Abstract
Transplastomic tobacco (Nicotiana tabacum) plants expressing β-glucosidase (Bgl-1) show modified development. They flower 1 month earlier with an increase in biomass (1.9-fold), height (1.5-fold), and leaf area (1.6-fold) than untransformed plants. Trichome density on the upper and lower leaf surfaces of BGL-1 plants increase by 10- and 7-fold, respectively, harboring 5-fold more glandular trichomes (as determined by rhodamine B staining), suggesting that BGL-1 lines produce more sugar esters than control plants. Gibberellin (GA) levels were investigated because it is a known regulator of flowering time, plant height, and trichome development. Both GA(1) and GA(4) levels are 2-fold higher in BGL-1 leaves than in untransformed plants but do not increase in other organs. In addition, elevated levels of other plant hormones, including zeatin and indole-3-acetic acid, are observed in BGL-1 lines. Protoplasts from BGL-1 lines divide and form calli without exogenous hormones. Cell division in protoplasts is enhanced 7-fold in the presence of exogenously applied zeatin-O-glucoside conjugate, indicating the release of active hormones from their conjugates. Whitefly (Bemisia tabaci) and aphid (Myzus persicae) populations in control plants are 18 and 15 times higher than in transplastomic lines, respectively. Lethal dose to kill 50% of the test population values of 26.3 and 39.2 μg per whitefly and 23.1 and 35.2 μg per aphid for BGL-1 and untransformed control exudates, respectively, confirm the enhanced toxicity of transplastomic exudates. These data indicate that increase in sugar ester levels in BGL-1 lines might function as an effective biopesticide. This study provides a novel strategy for designing plants for enhanced biomass production and insect control by releasing plant hormones or sugar esters from their conjugates stored within their chloroplasts.
Highlights
In plants, b-glucosidases have been implicated in key developmental processes, such as growth, pathogen defense, and hormone hydrolysis (Esen, 1993; Kleczkowski and Schell, 1995)
Their regulating properties appear in the course of the biosynthetic and signaling pathways and are followed by catabolic processes. All these metabolic steps are irreversible except for some processes including the formation of glucoside ester or ether conjugates, where the free hormone can be liberated by b-glucosidase enzymatic hydrolysis
After characterization of the first GA glucoside, GA8-2-O-b-D-glucoside from Phaseolus coccineus fruits (Schreiber et al, 1970), the term GA conjugate was used for a GA covalently bound to another low-molecular-weight compound
Summary
B-glucosidases have been implicated in key developmental processes, such as growth, pathogen defense, and hormone hydrolysis (Esen, 1993; Kleczkowski and Schell, 1995). Their regulating properties appear in the course of the biosynthetic and signaling pathways and are followed by catabolic processes All these metabolic steps are irreversible except for some processes including the formation of glucoside ester or ether conjugates, where the free hormone can be liberated by b-glucosidase enzymatic hydrolysis. The loss of biological activity in the course of the conjugation process and the increased polarity of GA glucosyl conjugates favor GA conjugates for their deposition into the plant cell vacuole, but their storage within chloroplasts has not yet been investigated Because of their preferential formation and accumulation during seed maturation, it has been proposed that GA Glc conjugates may function as storage products (Schneider et al, 1992). The easy formation and hydrolysis of GA glucosyl conjugates results in reversible deactivation/ activation and facilitates the regulation of free GA pools
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