these proteins could be used in the selective transport of large molecules between the two cells. Transport Plasmodesmata have been shown to transport proteins (including transcription factors), short interfering RNA, messenger RNA, viroids, and viral genomes from cell to cell. One example of a viral movement proteins is the tobacco mosaic virus MP-30. MP-30 is thought to bind to the virus's own genome and shuttle it from infected cells to uninfected cells through plasmodesmata. Flowering Locus T protein moves from leaves to the shoot apical meristem through plasmodesmata to initiate flowering. Plasmodesmata are also used by cells in phloem, and symplastic transport is used to regulate the sieve-tube cells by the companion cells. The size of molecules that can pass through plasmodesmata is determined by the size exclusion limit. This limit is highly variable and is subject to active modification. For example, MP-30 is able to increase the size exclusion limit from 700 Daltons to 9400 Daltons thereby aiding its movement through a plant. Also, increasing calcium concentrations in the cytoplasm, either by injection or by cold-induction, has been shown to constrict the opening of surrounding plasmodesmata and limit transport. Several models for possible active transport through plasmodesmata exist. It has been suggested that such transport is mediated by interactions with proteins localized on the desmotubule, and/or by chaperones partially unfolding proteins, allowing them to fit through the narrow passage. A similar mechanism may be involved in transporting viral nucleic acids through the plasmodesmata. A number of mathematical models have been suggested for estimating transport across plasmodesmata. These models have primarily treated transport as a diffusion problem with some added hindrance. Cytoskeletal components of Plasmodesmata Plasmodesmata link almost every cell within a plant, which can cause negative effects such as the spread of viruses. In order to understand this we must first look at cytoskeletal components, such as actin microfilaments, microtubules, and myosin proteins, and how they are related to cell to cell transport. Actin microfilaments are linked to the transport of viral movement proteins to plasmodesmata which allow for cell to cell transport through the plasmodesmata. Fluorescent tagging for co-expression in tobacco leaves showed that actin filaments are responsible for transporting viral movement proteins to the plasmodesmata. When actin polymerization was blocked it caused a decrease in plasmodesmata targeting of the movement proteins in the tobacco and allowed for 10-kDa (rather than 126-kDa) components to move between tobacco mesophyll