Xing Liu

Hibiscus hamabo is an important coastal ornamental plant and a pioneer tree species for coastal protection forests. Prolonged waterlogging severely restricts its ecological expansion toward the intertidal‑zone forefront and limits population regeneration; clarifying its waterlogging‑adaptation mechanisms is essential for coastal wetland ecological restoration. In this study, phenotypic, physiological and transcriptomic responses of H. hamabo under 3-, 9-, and 18-day waterlogging treatments were characterized. Prolonged waterlogging markedly suppressed plant growth and biomass accumulation, and impaired photosynthetic performance via pigment degradation and stomatal closure. To counteract oxidative injury, H. hamabo sequentially triggered osmotic‑adjustment and antioxidant‑defense systems, relying on soluble sugars, proline and ROS‑scavenging enzymes. Transcriptome profiling identified stage‑dependent gene expression changes mainly related to photosynthesis, hormone signaling and phenylpropanoid biosynthesis. WGCNA uncovered two core co‑expression modules with opposite functional roles, representing stress‑defense activation and growth repression, respectively, and hub genes for osmotic homeostasis, ROS detoxification and photosynthesis preservation were screened. Collectively, our findings illustrate the coordinated adaptive strategy of H. hamabo against waterlogging stress and supply promising candidate genes for breeding waterlogging‑tolerant coastal woody plants.

Background/Objectives: bHLH transcription factors regulate key plant processes including development, stress adaptation, and metabolism. In Kandelia obovata, a mangrove species well known for its high tolerance to harsh environments, the bHLH gene family has not been systematically investigated. Here, this study aims to perform genome-wide identification and expression profiling of KobHLH genes under normal and saline conditions. Methods: bHLH members were identified by homology searches (HMMER and BLASTp) against the K. obovata protein dataset and verified with SMART-based domain analysis. Subsequent analyses covered physicochemical features, conserved motif composition, and phylogenetic relationships. Transcript abundance was quantified via RNA-seq in various organs (roots, stems, leaves, flowers, and fruits) as well as under saline conditions. Differentially expressed transcripts were selected for co-expression network construction. Results: Phylogenetic analysis of the 118 identified KobHLH loci assigned them into 18 distinct clades. Organ-preferential expression patterns were observed, leading to their classification into three putative functional categories. Upon salinity exposure (10–30‰), 61 of the 118 KobHLH genes exhibited differential expression, grouped into ten significant clusters. A co-expression network comprising 20 hub genes and 198 edges was constructed. Conclusions: This study provides the first genome-wide characterization of the bHLH family in K. obovata, revealing its evolutionary divergence and identifying 61 salt-responsive candidates, notably KobHLH108 and KobHLH5 associated with secondary metabolism, for further functional validation. The findings provide a foundation for future functional studies on stress adaptation mechanisms in this ecologically valuable mangrove species.

Hibiscus hamabo, an endangered semi-mangrove species with critical ecological functions in coastal protection and biodiversity maintenance, faces research limitations due to unstable genetic transformation systems. Protoplasts, as a transient expression system, have proven crucial in multiple fields of research such as plant genomics, transcriptomics, genetic transformation, physiology, and biochemistry. Establishing an efficient protoplast separation system and transient transformation system is the basis for conducting molecular biology and related research. In particular, woody plants are highly valuable because of the difficulty in constructing a stable transgenic system. Here, we establish a robust protoplast isolation and transfection protocol using stem tissues, filling the gap in protoplast technology for Hibiscus hamabo. This approach includes tissue preparation, protoplast isolation, protoplast transfection, and incubation. The protoplast yield reached ~2.0 × 10⁷ per g, and the protoplast activity reached 93.61% ± 1.09%. The establishment of this cell system can immediately trigger transcriptome responses to gene expression to characterize and quantify genetic networks and gene functions, enabling rapid and efficient flux experiments. This study provides a reference for protoplast research on other difficult-to-transform woody plants, and offers guidance for further analysis of plant growth/development and genetic improvement.

