The regulation of component processes of photosynthesis in transgenic tobacco with decreased phosphoribulokinase activity

Habash, Dimah Z. and Parry, Martin A J and Parmar, Saroj and Paul, Matthew J. and Driscoll, Simon and Knight, Jacqueline and Gray, John C. and Lawlor, David W. (1996) The regulation of component processes of photosynthesis in transgenic tobacco with decreased phosphoribulokinase activity. Photosynthesis Research, 49 (2). pp. 159-167. ISSN 0166-8595

Abstract

Tobacco plants (Nicotiana tabacum L.) transformed with an inverted cDNA encoding ribulose 5-phosphate kinase (phosphoribulokinase,PRK; EC 2.7.1.19) were employed to study the in vivo relationship between photosynthetic electron transport and the partitioning of electron transport products to major carbon metabolism sinks under conditions of elevated ATP concentrations and limited ribulose 1,5-bisphosphate (RuBP) regeneration. Simultaneous measurements of room temperature chlorophyll fluorescence and CO2 gas exchange were conducted on intact leaves. Under ambient CO2 concentrations and light intensities above those at which the plants were grown, transformants with only 5% of PRK activity showed 'down-regulation' of PS II activity and electron transport in response to a decrease in net carbon assimilation when compared to wild-type. This was manifested as a decline in the efficiency of PS II electron transport (Φ(PS II)), an increase in dissipation of excess absorbed light in the antennae of PS II and a decline in : total linear electron transport (J(I)), electron transport dedicated to carbon assimilation (J(A)) and electron transport allocated to photorespiration (J(L)). The transformants showed no alteration in the Rubisco specificity factor measured in vitro and calculated in vivo but had a relatively smaller ratio of RuBP oxygenation to carboxylation rates (v(o)/v(c)), due to a higher CO2 concentration at the carboxylation site (C(c)). The relationship between Φ(PS II) and Φ(CO2) was similar in transformants and wild-type under photorespiratory conditions demonstrating no change in the intrinsic relationship between PS II function and carbon assimilation, however, a novel result of this study is that this similar relationship occurred at different values of quantum flux, J(I), J(A), J(L) and v(o)/v(c) in the transformant. For both wild-type and transformants, an assessment was made of the possible presence of a third major sink for electron transport products, beside RuBP oxygenation and carboxylation, the data provided no evidence for such a sink.