Ethylene (C2H4) is a gaseous hormone that is produced by plants during growth and development and in response to environmental stress (Abeles et al., 1992). At the seedling stage, ethylene causes reduced hypocotyl elongation, increased hypocotyl thickening, and an exaggerated apical hook. Collectively, these symptoms are known as the triple response (Knight et al., 1910). Exposure to ethylene at plant maturity can lead to flower, bud, or leaf abscission; flower senescence; leaf chlorosis (yellowing); or epinasty (downward curvature of the leaf or petiole) (Abeles et al., 1992). The quality of ornamental plants may be reduced by exposure to ethylene during production, shipping, and retailing (Jones and Edelman, 2013; Jones and Ling, 2012). Ethylene also controls fruit ripening in many species, and the ripening process is accompanied by a burst of ethylene production (Alexander and Grierson, 2002; Burg and Burg, 1965). Decreasing ethylene production and inhibiting ethylene perception increase the postharvest shelf life of ethylene-sensitive fruits, cut flowers, and potted plants.
Most evaluations of ethylene sensitivity have been performed on mature plants or on detached organs (i.e., fruits or cut flowers) (Archambault et al., 2006; Dole et al., 2009; Goto et al., 1999; Serek et al., 1994, 1995; van Doorn, 2001, 2002; Woltering, 1987; Woltering and van Doorn, 1988). A few studies have used the seedling triple response to evaluate differences in ethylene sensitivity between plants (Clark et al., 2001; Edelman et al., 2014; Lanahan et al., 1994). The seedling triple response screen has been extremely successful at identifying ethylene biosynthesis and signaling mutants in tomato (Solanum lycopersicum) and arabidopsis (Arabidopsis thaliana) (Alexander and Grierson, 2002; Binder et al., 2004). In dark-grown seedlings, hypocotyl length decreases at increasing concentrations of ethylene or ACC, the immediate precursor to ethylene. This component of the triple response provides an easy screen for evaluating ethylene sensitivity. Never ripe (nr) tomato mutants were determined to be ethylene-insensitive because seedlings did not exhibit any symptoms of the triple response when grown on media containing ACC (Lanahan et al., 1994). In contrast, ACC-treated wild-type ‘Pearson’ seedlings exhibited a significant reduction in hypocotyl length. When mature plants were infiltrated with ACC, epinasty was also observed in the wild-type plants but not the nr mutants.
Solanaceae is a relatively large and very diverse family that includes many plants that are highly sensitive to ethylene. The family contains ≈90 genera consisting of 3000 to 4000 species, which include edible, ornamental, and medicinal plants (Bombarely et al., 2011). Solanum melongena (eggplant), Capsicum annum (pepper), Solanum tuberosum (potato), Solanum lycopersicum (tomato), and Physalis ixocarpa (tomatillo) are important food crops, whereas Calibrachoa ×hybrida, Petunia ×hybrida, and the genus Nicotiana include popular ornamentals. Nicotiana, petunia, and tomato are also widely used as model experimental plants for studying development and plant–pathogen interactions (Gerats and Vandenbussche, 2005; Goodin et al., 2008; Wing et al., 1994). Although studies evaluating ethylene sensitivity have been published for multiple Solanaceous plants at the seedling or mature plant stages (Chaabouni et al., 2009; Edelman et al., 2014; Lanahan et al., 1994; van Doorn, 2001, 2002), a more comprehensive screen comparing sensitivity among genera, species, and cultivars within this family has not been conducted.
We have evaluated ethylene sensitivity in 41 different plant accessions. This study included two objectives: 1) to identify ethylene sensitivity differences (levels of sensitivity and symptoms) between accessions within the Solanaceae family; and 2) to identify ethylene sensitivity differences at different developmental stages (seedling, juvenile, and mature plants). This research will determine if the seedling hypocotyl elongation assay can be used to predict mature plant sensitivity within the family Solanaceae.
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