The role of chloroplast electron transport and metabolites in modulating rubisco activity in tobacco. Insights from transgenic plants with reduced amounts of cytochrome b/f complex or glyceraldehyde 3-phosphate dehydrogenase

Leaf metabolites, adenylates, and Rubisco activation were studied in two tr ansgenic tobacco (Nicotiana tabacum L. cv W38) types. Plants with reduced a mounts of cytochrome b/f complex (anti-b/f) have impaired electron transpor t and a low transthylakoid pH gradient that restrict ATP and NADPH synthesi s. Plants with reduced glyceraldehyde 3-phosphate dehydrogenase (anti-GAPDH ) have a decreased capacity to use ATP and NADPH in carbon assimilation. Th e activation of the chloroplast NADP-malate dehydrogenase decreased in anti -b/f plants, indicating a low NADPH/ NADP(+) ratio. The whole-leaf ATP/ADP in anti-b/f plants was similar to wild type, while it increased in anti-GAP DH plants. In both plant types, the CO2 assimilation rates decreased with d ecreasing ribulose 1,5-bisphosphate concentrations. In anti-b/f plants, CO2 assimilation was further compromised by reduced carbamylation of Rubisco, whereas in anti-GAPDH plants the carbamylation remained high even at subsat urating ribulose 1,5-bisphosphate concentrations. We propose that the low c arbamylation in anti-b/f plants is due to reduced activity of Rubisco activ ase. The results suggest that light modulation of activase is not directly mediated via the electron transport rate or stromal ATP/ADP, but some other manifestation of the balance between electron transport and the consumptio n of its products. Possibilities include the transthylakoid pH gradient and the reduction state of the acceptor side of photosystem I and/or the degre e of reduction of the thioredoxin pathway.