Plasmodesmata: Dynamic Channels for Symplastic Transport

Plasmodesmata (PDs) are cytoplasmic structures that link adjacent cells to form the symplast of a plant. PDs are involved extensively in a plant's life by mediating symplastic transport of a wide range of ions and molecules. Major components of a plasmodesma (PD) include a plasma membrane, a desmotubule, and a cytoplasmic annulus, all of which are readily detectable by electron microscopy. Both the plasma membrane and the desmotubule contain proteinaceous particles, thought to be involved in altering the size of the cytoplasmic annulus. Cytoskeleton elements (actin and myosin) are essential for maintaining the integrity of PDs. Together with these elements, calcium_binding proteins probably play a significant role in regulating PD function. Symplastic transport occurs through the cytoplasmic annulus for the great majority of solutes, while other substances may traverse through the desmotubule internal compartment, the desmotubule shell, or the plasma membrane. The symplast is subdivided into several domains with varying molecular size exclusion limits (ranging from <1 kD to >10 kD). Plasmodesmata can be either primary or secondary; the former are developed during new wall formation and the latter are made in existing walls. The dynamic nature of plasmodesmata is also reflected by their changing frequencies, which, in turn, depend on the developmental and physiological status of the tissue or the entire plant. While diffusion is the major mechanism of symplastic transport, plasmodesmata are selective for certain ions and molecules. Upon viral infection, viral movement proteins interact with PD receptor proteins and, as a result of yet unknown mechanisms, the plasmodesmata are remarkably dilated to allow viral movement proteins and the bound viral genome to enter healthy cells. Some proteins of plant origin are also able to traverse plasmodesmata, presumably in ways similar to viral movement proteins. Some of these plant proteins are probably signal molecules contributing to cell differentiation and other activities. Other proteins move cell_to_cell in a non_specific manner.

Symplastic communication in the root cap directs auxin distribution to modulate root development

Root cap not only protects root meristem,but also detects and transduces the signals of environmental changes to affect root development.The symplastic communication is an important way for plants to transduce signals to coordinate the development and physiology in response to the changing enviroments.However,it is unclear how the symplastic communication between root cap cells affects root growth.Here we exploit an inducible system to specifically block the symplastic communication in the root cap.Transient blockage of plasmodesmata(PD)in differentiated collumella cells severely impairs the root development in Arabidopsis,in particular in the stem cell niche and the proximal meristem.The neighboring stem cell niche is the region that is most sensitive to the disrupted symplastic communication and responds rapidly via the alteration of auxin distribution.In the later stage,the cell division in proximal meristem is inhibited,presumably due to the reduced auxin level in the root cap.Our results reveal the essential role of the differentiated collumella cells in the root cap mediated signaling system that directs root development.

Different effects of foliar application of silica sol on arsenic translocation in rice under low and high arsenite stress

Foliar application of Si can generally reduce As translocation from roots to shoots in rice;however, it does not always work, particularly under high As stress. Here, the effects of foliar application of nanoscale silica sol on As accumulation in rice were investigated under low(2 μmol/L) and high(8 μmol/L) arsenite stress. The results revealed that foliar Si application significantly decreased the As concentration in shoots under low arsenite stress, but showed different effects under high arsenite stress after 7 days of incubation. The reduction in root-to-shoot As translocation under the 2 As + Si treatment was related to the down-regulation of Os Lsi1 and Os Lsi2 expression and up-regulation of Os ABCC1 expression in roots. In the 8 As + Si treatment, the expressions of Os Lsi1, Os Lsi2, and Os ABCC1 were significantly promoted, which resulted in substantially higher As accumulation in both the roots and shoots. In the roots, As predominantly accumulated in the symplasts(90.6%–98.3%), in which the majority of As was sequestered in vacuoles(79.0%–94.0%) under both levels of arsenite stress. Compared with that of the 8 As treatment, the 8 As + Si treatment significantly increased the As concentration in cell walls, but showed no difference in the vacuolar As concentration, which remained constant at approximately 69.1–71.7 mg/kg during days 4–7. It appeared that the capacity of root cells to sequester As in the vacuoles had a threshold, and the excess As tended to accumulate in the cell walls and transfer to the shoots via apoplasts under high arsenite stress. This study provides a better understanding of the different effects of foliar Si application on As accumulation in rice from the view of arseniterelated gene expression and As subcellular distribution in roots.