PHLOEM-SPECIFIC EXPRESSION OF PYROPHOSPHATASE INHIBITS LONG-DISTANCE TRANSPORT OF CARBOHYDRATES AND AMINO-ACIDS IN TOBACCO PLANTS

As reported in a previous paper [Lerchl et al. (1995) Plant Cell, 7, 2 59-270], expression of Escherichia call inorganic pyrophosphatase in t he cytosol under the control of the phloem-specific rolC promoter from Agrobacterium rhizogenes results in decreased growth of transgenic to bacco plants. In this paper we investigate the effect of the phloem-sp ecific expression of pyrophosphatase on phloem metabolism, and on plan t growth and allocation. A small decrease in the hexose phosphate/UDP- glucose ratio, the ATP/ADP ratio and the respiration rate in the midri bs of the transformants provides evidence that mobilization of sucrose via pyrophosphate-dependent reactions is necessary for phloem energy metabolism. The source leaves of the transformants had higher levels o f carbohydrates and amino acids and a much higher glutamine/glutamate ratio than the wild type, showing that export was inhibited and that t he growth inhibition was not due to a lack of photoassimilates or orga nic nitrogen in the leaves. The accumulation of photoassimilates was p aralleled by a decrease in photosynthesis, chlorophyll content and rib ulose bisphosphate carboxylase/oxygenase (Rubisco) activity, a small i ncrease in hexose phosphates and triose phosphates and a decrease in g lycerate 3-phosphate in the source leaves. There was a decrease of sol uble sugars and amino acids in sink leaves of the transformants. In si nk leaves amino acids decreased more than carbohydrates and a decrease in the glutamine/ glutamate ratio was observed. This was accompanied by a large decrease of nitrate. Sugars and amino acids were also reduc ed in the root tips of the transformants. The carbohydrate /amino acid ratio decreased 5-fold in the root tips, indicating a particularly ac ute shortage of carbohydrates. Relatively high levels of sugars and am ino acids in the basal regions of the root and the increase in sugars in the midrib indicate that there is also increased leakage of assimil ates out of the phloem during long-distance transport. Metabolism is r equired to maintain phloem function along the transport route, as well as for the initial step of loading. The transformants showed decrease d stem and root growth. The growth inhibition was largest in condition s allowing rapid growth of the wild type (high light and nitrogen supp ly).