Non-targeted and targeted protein movement through plasmodesmata in leavesin different developmental and physiological states

Plant cells rely on plasmodesmata for intercellular transport of small sign aling molecules as well as larger informational macromolecules such as prot eins. A green fluorescent protein (GFP) reporter and low-pressure microproj ectile bombardment were used to quantify the degree of symplastic continuit y between cells of the leaf at different developmental stages and under dif ferent growth conditions. Plasmodesmata were observed to be closed to the t ransport of GFP or dilated to allow the traffic of GFP. In sink leaves, bet ween 34% and 67% of the cells transport GFP (27 kD), and between 30% and 46 % of the cells transport double GFP (54 kD). In leaves in transition transp ort was reduced; between 21% and 46% and between 2% and 9% of cells transpo rt single and double GFP, respectively. Thus, leaf age dramatically affects the ability of cells to exchange proteins nonselectively. Further, the num ber of cells allowing GFP or double GFP movement was sensitive to growth co nditions because greenhouse-grown plants exhibited higher diffusion rates t han culture-grown plants. These studies reveal that leaf cell plasmodesmata are dynamic and do not have a set size exclusion limit. We also examined t argeted movement of the movement protein of tobacco mosaic virus fused to G FP, P30::GFP. This 58-kD fusion protein localizes to plasmodesmata, consist ently transits from up to 78% of transfected cells, and was not sensitive t o developmental age or growth conditions. The relative number of cells cont aining dilated plasmodesmata varies between different species of tobacco, w ith Nicotiana clevelandii exhibiting greater diffusion of proteins than Nic otiana tabacum.