To evaluate the physiological significance of cyclic electron f low around photosystem (PS) I, we used a reverse genetic approach to focus on 11 chloroplast genes that encode homologs of mitochondrial complex I subunits (ndhA-K). Since their discovery, the exact function of the respiratory components in plant chloroplasts has been a matter of discussion. We disrupted one of these genes (ndhB) in tobacco by chloroplast transformation. Analysis of the transient increase in chlorophyll f luorescence after actinic light illumination and the redox kinetics of P700 (reaction center chlorophylls of PS I) suggest that the cyclic electron f low around PS I is impaired in the ndhB-deficient transformants. Transformants grew normally in a greenhouse, suggesting that the cyclic electron f low around PS I mediated by ndh gene products is dispensable in tobacco under mild environmental conditions.Photosynthetic electron flow provides the first stable products of photosynthesis: NADPH and ATP. Despite the importance of this electron flow, a fundamental problem remains unsolved; that is, how an appropriate balance between the production of NADPH and ATP is maintained. To answer this question, the contributions of the Q cycle, cyclic electron flow around photosystem (PS) I, and pseudocyclic electron flow (water-water cycle) in chloroplast energetics must be evaluated quantitatively (1). There is little doubt that cyclic electron flow around PS I provides extra ATP in some cellular processes, such as N 2 fixation in cyanobacterial heterocysts (2) and CO 2 concentration in cyanobacterial and C4 photosynthesis (3-8). However, it is unclear whether this cyclic electron flow contributes to the supply of ATP during steady-state photosynthesis in nonspecialized photosynthetic cells of higher plants (1, 9, 10).Although molecular biological dissection using a reverse genetic approach is an effective means to evaluate the physiological significance of cyclic electron flow around PS I, it has not been attempted because of a lack of information about the genes responsible for the electron flow. However, the discovery of an ndhB-deficient mutant of Synechocystis PCC6803 that lacked cyclic electron flow around PS I led to the idea that electron f low is mediated by the respiratory complex, NAD(P)H dehydrogenase, in cyanobacteria (4-8).Eleven ndh genes encoding homologs of mitochondrial complex I subunits are also present in the chloroplast genome of higher plants (11,12). Although respiratory function is limited to the mitochondria, a respiratory complex, NAD(P)H dehydrogenase, may catalyze cyclic electron flow around PS I in chloroplasts, as in cyanobacteria. However, the existence of NAD(P)H dehydrogenase-mediated electron flow in higher plants is still a matter of controversy (1, 13), because genes for the crucial flavoprotein subunits have not yet been identified (14). Moreover, physiological evidence alone has been insufficient to show an NAD(P)H dehydrogenase-mediated pathway for cyclic electron flow around PS I in higher plants...

Two nuclear genes, Nicl and Nic2, regulate nicotine levels in tobacco. nicl and nic2 are semidominant mutations in Burley 21 that reduce leaf nicotine levels and the activities of multiple enzymes in the nicotine pathway and simultaneously increase polyamine levels in cultured roots. Cultured roots homozygous for both mutations were used to isolate two cDNAs by subtraction hybridization; the transcript levels of these two cDNAs were much lower in the mutant roots than in the wild-type roots. The A411 gene encodes a 41-kD protein with considerable homology to mammalian spermidine synthase, whereas the A622 gene encodes a 35-kD protein with high homology to isoflavone reductase. When these genes were expressed in Escherichia coli, A411 had no spermidine synthase activity but did show putrescine N-methyltransferase activity, which is the first enzyme committed to the nicotine biosynthetic pathway, and A622 did not show isoflavone reductase activity. Both the methyltransferase and A622 genes are predominantly expressed in the root, and their expression levels in cultured roots are coordinately decreased by the nic mutations in the order of wild type > nic2 > nicl > nicl nic2. Removal of tobacco flower heads and young leaves rapidly and coordinately induced both genes in the root. Further, exogenous supply of auxin down-regulated both genes in cultured tobacco roots. These results suggest that N i c l and Nic2 are regulatory genes for nicotine biosynthesis.