A MODERATE DECREASE OF PLASTID ALDOLASE ACTIVITY INHIBITS PHOTOSYNTHESIS, ALTERS THE LEVELS OF SUGARS AND STARCH, AND INHIBITS GROWTH OF POTATO PLANTS

Antisense expression of a full length cDNA encoding plastid aldolase l ed to decreased expression of aldolase at the transcript and protein l evel in several 'antisense' potato transformants. To quantify the inhi bition, activity was compared in corresponding leaves down a plant and in plants of different ages. Aldolase activity was decreased by 32-43 %, 56-71%, 79-83% and 91-97% in A-70, A-3, A-51 and A-2. Separation on a Q-Sepharose-FF column showed the decrease was due to inhibition of plastid aldolase. The transformants showed a small increase of Rubisco activity, a small decrease of phosphoribulokinase activity, and large r but subproportional decreases of sedoheptulose-1,7-biphosphatase and plastid fructose-1,6-bisphosphatase activity. Ambient photosynthesis was inhibited by 10%, 40%, 66% and 85% in A-70, A-3, A-51 and A-2. The transformants contained increased triose phosphates, and very low rib ulose-1,5-bisphosphate and glycerate-3-phosphate. Chlorophyll fluoresc ence indicated that photosystem II was more reduced and thylakoid ener gization was increased. Starch synthesis was decreased by 16% and 36% in A-70 and A-3, whereas sucrose synthesis was less strongly inhibited . Plant growth was not significantly altered in A-70, was decreased by 41% in A-3, and was severely inhibited in plants with under 20% of wi ld-type aldolase activity. Although plastid aldolase catalyses a readi ly reversible reaction, possesses no known regulatory properties, and would appear irrelevant for the control of metabolism and growth, smal l changes in its activity have marked consequences for photosynthesis, carbon partitioning and growth.