During tomato fruit ripening, photosynthetically competent thylakoid membranes are broken down and replaced by membranous deposits of carotenoids. Few of the proteins involved in this transition have been identified. We have used chloroplast protein import assays as a means to identify two cDNAs that encode proteins destined for the developing chromoplast. One of the cDNAs had unexpected properties and its biological function has not been determined. However, the other cDNA encodes a plastid-localized low-MW heat shock protein (hsp). The steady-state level of RNA corresponding to this cDNA increased several-fold during tomato ripening, and the amount of RNA induced by heat stress increased dramatically during this process. These observations suggest a new role for this stress protein in protecting the plastid during the dismantling of the thylakoid membranes or during the buildup of carotenoids.

Heat treatment of tomato fruits induces tolerance to chilling injury. We have previously shown that specific heat-shock proteins (HSPs) are expressed in heated tomato fruits after cold storage. To search for heat-induced genes that are expressed at low temperatures, a cDNA library prepared from pre-heated chilled tomato fruits was differentially screened. A novel cDNA clone, hcit2, encoding a protein of ca. 16.5 kDa, was isolated. The predicted protein contains three putative trans-membrane hydrophobic sequences, suggesting that the protein is membrane-localized. The expression of hcit2 in fruits was induced by high temperature, but not by other stresses such as low temperature, drought or anaerobic conditions, and not during fruit ripening. A high level of hcit2 transcript was found in heated fruits after 2 weeks at 2 degrees C. High temperatures also induced hcit2 expression in tomato leaves, flowers and stems. The HCIT2 protein may be involved in the acquisition of tolerance to chilling injury.

The tomato (Lycopersicon esculentum) chloroplast small heat shock protein (sHSP), HSP21, is induced by heat treatment in leaves, but also under normal growth conditions in developing fruits during the transition of chloroplasts to chromoplasts. We used transgenic tomato plants constitutively expressing HSP21 to study the role of the protein under stress conditions and during fruit maturation. Although we did not find any effect for the transgene on photosystem II (PSII) thermotolerance, our results show that the protein protects PSII from temperature-dependent oxidative stress. In addition, we found direct evidence of the protein's role in fruit reddening and the conversion of chloroplasts to chromoplasts. When plants were grown under normal growth temperature, transgenic fruits accumulated carotenoids earlier than controls. Furthermore, when detached mature green fruits were stored for 2 weeks at 2 degrees C and then transferred to room temperature, the natural accumulation of carotenoids was blocked. In a previous study, we showed that preheat treatment, which induces HSP21, allowed fruit color change at room temperature, after a cold treatment. Here, we show that mature green transgenic fruits constitutively expressing HSP21 do not require the heat treatment to maintain the ability to accumulate carotenoids after cold storage. This study demonstrates that a sHSP plays a role in plant development under normal growth conditions, in addition to its protective effect under stress conditions.