Flaveria bidentis (L.) Kuntze Asteraceae

The NADP-malic enzyme type C4 dicot Flaveria bidentis (L.) Kuntze was transformed with antisense and cosense gene constructs that resulted in specific decreases in single photosynthetic enzymes. The enzymes targeted were ribulose-1,5-bisphosphate carboxylase/oxygenase [EC 4.1.1.39] (Rubisco), pyruvate, Pi dikinase [EC 2.7.9.1] (PPDK) and NADP malate dehydrogenase [EC 1.1.1.82] (NADP-MDH). These enzymes were chosen as targets because they have low activity compared to photosynthetic rates (Rubisco), are subject to complex covalent regulation (NADP-MDH), or both (PPDK). T1 progeny of a number of lines of these transformants were examined for the effects of these gene constructs on enzyme levels and photosynthetic performance. Rubisco antisense transformants expressing between 15 and 100% of wild-type enzyme activity were obtained. Pyruvate, Pi dikinase antisense lines were obtained with 40–100% wild-type levels. NADP malate dehydrogenase sense constructs caused a co-suppression of enzyme activity with some lines containing less than 2% of wild- type activity. Under saturating illumination, the control coefficients for these enzymes were determined to be up to 0.7 for Rubisco, 0.2–0.3 for PPDK and effectively zero for NADP-MDH. The implications of these observations for the regulation of photosynthetic flux and metabolism in C4 plants and the role of regulation by covalent modification are discussed.

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Effects of Exogenous Sucrose Feeding on Photosynthesis in the C3 Plant Tobacco and the C4 Plant Flaveria bidentis

Functional Plant Biology ◽

10.1071/pp96117 ◽

1997 ◽

Vol 24 (3) ◽

pp. 291 ◽

Cited By ~ 7

Author(s):

Robert T. Furbank ◽

Jeni Pritchard ◽

Colin L.D. Jenkins

Keyword(s):

Gene Expression ◽

Leaf Area ◽

Experimental System ◽

Threshold Level ◽

C3 Plant ◽

Sterile Culture ◽

Unit Leaf ◽

Photosynthetic Genes ◽

Sucrose Feeding ◽

Flaveria Bidentis

Seeds of the C4 dicot Flaveria bidentis and the C3 dicot Nicotiana tabacum were germinated on expanded cotton plugs and grown for 28–35 days in a sterile culture system with liquid nutrients supplied to the roots in the presence or absence of 5% (w/v) sucrose. Despite previous reports that supplying sucrose exogenously to plants inhibits expression of photosynthetic genes, sucrose application in this system stimulated maximum photosynthetic rates and these were comparable to those measured in glasshouse-grown material. The activities of many photosynthetic enzymes in the youngest fully expanded leaves of both Flaveria and tobacco plants were increased by sucrose feeding. Sucrose levels were increased more than 2-fold in tobacco and 3-fold in Flaveria leaves (on a leaf area basis) by sucrose application, and both hexose and starch levels also increased. Chlorophyll per unit leaf area in sucrose-fed plants increased by approximately 20% in tobacco and 2-fold in Flaveria. There was no evidence of sucrose repression of photosynthetic competence in this experimental system. We propose that, over the concentration range present in the leaves in these experiments, sucrose acts as a positive signal for photosynthetic gene expression but that, above a threshold level, inhibition of gene expression seen in other studies could still occur.

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