miR397 regulates cadmium stress response by coordinating lignin polymerization in the root exodermis in Kandelia obovata.

Secondary lignification of the root exodermis of Kandelia obovata is crucial for its response to adversity such as high salinity and anaerobic environment, and this lignification is also effective in blocking cadmium transport to the roots. However, how the differences in lignification of root exodermis at different developmental stages respond to Cd stress and its regulatory mechanisms have not been revealed. In this study, after analyzing the root structure and cell wall thickness using a Phenom scanning electron microscope as well as measuring cadmium content in the root cell wall, we found that the exodermis of young and mature roots of K. obovata responded to Cd stress through the polymerization of different lignin monomers, forming two different mechanisms: chelation and blocking. Through small RNA sequencing, RLM-5'-RACE and dual luciferase transient expression system, we found that miR397 targets and regulates KoLAC4/17/7 expression. The expression of KoLAC4/17 promoted the accumulation of guaiacyl lignin during lignification and enhanced the binding of cadmium to the cell wall. Meanwhile, KoLAC7 expression promotes the accumulation of syringyl lignin during lignification, which enhances the obstruction of cadmium and improves the tolerance to cadmium. These findings enhance our understanding of the molecular mechanisms underlying the differential lignification of the root exodermis of K. obovata in response to cadmium stress, and provide scientific guidance for the conservation of mangrove forests under heavy metal pollution.

Function verification of a chlorophyll a/b binding protein gene through a newly established tobacco rattle virus-induced gene silencing system in Kandelia obovata.

As an important mangrove species, Kandelia obovata plays an irreplaceable role in the coastal ecosystem. However, due to a lack of genetic technology, there is limited research on its functional genes. As such, establishing an efficient and rapid functional verification system is particularly important. In this study,tobacco rattle virus (TRV) and the phytoene desaturase gene KoPDS were used as the vector and target gene, respectively, to establish a virus-induced gene silencing system (VIGS) in K. obovata. Besides, the system was also used to verify the role of a Chlorophyll a/b binding protein (Cab) gene KoCAB in leaf carbon sequestration of K. obovata. RNA-Seq and qRT-PCR showed that the highest gene-silencing efficiency could reach 90% after 10 days of inoculation and maintain above 80% after 15 days, which was achieved with resuspension buffer at pH 5.8 and Agrobacterium culture at OD600 of 0.4-0.6. Taken together, the TRV-mediated VIGS system established herein is the first genetic analysis tool for mangroves, which may greatly impel functional genomics studies in mangrove plants.

Comparative transcriptome analysis reveals different functions of Kandelia obovata superoxide dismutases in regulation of cadmium translocation.

Kandelia obovata is a dominant mangrove species in southeastern of China. This species has a high tolerance to heavy metal stress that is mainly ascribed to the thickening of the secondary exodermis and high activity of superoxide dismutase (SOD) enzymes in the roots. The thickened exodermis inhibits entry of heavy metals into the roots, and oxidative stress due to Cd can be reduced efficiently by SOD activity. Although there are several proposed mechanisms by which genes encoding SODs in K. obovata (KoSODs) could induce lignification that contributes to thickening of the root exodermis, the exact functions of FeSOD2 (KoFSD2) and Cu/ZnSOD3 (KoCSD3) remain unclear. In this study we investigated the role of a thickened exodermis in K. obovata root tissues in response to Cd treatment, and determined functions of KoFSD2 and KoCSD3 by performing comparative transcriptome analysis on tobacco lines that overexpress these two enzymes. we found that a thickened exodermis can reduce Cd uptake, and that exposure to high concentrations Cd can promote secondary thickening processes in K. obovata roots exodermis. Transgenic Nicotiana benthamiana overexpressing of KoFSD2 or KoCSD3 exhibit different responses to Cd stress. KoCSD3 can promote thickening of the secondary cell wall of root vascular tissues to impede Cd entry. Meanwhile, KoFSD2 and KoCSD3 can also regulate translocation of copper and iron in response to Cd accumulation. Taken together, our results expand our understanding of the physiological roles of SODs in K. obovata against Cd stress.

Health risk assessment of heavy metal and its mitigation by glomalin-related soil protein in sediments along the South China coast.

Knowledge regarding the exposure risk of heavy metals in estuarine wetland is important for people (especially fishermen and consumers of local fish) living in the coastal area and the coastal reclamation. Here, we assessed the ecological and human health risks of sediment-associated metals in the large-scale estuary area based on different sediment textures (Mud, Mud-Sand, and Sand). To investigate the potential impact of terrestrial material on the contamination risks, glomalin-related soil protein (GRSP), a recalcitrant soil protein fraction, was used. Results showed that the estuarine sediment texture played a rather important role in the heavy metal distribution, risk assessment, and the metal sequestration capacity of the land-derived GRSP fraction. We found arsenic, Pb, Cd, and Cr had the highest enrichment in the estuarine wetlands by calculating multiple contamination indices, and that confirmed similar findings of heavy metal contents, except Cd. The average pollution load index (PLI) was 1.67 in all the estuarine sediments, indicating multi-element contamination, with the Muddy sediments (PLI = 2.07) significantly higher than the Mud-Sand mixed sediments (PLI = 1.85), and the lowest (0.78 < 1) in the Sandy sediments. The health risk assessment suggested that the potential cancer risk occurred (>1.00E-6) but no obvious non-carcinogenic effects occurred (<1). Arsenic was found to be the primary contributor to non-carcinogenic risk, accounting for 44.2% of hazard index, while Ni is the primary control metal for carcinogenic risk (except arsenic in the Sandy sediments). As a proxy of terrestrial organic matter, GRSP fraction possessed a high sequestration capacity for heavy metal, especially in the Sandy sediments, and it was significantly associated with the mitigation of ecological and health risks, which may provide new insights into the in situ remediation of anoxic estuarine wetlands.

SODs involved in the hormone mediated regulation of H2O2 content in Kandelia obovata root tissues under cadmium stress.

Cadmium (Cd) pollution in mangrove wetlands has received increasing attention as urbanization expands rapidly. As a dominant mangrove species, Kandelia obovata is highly tolerant to Cd toxicity. Plant hormones and superoxide dismutase (SODs) play critical roles in the response to heavy metal stress in K. obovata roots. Although theirs important influence have been reported, the regulation mechanism between SODs and plant hormones in Cd detoxification by K. obovata roots remains limited. Here, we investigated relationships among SOD, plant hormones, and Cd tolerance in K. obovata roots exposed to Cd. We found that Cd was retained in the epidermis and exodermis of roots, and the epidermis and exodermis had highest hydrogen peroxide (H2O2) content and SOD activity. Similarly, SOD isozymes also exhibited distinct activity in the different parts of root. Overexpressed KoCSD3 and KoFSD2 individually in Nicotiana benthamiana revealed that different SOD members contributed to H2O2 content regulation by promote the activity of downstream antioxidant enzymes under Cd treatment. In addition, assays on the effects of hormones showed that increased endogenous indole-3-acetic acid (IAA) was observed in the cortex and stele, whereas the abscisic acid (ABA) content was enhanced in the epidermis and exodermis in roots during Cd treatment. The results of exogenous hormones treatment indicated that KoFSD2 upregulated under ABA and IAA treatment, but KoCSD3 only induced by ABA stimulation. Taken together, our results reveal the relationship between SODs and plant hormones, which expands the knowledge base regarding KoSODs response to plant hormones and mediating H2O2 concentration under Cd stress.