A Genetic Transformation Method for Cadmium Hyperaccumulator Sedum plumbizincicola and Non-hyperaccumulating Ecotype of Sedum alfredii

Improving Sedum plumbizincicola genetic transformation with the SpGRF4-SpGIF1 gene and the self-excision CRE/LoxP system

MAIN CONCLUSION

S. plumbizincicola genetic transformation was optimized using a self-excision molecular-assisted transformation system by integrating the SpGRF4/SpGIF1 gene with XVE and Cre/loxP. Sedum plumbizincicola, despite being an excellent hyperaccumulator of cadmium and zinc with significant potential for soil pollution phytoremediation on farmland, has nonetheless trailed behind other major model plants in genetic transformation technology. In this study, different explants and SpGRF4-SpGIF1 genes were used to optimize the genetic transformation of S. plumbizincicola. We found that petiole and stem segments had higher genetic transformation efficiency than cluster buds. Overexpression of SpGRF4-SpGIF1 could significantly improve the genetic transformation efficiency and shorten the period of obtaining regenerated buds. However, molecular assistance with overexpression of SpGRF4-SpGIF1 leads to abnormal morphology, resulting in plant tissue enlargement and abnormal growth. Therefore, we combined SpGRF4-SpGIF1 with XVE and Cre/loxP to obtain DNA autocleavage transgenic plants induced by estradiol, thereby ensuring normal growth in transgenic plants. This study optimized the S. plumbizincicola genetic transformation system, improved the efficiency of genetic transformation, and established a self-excision molecular-assisted transformation system. This work also established the basis for studying S. plumbizincicola gene function, and for S. plumbizincicola breeding and germplasm innovation.

High-quality genome assembly and genetic transformation system of Lasiodiplodia theobromae strain LTTK16-3, a fungal pathogen of Chinese hickory

Lasiodiplodia theobromae, as one of the causative agents associated with Chinese hickory trunk cankers, has caused huge economic losses to the Chinese hickory industry. Although the biological characteristics of this pathogen and the occurrence pattern of this disease have been well studied, few studies have addressed the related mechanisms due to the poor molecular and genetic study basis of this fungus. In this study, we sequenced and assembled L. theobromae strain LTTK16-3, isolated from a Chinese hickory tree (cultivar of Linan) in Linan, Zhejiang province, China. Phylogenetic analysis and comparative genomics analysis presented crucial cues in the prediction of LTTK16-3, which shared similar regulatory mechanisms of transcription, DNA replication, and DNA damage response with the other four Chinese hickory trunk canker-associated Botryosphaeria strains including, Botryosphaeria dothidea, Botryosphaeria fabicerciana, Botryosphaeria qingyuanensis, and Botryosphaeria corticis. Moreover, it contained 18 strain-specific protein clusters (not conserved in the other L. theobromae strains, AM2As and CITRA15), with potential roles in specific host-pathogen interactions during the Chinese hickory infection. Additionally, an efficient system for L. theobromae protoplast preparation and polyethylene glycol (PEG) -mediated genetic transformation was firstly established as the foundation for its future mechanisms study. Collectively, the high-quality genome data and the efficient transformation system of L. theobromae here set up the possibility of targeted molecular improvements for Chinese hickory canker control.IMPORTANCEFungi with disparate genomic features are physiologically diverse, possessing species-specific survival strategies and environmental adaptation mechanisms. The high-quality genome data and related molecular genetic studies are the basis for revealing the mechanisms behind the physiological traits that are responsible for their environmental fitness. In this study, we sequenced and assembled the LTTK16-3 strain, the genome of Lasiodiplodia theobromae first obtained from a diseased Chinese hickory tree (cultivar of Linan) in Linan, Zhejiang province, China. Further phylogenetic analysis and comparative genomics analysis provide crucial cues in the prediction of the proteins with potential roles in specific host-pathogen interactions during the Chinese hickory infection. An efficient PEG-mediated genetic transformation system of L. theobromae was established as the foundation for the future mechanisms exploration. The above genetic information and tools set up valuable clues to study L. theobromae pathogenesis and assist in Chinese hickory canker control.

