Ectopic overexpression of ShCBF1 and SlCBF1 in tomato suggests an alternative view of fruit responses to chilling stress postharvest

Postharvest chilling injury (PCI) is a physiological disorder that often impairs tomato fruit ripening; this reduces fruit quality and shelf-life, and even accelerates spoilage at low temperatures. The <i>CBF</i> gene family confers cold tolerance in <i>Arabidopsis thaliana</i>, and constitutive overexpression of <i>CBF</i> in tomato increases vegetative chilling tolerance, in part by retarding growth, but, whether CBF increases PCI tolerance in fruit is unknown. We hypothesized that <i>CBF1</i> overexpression (OE) would be induced in the cold and increase resistance to PCI. We induced high levels of <i>CBF1</i> in fruit undergoing postharvest chilling by cloning it from <i>S. lycopersicum</i> and <i>S. habrochaites</i>, using the stress-inducible <i>RD29A</i> promoter. Harvested fruit were cold-stored (2.5°C) for up to three weeks, then rewarmed at 20°C for three days. Transgene upregulation was triggered during cold storage from 8.6- to 28.6-fold in <i>SlCBF1</i>-OE, and between 3.1- to 8.3-fold in <i>ShCBF1</i>-OE fruit, but developmental abnormalities in the absence of cold induction were visible. Remarkably, transgenic fruit displayed worsening of PCI symptoms, i.e., failure to ripen after rewarming, comparatively higher susceptibility to decay relative to wild-type (WT) fruit, lower total soluble solids, and the accumulation of volatile compounds responsible for off-odors. These symptoms correlated with <i>CBF1</i> overexpression levels. Transcriptomic analysis revealed that the ripening and biotic and abiotic stress responses were altered in the cold-stored transgenic fruit. Seedlings grown from 'chilled' and 'non-chilled' WT fruit, in addition to 'non-chilled' transgenic fruit were also exposed to 0°C to test their photosynthetic response to chilling injury. Chilled WT seedlings adjusted their photosynthetic rates to reduce oxidative damage; 'non-chilled' WT seedlings did not. Photosynthetic parameters between transgenic seedlings were similar at 0°C, but <i>SlCBF1</i>-OE showed more severe photoinhibition than <i>ShCBF1</i>-OE, mirroring phenotypic observations. These results suggest that 1) <i>CBF1</i> overexpression accelerated fruit deterioration in response to cold storage, and 2) Chilling acclimation <i>in fructus</i> can increase chilling tolerance in seedling progeny of WT tomato.