Functional characterization of a tomato COBRA-like gene functioning in fruit development and ripening

BMC Plant Biology, Nov 2012

Background Extensive studies have demonstrated that the COBRA gene is critical for biosynthesis of cell wall constituents comprising structural tissues of roots, stalks, leaves and other vegetative organs, however, its role in fruit development and ripening remains largely unknown. Results We identified a tomato gene (SlCOBRA-like) homologous to Arabidopsis COBRA, and determined its role in fleshy fruit biology. The SlCOBRA-like gene is highly expressed in vegetative organs and in early fruit development, but its expression in fruit declines dramatically during ripening stages, implying a primary role in early fruit development. Fruit-specific suppression of SlCOBRA-like resulted in impaired cell wall integrity and up-regulation of genes encoding proteins involved in cell wall degradation during early fruit development. In contrast, fruit-specific overexpression of SlCOBRA-like resulted in increased wall thickness of fruit epidermal cells, more collenchymatous cells beneath the epidermis, elevated levels of cellulose and reduced pectin solubilization in the pericarp cells of red ripe fruits. Moreover, transgenic tomato fruits overexpressing SlCOBRA-like exhibited desirable early development phenotypes including enhanced firmness and a prolonged shelf life. Conclusions Our results suggest that SlCOBRA-like plays an important role in fruit cell wall architecture and provides a potential genetic tool for extending the shelf life of tomato and potentially additional fruits.

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Functional characterization of a tomato COBRA-like gene functioning in fruit development and ripening

Ying Cao 1 Xiaofeng Tang 1 Jim Giovannoni Fangming Xiao Yongsheng Liu 0 1 0 School of Biotechnology and food Engineering, Hefei University of Technology , Hefei 230009 , China 1 Ministry of Education Key Laboratory for Bio-resource and Eco-environment, College of Life Science, State Key Laboratory of Hydraulics and Mountain River Engineering, Sichuan University , Chengdu 610064 , China Background: Extensive studies have demonstrated that the COBRA gene is critical for biosynthesis of cell wall constituents comprising structural tissues of roots, stalks, leaves and other vegetative organs, however, its role in fruit development and ripening remains largely unknown. Results: We identified a tomato gene (SlCOBRA-like) homologous to Arabidopsis COBRA, and determined its role in fleshy fruit biology. The SlCOBRA-like gene is highly expressed in vegetative organs and in early fruit development, but its expression in fruit declines dramatically during ripening stages, implying a primary role in early fruit development. Fruit-specific suppression of SlCOBRA-like resulted in impaired cell wall integrity and up-regulation of genes encoding proteins involved in cell wall degradation during early fruit development. In contrast, fruit-specific overexpression of SlCOBRA-like resulted in increased wall thickness of fruit epidermal cells, more collenchymatous cells beneath the epidermis, elevated levels of cellulose and reduced pectin solubilization in the pericarp cells of red ripe fruits. Moreover, transgenic tomato fruits overexpressing SlCOBRA-like exhibited desirable early development phenotypes including enhanced firmness and a prolonged shelf life. Conclusions: Our results suggest that SlCOBRA-like plays an important role in fruit cell wall architecture and provides a potential genetic tool for extending the shelf life of tomato and potentially additional fruits. - Background The ripening of fleshy fruits involves a number of physiological processes including the production of aromatic compounds, nutrients, pigmentation, and softening of flesh to an edible texture [1,2]. These processes have direct impacts not only on fruit firmness, color, flavor and nutritional content, but also on shelf life, consumer acceptability, processing qualities, in addition to pre- and postharvest disease resistance [1,2]. Excessive fruit softening is the main factor contributing to damage during shipping, storage and post-harvest handling [3]. Fruit firmness and texture also affect the integrity of chopped and diced fruit used for canning and fruit products [4]. Because postharvest losses of fresh fruits due to excessive softening can account for as much as 30~40% of total production, considerable research had focused on mechanisms of fruit softening, often using tomato (Solanum lycopersicum) as a model system [3]. Fruit softening during the ripening process results in part from disassembly of the cell walls, leading to a reduction in intercellular adhesion, depolymerization and solubilization of pectins, depolymerization of hemicelluloses, and loss of pectic galactose side chains [3]. Generally, the decline in fruit firmness due to softening is accompanied by elevated expression of numerous cell metabolism enzymes, including polygalacturonase (PG) [5,6], pectin methylesterase (PME) [7], -galactosidase [8], as well as cell wall loosening proteins such as expansin [9,10]. Suppression of single genes encoding fruit PG [3,11] or PME [7,12] in transgenic tomato plants had limited impact on fruit softening during ripening, but conferred longer shelf life resulting from reduced susceptibility to postharvest pathogens. These results suggest that suppression of certain enzymes acting on cellulose, hemicellulose or pectin alone are not sufficient to prevent softening, likely due to functional redundancy of enzymes involved in what is likely a complex metabolic process [1]. Nevertheless, a recent study has shown that down-regulation of genes encoding the N-glycan processing enzymes -mannosidase and -D-N-acetylhexosaminidase significantly increased fruit shelf life, which was attributed to decreased softening during ripening [2]. These enzymes have been shown to break glycosidic bonds between carbohydrates, or between carbohydrates and noncarbohydrate structural molecules [13]. Expansins are cell wall-localized proteins faciliating wall loosening. They are involved in many aspects of cell wall modification during development through disruption of non-covalent bonds between matrix glycans and cellulose microfibrils [9,10,14,15]. Transgenic silencing of the tomato expansin gene LeExp1 resulted in increased fruit firmness throughout ripening and improved fruit integrity during storage [16]. Molecular and genetic investigations have identified additional regulators of cell wall biosynthesis and regulation of cell expansion. One such activity is encoded by the COBRA gene previously reported in Arabidopsis, rice and maize [17-20]. The COBRA gene (...truncated)


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Ying Cao, Xiaofeng Tang, Jim Giovannoni, Fangming Xiao, Yongsheng Liu. Functional characterization of a tomato COBRA-like gene functioning in fruit development and ripening, BMC Plant Biology, 2012, pp. 211, 12, DOI: 10.1186/1471-2229-12-211