Cloning and Functional Characterization of SpZIP2

Zinc (Zn)-regulated and iron (Fe)-regulated transporter-like proteins (ZIP) are key players involved in the accumulation of cadmium (Cd) and Zn in plants. Sedum plumbizincicola X.H. Guo et S.B. Zhou ex L.H. Wu (S. plumbizincicola) is a Crassulaceae Cd/Zn hyperaccumulator found in China, but the role of ZIPs in S. plumbizincicola remains largely unexplored. Here, we identified 12 members of ZIP family genes by transcriptome analysis in S. plumbizincicola and cloned the SpZIP2 gene with functional analysis. The expression of SpZIP2 in roots was higher than that in the shoots, and Cd stress significantly decreased its expression in the roots but increased its expression in leaves. Protein sequence characteristics and structural analysis showed that the content of alanine and leucine residues in the SpZIP2 sequence was higher than other residues, and several serine, threonine and tyrosine sites can be phosphorylated. Transmembrane domain analysis showed that SpZIP2 has the classic eight transmembrane regions. The evolutionary analysis found that SpZIP2 is closely related to OsZIP2, followed by AtZIP11, OsZIP1 and AtZIP2. Sequence alignment showed that most of the conserved sequences among these members were located in the transmembrane regions. A further metal sensitivity assay using yeast mutant Δyap1 showed that the expression of SpZIP2 increased the sensitivity of the transformants to Cd but failed to change the resistance to Zn. The subsequent ion content determination showed that the expression of SpZIP2 increased the accumulation of Cd in yeast. Subcellular localization showed that SpZIP2 was localized to membrane systems, including the plasma membrane and endoplasmic reticulum. The above results indicate that ZIP member SpZIP2 participates in the uptake and accumulation of Cd into cells and might contribute to Cd hyperaccumulation in S. plumbizincicola.

Maintenance of grafting reducing cadmium accumulation in soybean (Glycinemax) is mediated by DNA methylation

Cadmium (Cd) pollution in farmland soil increases the probability of wastage of land resources and compromised food safety. Grafting can change the absorption rates of elements in crops; however, there are few studies on grafting in bulk grain and cash crops. In this study, Glycine max was used as a scion and Luffa aegyptiaca as a rootstock for grafting experiments. The changes in total sulfur and Cd content in the leaves and grains of grafted species were determined for three consecutive generations, and the gene expression and DNA methylation status of the leaves were analyzed. The results show that grafting significantly reduced the total sulfur and Cd content in soybean leaves and grains; the Cd content in soybean leaves and grains decreased by >50 %. The plant's primary sulfur metabolism pathway was not significantly affected. Glucosinolates and DNA methylation may play important roles in reducing total sulfur and Cd accumulation. Notably, low sulfur and low Cd traits can be maintained over two generations. Our study establishes that grafting can reduce the total sulfur and Cd content in soybean, and these traits can be inherited. In summary, grafting technology can be used to prevent soybean from accumulating Cd in farmland soil. This provides a theoretical basis for grafting to cultivate crops with low Cd accumulation.

Quantitative proteome analysis reveals changes of membrane transport proteins in Sedum plumbizincicola under cadmium stress

Sedum plumbizincicola is an herbaceous species tolerant of excessive cadmium accumulation in above-ground tissues. The implications of membrane proteins, especially integrative membrane proteins, in Cd detoxification of plants have received attention in recent years, but a comprehensive profiling of Cd-responsive membrane proteins from Cd hyperaccumulator plants is lacking. In this study, the membrane proteins of root, stem, and leaf tissues of S. plumbizincicola seedlings treated with Cd solution for 0, 1 or 4 days were analyzed by Tandem Mass Tag (TMT) labeling-based proteome quantification (Data are available via ProteomeXchange with identifier PXD025302). Total 3353 proteins with predicted transmembrane helices were identified and quantified in at least one tissue group. 1667 proteins were defined as DAPs (differentially abundant proteins) using fold change >1.5 with p-values <0.05. The number of DAPs involved in metabolism, transport protein, and signal transduction was significantly increased after exposure to Cd, suggesting that the synthesis and decomposition of organic compounds and the transport of ions were actively involved in the Cd tolerance process. The number of up-regulated transport proteins increased significantly from 1-day exposure to 4-day exposure, from 5 to 112, 16 to 42, 18 to 44, in root, stem, and leaf, respectively. Total 352 Cd-regulated transport proteins were identified, including ABC transporters, ion transport proteins, aquaporins, proton pumps, and organic transport proteins. Heterologous expression of SpABCB28, SpMTP5, SpNRAMP5, and SpHMA2 in yeast and subcellular localization showed the Cd-specific transport activity. The results will enhance our understanding of the molecular mechanism of Cd hypertolerance and hyperaccumulation in S. plumbizincicola and will be benefit for future genetic engineering in phytoremediation.

Comparative understanding of metal hyperaccumulation in plants: a mini-review

Hyperaccumulator plants are ideal models for investigating the regulatory mechanisms of plant metal homeostasis and environmental adaptation due to their notable traits of metal accumulation and tolerance. These traits may benefit either the biofortification of essential mineral nutrients or the phytoremediation of nonessential toxic metals. A common mechanism by which elevated expression of key genes involved in metal transport or chelation contributes to hyperaccumulation and hypertolerance was proposed mainly from studies examining two Brassicaceae hyperaccumulators, namely Arabidopsis halleri and Noccaea caerulescens (formerly Thlaspi caerulescens). Meanwhile, recent findings regarding systems outside the Brassicaceae hyperaccumulators indicated that functional enhancement of key genes might represent a strategy evolved by hyperaccumulator plants. This review provides a brief outline of metal hyperaccumulation in plants and highlights commonalities and differences among various hyperaccumulators.

