Current Understanding of Role of Vesicular Transport in Salt Secretion by Salt Glands in Recretohalophytes

Abstract

Soil salinization is a serious and growing problem around the world. Some plants, recognized as the recretohalophytes, can normally grow on saline–alkali soil without adverse effects by secreting excessive salt out of the body. The elucidation of the salt secretion process is of great significance for understanding the salt tolerance mechanism adopted by the recretohalophytes. Between the 1950s and the 1970s, three hypotheses, including the osmotic potential hypothesis, the transfer system similar to liquid flow in animals, and vesicle-mediated exocytosis, were proposed to explain the salt secretion process of plant salt glands. More recently, increasing evidence has indicated that vesicular transport plays vital roles in salt secretion of recretohalophytes. Here, we summarize recent findings, especially regarding the molecular evidence on the functional roles of vesicular trafficking in the salt secretion process of plant salt glands. A model of salt secretion in salt gland is also proposed.

Adaptive Mechanisms of Halophytes and Their Potential in Improving Salinity Tolerance in Plants

“…Recretohalophytes have unique salt-secreting structures, such as salt glands and salt bladders, whereas euhalophytes have the ability to dilute absorbed salts in their succulent leaves or stems. Salt excluders, on the other hand, can prevent the uptake of salt ions from soils or shed leaves containing toxic levels of salt [ 49 , 50 ]. With these complex classifications, it is important to acquire knowledge on their extent of salt tolerance levels, and what the intrinsic mechanisms they adopt to combat salinity effects.…”

“…Species of another recretohalophyte genus, Tamarix , are salt-tolerant by increasing the density of salt glands and the rate of salt secretion per salt gland [ 69 ] ( Table 1 ). However, salt glands in different halophytes species possess diverse structural characteristics, and the number of cells that make up salt glands are different [ 50 ]. Still, their common features include the followings: (i) salt glands are surrounded by a thickened cuticle; (ii) having many plasmodesmata between the cells, a large number of highly developed mitochondria and many small vesicles in the cytoplasm; and (iii) having no chloroplasts in the cells [ 50 , 70 ].…”

“…Multi-cellular salt glands varying from 4 to 40 cells are found in dicotyledonous recretohalophytes, including L. bicolor [ 66 ], whereas bi-cellular salt glands are prominent in monocotyledonous recretohalophytes of the Poaceae family [ 71 ]. Ions secreted through salt glands include various cations (Na + , K + , Ca 2+ , Mg 2+ , Fe 2+ , Mn 2+ , and Zn 2+ ) and anions (Cl − , Br − , I − , SO 4 2− , PO 4 3− , and NO 3 − ), depending largely on the environment [ 50 ]. The actual pathways of salt secretion from salt glands remain to be determined, although several possible pathways based on recent studies have been suggested by Yuan et al [ 70 ] and Lu et al [ 50 ].…”

“…Ions secreted through salt glands include various cations (Na + , K + , Ca 2+ , Mg 2+ , Fe 2+ , Mn 2+ , and Zn 2+ ) and anions (Cl − , Br − , I − , SO 4 2− , PO 4 3− , and NO 3 − ), depending largely on the environment [ 50 ]. The actual pathways of salt secretion from salt glands remain to be determined, although several possible pathways based on recent studies have been suggested by Yuan et al [ 70 ] and Lu et al [ 50 ]. Nevertheless, it was also demonstrated that salt glands might impose detrimental threats to plants by causing leaf dehydration because the glands produce salt crystals on the leaf surface [ 65 ].…”

“…Halophytes, particularly recretohalophytes, directly secrete remarkably higher contents of salt ions onto the leaf surface, for dealing with maintenance of cellular ion homeostasis, through a unique epidermal structure called salt glands, making them superior compared with other classes of halophytes and non-halophytes [50] (Figure 3). Several species of the Limonium genus, including L. bicolor, L. aureum, L. gmelinii, L. otolepis, and L. sinuatum, are good examples of recretohalophytes, which boost salt tolerance by enhancing salt secretion via the salt glands [66][67][68] (Table 1).…”