Abstract
To survive in a saline environment, halophytes have evolved many strategies to resist salt stress. The salt glands of recretohalophytes are exceptional features for directly secreting salt out of a plant. Knowledge of the pathway(s) of salt secretion in relation to the function of salt glands may help us to change the salt-tolerance of crops and to cultivate the extensive saline lands that are available. Recently, ultrastructural studies of salt glands and the mechanism of salt secretion, particularly the candidate genes involved in salt secretion, have been illustrated in detail. In this review, we summarize current researches on salt gland structure, salt secretion mechanism and candidate genes involved, and provide an overview of the salt secretion pathway and the asymmetric ion transport of the salt gland. A new model recretohalophyte is also proposed.
Highlights
Soil salinization has long been known as an environmental problem (Jacobsen and Adams, 1958), and approximately 6% of the planet’s total land, or more than 800 million ha, is affected (Shabala, 2013)
This special structure determines the functionality of bladders and glands; an ion can be rapidly transported from a mesophyll cells into a salt gland and secreted out of the salt gland with a force that is generated by mitochondrial activity and is transported in vesicles, eventually being excluded through pores (Feng et al, 2014)
Using the muti-cellular salt gland such as L. bicolor as the model, Figure 4 shows the possible pathway for an ion (e.g., Na+) transport from the mesophyll cell into the salt gland and the pathway of ion secretion
Summary
Soil salinization has long been known as an environmental problem (Jacobsen and Adams, 1958), and approximately 6% of the planet’s total land, or more than 800 million ha, is affected (Shabala, 2013). Halophytes, which constitute 0.4% of the total plants in the world, are plants that can survive and complete their life cycle in media containing more than 200 mM NaCl (Flowers and Colmer, 2008; Santos et al, 2015) Amongst these halophytes there is a small number that are able to secrete salt from their leaves, the so-called recretohalophytes (Flowers et al, 2015). By brushing the salt bladders from the surface of leaves in A. canescens, the Na+ concentration in bladders significantly raised with the increasing of external NaCl (Pan et al, 2016) These methods do not provide direct evidence of salt secretion by a single salt gland, so credible techniques are required in order to determine the salt secretion of a single salt gland. Over the past 5 years, a new method called Non-invasive Microsensing System (NMS)-BIO-001A (Younger USA Sci.&Tech) has been widely applied to measure the secretion
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