SaHsfA4c From Sedum alfredii Hance Enhances Cadmium Tolerance by Regulating ROS-Scavenger Activities and Heat Shock Proteins Expression

The heat shock transcription factor (Hsf) family, an important member in plant stress response, affects cadmium (Cd) tolerance in plants. In this study, we identified and functionally characterized a transcript of the Hsf A4 subgroup from Sedum alfredii. Designated as SaHsfA4c, the open reading frame was 1,302 bp long and encoded a putative protein of 433 amino acids containing a complete DNA-binding domain (DBD). Heterologous expression of SaHsfA4c in yeast enhanced Cd stress tolerance and accumulation, whereas expression of the alternatively spliced transcript InSaHsfA4c which contained an intron and harbored an incomplete DBD, resulted in relatively poor Cd stress tolerance and low Cd accumulation in transgenic yeast. The function of SaHsfA4c under Cd stress was characterized in transgenic Arabidopsis and non-hyperaccumulation ecotype S. alfredii. SaHsfA4c was able to rescue the Cd sensitivity of the Arabidopsis athsfa4c mutant. SaHsfA4c reduced reactive oxygen species (ROS) accumulation and increased the expression of ROS-scavenging enzyme genes and Hsps in transgenic lines. The present results suggest that SaHsfA4c increases plant resistance to stress by up-regulating the activities of ROS-scavenging enzyme and the expression of Hsps.

SpHMA1 is a chloroplast cadmium exporter protecting photochemical reactions in the Cd hyperaccumulator Sedum plumbizincicola

Sedum plumbizincicola is able to hyperaccumulate cadmium (Cd), a nonessential and highly toxic metal, in the above-ground tissues, but the mechanisms for its Cd hypertolerance are not fully understood. Here, we show that the heavy metal ATPase 1 (SpHMA1) of S. plumbizincicola plays an important role in chloroplast Cd detoxification. Compared with the HMA1 ortholog in the Cd nonhyperaccumulating ecotype of Sedum alfredii, the expression of SpHMA1 in the leaves of S. plumbizincicola was >200 times higher. Heterologous expression of SpHMA1 in Saccharomyces cerevisiae increased Cd sensitivity and Cd transport activity in the yeast cells. The SpHMA1 protein was localized to the chloroplast envelope. SpHMA1 RNA interference transgenic plants and CRISPR/Cas9-induced mutant lines showed significantly increased Cd accumulation in the chloroplasts compared with wild-type plants. Chlorophyll fluorescence imaging analysis revealed that the photosystem II of SpHMA1 knockdown and knockout lines suffered from a much higher degree of Cd toxicity than wild type. Taken together, these results suggest that SpHMA1 functions as a chloroplast Cd exporter and protects photosynthesis by preventing Cd accumulation in the chloroplast in S. plumbizincicola and hyperexpression of SpHMA1 is an important component contributing to Cd hypertolerance in S. plumbizincicola.

Micropropagation and Subsequent Enrichment of Carotenoids, Fatty Acids, and Tocopherol Contents in Sedum dasyphyllum L

A promising micropropagation protocol has been systematically established and demonstrated for the enhanced production of carotenoids, tocopherol and fatty acids in shoot tissues of Sedum dasyphyllum. Shoot tip explants were grown on Murashige and Skoog (MS) medium. Different concentrations of N⁶-benzyladenine (BA) or thidiazuron (TDZ) alone or in combination with α-naphthaleneacetic acid (NAA) were tested in order to stimulate multiple shoot production. Ideal shoot induction (100%) and maximized shoot numbers (36.4) were obtained on explants cultured on media incorporated with 2 μM BA and 1 μM NAA combinations. The in vitro-developed shoots rooted best on half-strength MS media incorporated with 2 μM indole 3-butyric acid. Plantlets were effectively acclimatized in the greenhouse with 100% survival rate. The composition and contents of bioactive compounds such as carotenoids, tocopherol and fatty acids in shoot tissues of S. dasyphyllum were investigated using HPLC and GC-MS. The most abundant carotenoid in the shoot tissue was all-E-lutein (40.3-70.5 μg g^-1 FW) followed by 9'-Z-neoxanthin (5.3-9.9 μg g^-1 FW), all-E-violaxanthin (4.4-8.2 μg g^-1 FW), and all-E-β-carotene (1.6-3.6 μg g^-1 FW). The α-tocopherol contents of in vitro-raised shoots was 6.5-fold higher than shoots of greenhouse-grown plants. The primary fatty acids found in shoot tissues were α-linolenic acid (32.0-39.3%), linoleic acid (27.4-38.2%), palmitic acid (13.3-15.5%), and stearic acid (5.2-12.2%). In all, summarizing the findings, the micropropagated S. dasyphyllum showed significant enrichment of valuable bioactive carotenoids (92.3 μg g^-1 FW), tocopherols (14.6 μg g^-1 FW), and α-linolenic acid (39.3%) compared to their greenhouse counterparts. The protocol demonstrated here could be applied for the mass propagation and production of enhanced bioactive compounds from S. dasyphyllum with credibility.