Heating tomato fruit (Lycopersicon esculentum) for 48 h at 38°Cprevented chilling injury from developing after 21 d at 2"C, whereas unheated fruit developed high levels of injury. Although the overall protein pattern as seen by Coomassie blue staining was similar from heated and unheated fruit, some high-and many low-molecularmass proteins were observed in the heated fruit that were absent or present in reduced amounts in unheated fruit. When fruit were injected with [35S]methionine at harvest and then heated, they accumulated high levels of specific radiolabeled proteins that could still be detected after 21 d at 2°C. If the fruit were held at 20°C after heating, the label in the proteins declined rapidly and these fruit were also sensitive to chilling injury. Hsp70 antibody reacted more strongly with proteins from heated and chilled fruit than with proteins from chilled fruit. Hspl8.1 antibody reacted strongly with proteins from heated fruit but not with those from unheated fruit. A 23-kD protein, highly labeled in heated fruit but not in unheated fruit, had its amino terminus sequenced. To our knowledge, this is the first report showing a relationship between the persistence of heat-shock proteins and chilling tolerance in a plant tissue.

Tissue softening accompanies the ripening of many fruit and initiates the processes of irreversible deterioration. Expansins are plant cell wall proteins proposed to disrupt hydrogen bonds within the cell wall polymer matrix. Expression of specific expansin genes has been observed in tomato (Lycopersicon esculentum) meristems, expanding tissues, and ripening fruit. It has been proposed that a tomato ripening-regulated expansin might contribute to cell wall polymer disassembly and fruit softening by increasing the accessibility of specific cell wall polymers to hydrolase action. To assess whether ripening-regulated expansins are present in all ripening fruit, we examined expansin gene expression in strawberry (Fragaria x ananassa Duch.). Strawberry differs significantly from tomato in that the fruit is derived from receptacle rather than ovary tissue and strawberry is non-climacteric. A full-length cDNA encoding a ripening-regulated expansin, FaExp2, was isolated from strawberry fruit. The deduced amino acid sequence of FaExp2 is most closely related to an expansin expressed in early tomato development and to expansins expressed in apricot fruit rather than the previously identified tomato ripening-regulated expansin, LeExp1. Nearly all previously identified ripening-regulated genes in strawberry are negatively regulated by auxin. Surprisingly, FaExp2 expression was largely unaffected by auxin. Overall, our results suggest that expansins are a common component of ripening and that non-climacteric signals other than auxin may coordinate the onset of ripening in strawberry.

Environmental stresses dramatically affect plant survival and productivity. Because plants are immobile, presumably different strategies are required for protection against transient stresses. Under stress, plants synthesize specific proteins, and their accumulation has a role in protecting the tissue from possible damage. An increasing number of studies show the existence of cross‐tolerance in plants: Exposure of tissue to moderate stress conditions often induces resistance to other stresses. Many varied mechanisms explaining the phenomenon of cross‐tolerance have been proposed, and they often, but not always, suggest that specific proteins are induced by one kind of stress and are involved in the protection against other kinds. Although various cross‐protections have been demonstrated in a number of plants, a common mechanism has not been found. This review discusses heat‐shock proteins and their possible roles in protecting the plant under heat and other stresses.

We previously reported that short exposure of tomato (Lycopersicon esculentum L.) fruits to high temperature protects them from chilling injury. To study the involvement of heat-shock proteins (HSPs) in the acquisition of low-temperature tolerance, we cloned two heat-shock-induced genes that are also expressed at low temperatures. The cloned cDNAs belong to the small HSP group. Sequence analyses of the clones showed perfect homology to the tomato-ripening gene tom66 and to the tomato chloroplastic HSP21 gene tom111. The expression of both genes was induced by high temperature in fruits, flowers, leaves, and stems, but not by low or ambient temperatures or by other stresses such as drought and anaerobic conditions. When the heated fruits were transferred to low temperature, tom66 and tom111 mRNA levels first decreased but were then reinduced. Induction was not observed in nonheated fruits at low temperature. Immunodetection of tom111-encoded protein indicated that this protein is present at low temperatures in the heated fruits. The results of this study show that the expression of tom66 and tom111 is correlated with protection against some, but not all, symptoms of chilling injury.Plants are sensitive to both high and low temperatures. Both extremes inhibit photosynthesis, growth, pollination, fruit set, and fruit development (Vierling, 1991). Plants from temperate zones are less sensitive to low than to high temperatures, whereas the opposite is true for plants originating from tropical or subtropical areas. The latter include tree crops such as mango, avocado, and banana, as well as many vegetables such as cucumber, pepper, squash, and tomato. However, plants from both groups can be damaged by extended exposure to both high and low temperatures (Wang, 1994). Plants can be partially protected against extreme temperatures if the conditions are gradually changed. Gradually raising the temperature to 38°C allows plants to acclimate and tolerate further increases to temperatures that are normally lethal (Vierling, 1991). Conversely, holding plants at 15°C acclimates them to lower temperatures, which would normally cause chilling injuries (Wang, 1994). Since these adaptations can be prevented by cycloheximide, it is reasonable to assume that de novo protein synthesis is required for protection against both high and low temperatures (Vierling, 1991).All organisms respond to high temperatures by inducing the synthesis of a small group of evolutionarily conserved polypeptides known as HSPs. Some HSPs are required for normal growth at the upper end of their normal growthtemperature range, whereas others help cells withstand the toxic effects of extreme temperatures (Yost et al., 1990). Plants synthesize numerous smHSPs, ranging from 15 to 30 kD, that are related to the smHSPs of other organisms and to the ␣-crystallins of the vertebrate eye lens (Inoglia and Craig, 1982). In contrast to mammalian smHSPs, those from plants constitute the most abundant and diverse group of proteins synthesized in response to heat stress...

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