ULTRASTRUCTURAL EFFECTS IN POTATO LEAVES DUE TO ANTISENSE-INHIBITION OF THE SUCROSE TRANSPORTER INDICATE AN APOPLASMIC MODE OF PHLOEM LOADING

To study the export of sugars from leaves and their long-distance tran sport, sucrose-proton/co-transporter activity of potato was inhibited by antisense repression of StSUT1 under control of either a ubiquitous ly active (CaMV 35S) or a companion-cell-specific (rolC) promotor in t ransgenic plants. Transformants exhibiting reduced levels of the sucro se-transporter mRNA and showing a dramatic reduction in root and tuber growth, were chosen to investigate the ultrastructure of their source leaves. The transformants had a regular leaf anatomy with a single-la yered palisade parenchyma, and bicollateral minor veins within the spo ngy parenchyma. Regardless of the promoter used, source leaves from tr ansformants showed an altered leaf phenotype and a permanent accumulat ion of assimilates as indicated by the number and size of starch grain s, and by the occurrence of lipid-storing oleosomes. Starch accumulate d throughout the leaf: in epidermis, mesophyll and, to a smaller degre e, in phloem parenchyma cells of minor veins. Oleosomes were observed equally in mesophyll and phloem parenchyma cells. Companion cells were not involved in lipid accmulation and their chloroplasts developed on ly small starch grains. The similarity of ultrastructural symptoms und er both promotors corresponds to, rather than contradicts, the hypothe sis that assimilates can move symplasmically from mesophyll, via the b undle sheath, up to the phloem. The microscopical symptoms of a consti tutively high sugar level in the transformant leaves were compared wit h those in wild-type plants after cold-girdling of the petiole. Inhibi tion of sugar export, both by a reduction of sucrose carriers in the s ieve element/companion cell complex (se/cc complex), or further downst ream by cold-girdling, equally evokes the accumulation of assimilates in all leaf tissues up to the se/cc complex border. However, microscop y revealed that antisense inhibition of loading produces a persistentl y high sugar level throughout the leaf, while cold-girdling leads only to local patches containing high levels of sugar.