IMPAIRED PHOTOASSIMILATE PARTITIONING CAUSED BY PHLOEM-SPECIFIC REMOVAL OF PYROPHOSPHATE CAN BE COMPLEMENTED BY A PHLOEM-SPECIFIC CYTOSOLICYEAST-DERIVED INVERTASE IN TRANSGENIC PLANTS

Constitutive expression of the Escherichia coli ppa gene encoding inor ganic pyrophosphatase resulted in sugar accumulation in source leaves and stunted growth of transgenic tobacco plants, The reason for this p henotype was hypothesized to be reduced sucrose utilization and loadin g into the phloem. To study the role of PPI in phloem cells, a chimeri c gene was constructed using the phloem-specific rolC promoter of Agro bacterium rhizogenes to drive the expression of the ppa gene. Removal of cytosolic PPI in those cells resulted in photoassimilate accumulati on in source leaves, chlorophyll loss, and reduced plant growth, From these data, it was postulated that sucrose hydrolysis via sucrose synt hase is essential for assimilate partitioning, To bypass the PPi-depen dent sucrose synthase step, transgenic plants were produced that expre ss various levels of the yeast suc2 gene, which encodes cytosolic inve rtase, in their phloem cells, To combine the phloem-specific expressio n of the ppa gene and the suc2 gene, crosses between invertase- and py rophosphatase-containing transgenic plants were performed. Analysis of their offspring revealed that invertase can complement the phenotypic effects caused by the removal of PPI in phloem cells.