Structural, developmental and functional analyses of leaf salt glands of mangrove recretohalophyte Aegiceras corniculatum

Salt secretion is an important strategy for mangrove plant Aegiceras corniculatum to adapt the coastal intertidal environment. However, the structural, developmental and functional analyses on the leaf salt glands, particularly the salt secretion mechanism are not well documented. In this study, we investigated the structural, developmental and degenerative characteristics, and the salt secretion mechanisms of salt glands to further elucidate the mechanisms of salt tolerance of A. corniculatum. The results showed that the salt gland cells have a large number of mitochondria and vesicles, and plenty of plasmodesmata as well, while chloroplasts were found in the collecting cells. The salt glands developed early and began to differentiate at the leaf primordium stage. We observed and defined three stages of salt gland degradation for the first time in A. corniculatum, where the secretory cells gradually twisted and wrinkled inwards and collapsed downwards as the salt gland degeneration increased and the intensity of salt gland autofluorescence gradually diminished. Besides, we found that the salt secretion rate from salt glands increased with the treated concentration of NaCl increased, reaching the maximum at 400 mM NaCl. The salt-secreting capacity from salt glands of the adaxial epidermis is significantly greater than that of the abaxial epidermis. The qRT-PCR results indicate that SAD2, TTG1, GL2 and RBR1 may be involved in regulating the development of the salt glands of A. corniculatum. Moreover, Na+/H+ antiporter, H+-ATPase, K+ channel, and Cl- channel may be play important roles in salt secretion from salt glands. In sum, this study strengthens the understanding of the structural, developmental, and degenerative patterns of salt glands and salt secretion mechanisms in mangrove recretohalophyte A. corniculatum, providing an important reference for further studies at the molecular level.