| Scope and method of study. The objective of this study was to investigate the influence of developmental stages and cellular types on plasmodesmata (PD)-mediated intercellular protein trafficking in plants. The fusion protein between cucumber mosaic virus 3a movement protein (3a MP) and green fluorescent protein (GFP) as well as free GFP were used as probes for protein trafficking. Tobacco, Arabidopsis thaliana, tomato, and cucumber plants were used. The proteins were expressed in epidermal cells by biolistic bombardment or in the phloem in transgenic plants. Intercellular trafficking of the proteins during plant development and at specific cellular boundaries was analyzed by multiple approaches including fluorescence microscopy, electron microscopy, and immunolabeling.; Findings and conclusions. Two basic mechanisms of protein trafficking, specific and nonspecific trafficking, were discovered. Specific trafficking requires interaction between trafficking molecules and cellular components, whereas nonspecific trafficking occurs by diffusion. Specific trafficking of 3a MP:GFP fusion protein occurs via complex secondary PD in tobacco source leaves, whereas it is restricted in sink leaves where most of the PD are primary. In contrast, nonspecific trafficking of GFP occurs via primary PD, but not complex secondary PD in tobacco. However, GFP trafficking is not influenced by developmental stages in A. thaliana. 3a MP:GFP is confined in the phloem, and its trafficking is blocked at the phloem and bundle sheath interface regardless of developmental stages in transgenic tobacco. GFP traffic is restricted at epidermis and mesophyll interface in cucumber cotyledons. Our findings indicate that PD-mediated trafficking involves complex mechanisms which may be unique to species, developmental stages, and cell or organ types. Transgenic A. thaliana plants expressing 3a MP:GFP were created for genetic analysis to investigate endogenous factors involved in intercellular protein trafficking. |
