Study On The Salt Secretion Mechanism Of Mangrove Plant Avicennia Marina

Mangroves forests,distributed along the tropical and subtropical coasts,are ecologically and economically important for coastal wetland ecosystems.They have evolved a number of strategies to thrive under highly saline conditions.Salt-secreting mangrove plants excrete Na+ions directly through salt glands,which are crucial in plants salt tolerance in saline conditions.Studying the mechanism of salt secretion by salt glands is of great significance to understand the salt tolerance of plants.Mangrove sediments in the coastline are typically known as anaerobic environments with high salinity,and are well equipped for H2S production.However,no evidence has yet been unraveled whether H2S is involved in the regulation of salt secretion in mangrove plants under saline environment and its regulatory role in mediating salt secretion still needs to be further investigated.Avicennia marina is a pioneer species in coastal mangrove wetlands and one of the most salt-tolerant salt-secreting mangrove plants.Therefore,in this study,A.marina seedlings were used as experimental materials to investigate the salt tolerance mechanism.The main findings were as follows.In the first part,we observed the distribution,structure,and developmental pattern of salt glands in A.marina leaves.In addition,we also explored the method of isolating the salt glands from A.marina leaves.The results showed that the salt glands were sunken into adaxial epidermis and each salt gland is approximately 30～40 μm in diameter.While the salt glands in the abaxial epidermis were covered by massive nongland hairs.The salt gland originated from the proepidermal cells and formed a multicellular salt gland through cell divisions and consists of collecting cells,stalk cells,and secretory cells.The collecting cells,stalk cells and secretory cells have a large number of plasmodesmata between them.The stalk cells and secretory cells contain a large number of mitochondria,Golgi apparatus,and vesicles scattered in the cytoplasm.In addition,we described an enzymatic method for isolating salt glands from the adaxial epidermis of A.marina leaves.This method can be used for the subsequent imaging observations without interference from neighboring cells.In the second part,we investigated the effects of NaCl treatment on the photosynthetic characteristics of A.marina seedlings and salt secretion from the salt glands.The results showed that NaCl below 200 mmol/L favored growth,while NaCl above 400 mmol/L inhibited growth of A.marina seedlings.Salinity caused a decrease in net photosynthetic rate,stomatal conductance,light compensation point,light saturation point,maximal net photosynthetic rate,carbon dioxide saturation point,carbon dioxide compensate point and maximum carboxylation rate and 600 mmol/L NaCl treatment resulted in severely damaged chloroplasts and a significant decrease in Fv/Fm.In addition,the salt secretion rate of salt glands reached a maximum under 400 mmol/L NaCl treatment,with a significant increase in vesicles and mitochondrial density in the secretory cells.Further results showed that NaCl treatment regulated salt secretion from salt glands by affecting the expression of H+-ATPase,Na+/H+antiporter and K+channel genes.In the third part,we investigated the effect of H2S on salt secretion from the salt glands of A.marina under salt treatment.The results showed that H2S promoted salt secretion from A.marina salt glands by increasing the plasma membrane H+-ATPase activity and transcript levels of SOS1 and AHA1 genes,which contributed to enhancing Na+efflux and restricting K+leakage from salt glands.In addition,H2S is involved in the activation of tonoplast H+-ATPaSe activity and increasing the expression of NHY1 and VHA-c1 genes in salt-treated A.marina adaxial epidermis,which is critical to sequester Na+in the vacuoles and reduce Na+concentration in the cytoplasm.In the fourth part,to gain more insight into the salt secretion mechanism of A.marina salt glands,we collected salt gland cells for RNA-seq study using laser capture microdissection technique.A total of 1741 differentially expressed genes were obtained by RNA-seq sequencing,of which 1053 differentially expressed genes were upregulated and 688 differentially expressed genes were down-regulated.These differentially expressed genes were mainly enriched in transport,stimulus response,carbohydrate metabolism,and fatty acid metabolic pathways.GO and KEGG analyses revealed that gene related ion transport and vesicle trafficking,water and compatible solutes transport,ABA and ethylene signaling transduction,and cell wall and cuticle biosynthesis genes were significantly altered under salt treatment,suggesting that they may play an important role in salt secretion in salt glands of mangrove plant A.marina.