Small heat shock proteins and the postharvest chilling tolerance of tomato fruit

Plants have the largest number of small heat shock proteins (sHsps) (15-42 kDa) among eukaryotes, but little is known about their function in vivo. They accumulate in response to different stresses, and specific sHsps are also expressed during developmental processes such as seed development, germination, and ripening. The presence of organelle-specific sHsps appears to be unique to plants. The sHsps expression is regulated by heat stress transcription factors (Hsfs). In this work, it was explored the role of sHsps in the chilling injury of tomato fruit. The level of transcripts and proteins of cytoplasmic and organellar sHsps was monitored in fruit during ripening and after cold storage (4 weeks at 4°C). Expression of HsfA1, HsfA2, HsfA3, and HsfB1 was also examined. Two cultivars of tomato (Solanum lycopersicum) contrasting in chilling tolerance were assayed: Micro-Tom (chilling-tolerant) and Minitomato (chilling-sensitive). Results showed that sHsps were induced during ripening in fruit from both cultivars. However, sHsps were induced in Micro-Tom fruit but not in Minitomato fruit after storage at a low temperature. In particular, sHsp 17.4-CII and sHsp23.8-M transcripts strongly accumulated in Micro-Tom fruit and HsfA3 transcript diminished after cold storage. These data suggest that sHsps may be involved in the protection mechanisms against chilling stress and substantiate the hypothesis that sHsps may participate in the mechanism of tomato genotype chilling tolerance.