Dynamic changes in the frequency and architecture of plasmodesmata during the sink-source transition in tobacco leaves

The sink-source transition in tobacco leaves was studied noninvasively usin g transgenic plants expressing the green-fluorescent protein (GFP) under co ntrol of the Arabidopsis thaliana SUC2 promoter, and also by imaging transg enic plants that constitutively expressed a tobacco mosaic virus movement p rotein (MP) fused to GFP MP-GFP. The sink-source transition was measured on intact leaves and progressed basipetally at rates of up to 600 mum/h. The transition was most rapid on the largest sink leaves. However, leaf size wa s a poor indicator of the current position of the sink-source transition. A quantitative study of plasmodesmatal frequencies revealed the loss of enor mous numbers of simple plasmodemata during the sink-source transition. In c ontrast, branched plasmodesmata increased in frequency during the sink-sour ce transition, particularly between periclinal cell walls of the spongy mes ophyll. The progression of plasmodesmal branching, as mapped by the labelli ng of plasmodesmata with MP-GFP fusion, occurred asynchronously in differen t cell layers, commencing in trichomes and appearing lastly in periclinal c ell wails of the palisade layer. It appears that dividing cells retain simp le plasmodesmata for longer periods than nondividing cells. The rapid conve rsion of simple to branched plasmodesmata is discussed in relation to the c apacity for macromolecular trafficking in developing leaf tissues.