CORRELATION BETWEEN ARRESTED SECONDARY PLASMODESMAL DEVELOPMENT AND ONSET OF ACCELERATED LEAF SENESCENCE IN YEAST ACID INVERTASE TRANSGENICTOBACCO PLANTS

Mature leaves of a transgenic tobacco plant (Nicotiana tabacum L. var. Samsun, line A41-10) that constitutively express a yeast-derived acid invertase gene develop symptoms which are characterized by the presen ce of greenish-yellow and green sectors in the same leaf, and onset of early leaf senescence. Previous studies indicated that invertase acti vity was two- to threefold higher in the greenish-yellow sectors than in the green sectors. Our structural analyses revealed that developmen t of secondary plasmodesmata, via modification of existing primary pla smodesmata, between mesophyll cells was inhibited severely in the gree nish-yellow sectors, but only marginally in the green sectors. In cont rast, the structure and function of primary plasmodesmata in the same symptomatic sectors remained unaltered as determined by structural and dye coupling studies. It is hypothesized that secondary plasmodesmata differ from primary plasmodesmata in having special abilities to traf fic information molecules to coordinate leaf development and physiolog ical function(s). Arrest of secondary plasmodesmal development by high invertase activity in the transgenic tobacco leaf may have prevented this type of trafficking and hence resulted in early leaf senescence. The results also indicate that the yeast acid invertase-expressing tob acco may provide an effective experimental system for the molecular ch aracterization of cellular mechanisms that regulate the development, f unction, and possible turnover of secondary plasmodesmata.