The regulation of intercellular and interorgan communication is pivotal for cell fate decisions in plant development and probably plays a significant role in the systemic regulation of gene expression and in defense reactions against pathogens or other biotic and abiotic environmental factors. In plants, symplasmic cell-to-cell communication is provided by plasmodesmata (Pd), coaxial membranous tunnels that span cell walls interconnecting adjacent cytoplasms. Macromolecules, proteins, and RNA may be transported through Pd by passive diffusion or by a facilitated mechanism. A quantitative tool was developed to measure the coefficient of conductivity, C(Pd), for diffusion-driven transport via Pd and to assess changes in the coefficient induced by developmental, biotic and abiotic signals. (GFP)C(Pd), the coefficient of conductivity for cell-to-cell spread of green-fluorescent protein (GFP), a protein with a Stokes radius of 2.82 nm, was determined in epidermal cells of sink and source leaves of wild-type and transgenic Nicotiana benthamiana plants expressing the movement protein of tobacco mosaic virus (MP(TMV)) incubated both in dark and light and at 16 and 25 degrees C. Under all conditions, Pd in source leaves conducted macromolecules, with (GFP)C(Pd)sink>(GFP)C(Pd)source. Light down-regulated (GFP)C(Pd) (all conditions); down-regulation was stronger for sink cells. The effect of MP(TMV) on (GFP)C(Pd) between epidermal cells was dependent on temperature and leaf development; at 16 degrees C, MP(TMV) down-regulated (GFP)C(Pd) only in source leaves, while at 25 degrees C, MP(TMV) had no significant effect. This quantitative tool should be useful for investigating differences in Pd conductivity that are induced by mutations or silencing.