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An investigation of the physiological roles and enzymatic properties of invertases in tobacco and hybrid poplar Canam, Thomas Benjamin

Abstract

Plant invertases (EC 3.2.1.26) represent a multi-gene family of β-fructofuranosidases that perform integral roles in several biochemical processes. The central importance of this family of enzymes to plant growth and development has made them a primary target of investigation in plant biology. Research has principally focused on sink-source interactions, and the potential to increase sink capacity in several economically important crop species, including potato and tomato. However, studies exploring the impacts of invertase mis-regulation on cellulose and lignin, the two most abundant biopolymers on earth, had not been conducted. Consequently, we investigated the effects of overexpressing yeast-derived invertases in tobacco and hybrid poplar. Transgenic tobacco expressing the yeast-derived invertases showed reduced height and interference in sink-source metabolism. In addition, some transgenic lines showed significant changes in cellulose and lignin content, providing evidence that sink capacity can be altered via the overexpression of this class of enzyme. In contrast, hybrid poplar expressing foreign invertase genes showed no visible phenotype, with only minor changes to the structural polymers cellulose and lignin, suggesting the mechanism of carbohydrate transport differs between tobacco and hybrid poplar. However, there was evidence for post-translational modification of the foreign invertases in hybrid poplar, which may also explain the difference in phenotypes observed. We suggest that the yeast-derived invertases may not be the most effective target to alter sink biopolymers, and that mis-regulating endogenous invertases may be a more suitable alternative. Consequently, we identified three cell-wall invertase genes in hybrid poplar and investigated their spatial and temporal expression profiles during the complete first year of growth. In addition, we heterologously expressed and characterized two hybrid poplar cell-wall invertase genes involved in vegetative growth. Collectively, the expression and functional characterization data suggest that one floral-specific and two vegetative cell-wall invertases exist in hybrid poplar. Of the two vegetative cell-wall invertases, one (PaxgINV1) appears to be involved in processes relating to dormancy, while the other (PaxgINV2) appears to be involved in phloem unloading and the seasonal reallocation of carbohydrate. We therefore hypothesize that PaxgINV2 may be a suitable target for future mis-regulation studies aimed at altering sink capacity.

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