A small decrease of plastid transketolase activity in antisense tobacco transformants has dramatic effects on photosynthesis and phenylpropanoid metabolism

Transketolase (TK) catalyzes reactions in the Calvin cycle and the oxidativ e pentose phosphate pathway (OPPP) and produces erythrose-4-phosphate, whic h is a precursor for the shikimate pathway leading to phenylpropanoid metab olism. To investigate the consequences of decreased TK expression for prima ry and secondary metabolism, we transformed tobacco with a construct contai ning an antisense TK sequence. The results were as follows: (1) a 20 to 40% reduction of TK activity inhibited ribulose-l,5-bisphosphate regeneration and photosynthesis. The inhibition of photosynthesis became greater as irra diance increased across the range experienced in growth conditions (170 to 700 mu mol m(-2) sec(-1)). TK almost completely limited the maximum rate of photosynthesis in saturating light and saturating CO2. (2) Decreased expre ssion of TK led to a preferential decrease of sugars, whereas starch remain ed high until photosynthesis was strongly inhibited. One of the substrates of TK (fructose-6-phosphate) is the starting point for starch synthesis, an d one of the products (erythrose-4-phosphate) inhibits phosphoglucose isome rase, which catalyzes the first reaction leading to starch. (3) A 20 to 50% decrease of TK activity led to decreased levels of aromatic amino acids an d decreased levels of the intermediates (caffeic acid and hydroxycinnamic a cids) and products (chlorogenic acid, tocopherol, and lignin) of phenylprop anoid metabolism. (4) There was local loss of chlorophyll and carotene on t he midrib when TK activity was inhibited by >50%, spreading onto minor vein s and lamina in severely affected transformants. (5) OPPP activity was not strongly inhibited by decreased TK activity. These results identify TK acti vity as an important determinant of photosynthetic and phenylpropanoid meta bolism and show that the provision of precursors by primary metabolism coli mits flux into the shikimate pathway and phenylpropanoid metabolism.