enhance SA-related defense gene activation and potentially protect plants from temperature stress. However, the effect of SA on HSP expression in plants has not been investigated.

HSPs are also induced by other stresses such as cold and drought [14], [15], [16]. These HSPs are part of a group of proteins induced by environmental stresses either to protect the plant from damage or to help repair the damage caused by the stress. Similarly, pre-exposure of cells to low concentration of H₂O₂, UV, cold, drought, or salinity stresses often allows the development of tolerance towards a lethal concentration of that same reagent [17]. Some researchers dealing with stress physiology in plants have hypothesized that there is a common denominator for the mechanism of inducing tolerance with various stress treatments, e.g. when stress treatments are applied at mild levels that do not cause appreciable damage. According to this hypothesis, plants respond with similar defense systems to a wide range of stresses such as osmotic, chemical pollutants, oxidative, salinity, cold or heat, UV, low oxygen, pathogen infection, and wounding.

In order to determine whether the mechanism of alleviating CI involves the expression of plant HSP genes, this study was undertaken to investigate the relationships of MeJA, MeSA and heat treatments to the expression of plant HSP genes. It is important to determine which members of the HSP family respond to protect against CI in the future search for ‘stress-labile’ proteins or processes in chilling sensitive commodities.