Background: The planting of mangroves is extensively used to control the invasive plant Spartina alterniflora in coastal wetlands. Different plant species release diverse sets of small organic compounds that affect rhizosphere conditions and support high levels of microbial activity. The root-associated microbial community is crucial for plant health and soil nutrient cycling, and for maintaining the stability of the wetland ecosystem. Methods: High-throughput sequencing was used to assess the structure and function of the soil bacterial communities in mudflat soil and in the rhizosphere soils of S. alterniflora, mangroves, and native plants in the Oujiang estuarine wetland, China. A distance-based redundancy analysis (based on Bray–Curtis metrics) was used to identify key soil factors driving bacterial community structure. Results: S. alterniflora invasion and subsequent mangrove afforestation led to the formation of distinct bacterial communities. The main soil factors driving the structure of bacterial communities were electrical conductivity (EC), available potassium (AK), available phosphorus (AP), and organic matter (OM). S. alterniflora obviously increased EC, OM, available nitrogen (AN), and NO₃⁻-N contents, and consequently attracted copiotrophic Bacteroidates to conduct invasion in the coastal areas. Mangroves, especially Kandelia obovata, were suitable pioneer species for restoration and recruited beneficial Desulfobacterota and Bacilli to the rhizosphere. These conditions ultimately increased the contents of AP, available sulfur (AS), and AN in soil. The native plant species Carex scabrifolia and Suaeda glauca affected coastal saline soil primarily by decreasing the EC, rather than by increasing nutrient contents. The predicted functions of bacterial communities in rhizosphere soils were related to active catabolism, whereas those of the bacterial community in mudflat soil were related to synthesis and resistance to environmental factors. Conclusions: Ecological restoration using K. obovata has effectively improved a degraded coastal wetland mainly through increasing phosphorus availability and promoting the succession of the microbial community.

Kandelia obovata (Rhizophoraceae) is the most cold-tolerant mangrove species and has been widely used in coastal wetland ecological restoration due to its specific viviparous phenomenon, beautiful shape, and unique floral pattern. Due to long-term adaptation to their local environment, the phenotypic characteristics and stress resistance of widely distributed plants of the same species often differentiate across different locations. The capacity for cold resistance is closely linked to the physiological and structural characteristics of plants. Herein, we explored the temporal variations in the leaf structure and physiological status of K. obovata under −5.5 °C from different areas such as Jiulongjiang Estuary (JLJ, 24°25′ N), Fujian Province, and Longgang City (LG, 27°34′ N) and Jiaojiang District (JJ, 28°67′ N), Zhejiang Province. The morphological variations implied that the cold resistance of K. obovata obviously strengthened after the northward migration and acclimatization, in the following order: LG > JJ > JLJ. More specifically, after exposure to a sustained low temperature, the relative conductivity (REC), an index widely used to evaluate the degree of plant damage, remarkably increased from 33.62 ± 2.39 to 63.73 ± 3.81, 31.20 ± 1.63 to 49.48 ± 1.12, and 23.75 ± 0.13 to 54.24 ± 1.45 for JLJ, LG, and JJ, respectively (p < 0.05). Additionally, the palisade-to-spongy tissue ratio (P/I) of JLJ and JJ decreased from 0.78 ± 0.05 and 0.75 ± 0.03 to 0.5 ± 0.04 and 0.64 ± 0.02 (p < 0.05), whereas no significant changes were found in LG (p > 0.05). The SOD activity of LG significantly kept increasing, with values increased from 352.49 ± 10.38 to 477.65 ± 1.78 U·g⁻¹, whereas no apparent changes in JLJ and JJ were observed with the sustained low temperature. The results of this study improved our understanding of the response of K. obovata to freezing stress, which could provide a sound theoretical foundation for cultivating cold-resistant varieties, as well as expanding mangrove plantations in higher latitudes.

The scientific management of salinized agricultural lands and the use of undeveloped saline lands to ensure food security have become one of the most urgent tasks nowadays. Biochar contains rich carbon (C) and functional groups, and processes high alkalinity, porosity, and specific surface area (SSA). Thus, it has been widely used as an effective organic conditioner in acidic or neutral soils to improve their fertility. However, so far, the impacts of biochar application on properities of saline soils and the underlying mechanisms remain unveiled. Therefore, in this study, we focus on the investigation of the impacts of biochar on the physical, chemical, and biological properties of saline soils. We found that biochar could: (1) decrease soil bulk density (BD), increase soil porosity, promote the formation of soil aggregation and enhance the leaching of soil salts; (2) increase the cation exchange capacity (CEC) of soil, decrease the salinity of soil through ion exchange and adsorption; (3) directly act as the nutrient supplements, indirectly adsorb water and nutrients or improve nutrient availability (e.g., soil organic carbon (SOC) turnover and sequestration, nutrient cycling); and (4) improve the structure and functioning of the soil microbial community and therefore indirectly impact the C, nitrogen (N) and phosphorus (P) cycling in soil systems. However, these impacts heavily depend on the properties, the concentration of the biochar added to the soil, and the type and location of the soil. In fact, some studies have shown that the addition of biochar in soil could even increase the salinity of saline soils. Another issue is the lack of long-term and large-scale field experiments regarding the impact of biochar addition on properties of saline soils. Therefore, future studies should focus on long-term field experiments with the combination of traditional soil analytical methods and mordern molecular techniques (e.g., high-throughput sequencing, macro-genomics, and metabolomics) to comprehensively reveal the response mechanism of physicochemical properties and microbial characteristics of saline soils to exogenous biochar. Our study can provide a scientific foundation for the practical agricultural production and ecological management of biochar.