Dynamic function and regulation of apoplast in the plant body

Apoplast is the internal environment of plant. Our body posses the internal environment that consists of blood, lympha, and tissue fluid. Plant cells are also cultivated and surrounded by a liquid medium in the apoplast. As well as various important functions of the internal environment in our body, apoplast function is also prerequisite for the plant life. There are so far seven distinct functions of apoplast. (1) Growth regulation with apoplastic enzymes by altering cell-wall properties through degradation, synthesis, orientation and cross-linking of supra-molecules of cell walls, such as cellulose, non-cellulosic polysaccharides, proteins, and lignin; (2) Skeleton sustained by cellulose microfibrils, lignin and various types of structural proteins with distinctively high content of hydroxyproline, proline or glycine; (3) Skin to defend symplast from desiccation, pathogens' attack and harmful environmental factors, such as ozone and sulfur dioxide; (4) Transportation route for not only well-known molecules of water, inorganic ions, and sugar, but also plant hormones, oligosaccharides and proteins; (5) Homeostasis of the internal environment by controlling ionic balance, pH and water content; (6) Adhesion of cell to cell; (7) Gas exchange space of leaf for photosynthesis. The present article reviews the recent advances in studies of several aspects of the dynamic function and regulation of apoplast.