Heat stress interferes with formation of double-strand breaks and homolog synapsis.

Meiotic recombination (MR) drives novel combinations of alleles and contributes to genomic diversity in eukaryotes. In this study, we showed that heat stress (36°C-38°C) over the fertile threshold fully abolished crossover formation in Arabidopsis (Arabidopsis thaliana). Cytological and genetic studies in wild-type plants and syn1 and rad51 mutants suggested that heat stress reduces generation of SPO11-dependent double-strand breaks (DSBs). In support, the abundance of recombinase DMC1, which is required for MR-specific DSB repair, was significantly reduced under heat stress. In addition, high temperatures induced disassembly and/or instability of the ASY4- but not the SYN1-mediated chromosome axis. At the same time, the ASY1-associated lateral element of the synaptonemal complex (SC) was partially affected, while the ZYP1-dependent central element of SC was disrupted, indicating that heat stress impairs SC formation. Moreover, expression of genes involved in DSB formation; e.g. SPO11-1, PRD1, 2, and 3 was not impacted; however, recombinase RAD51 and chromosome axis factors ASY3 and ASY4 were significantly downregulated under heat stress. Taken together, these findings revealed that heat stress inhibits MR via compromised DSB formation and homolog synapsis, which are possible downstream effects of the impacted chromosome axis. Our study thus provides evidence shedding light on how increasing environmental temperature influences MR in Arabidopsis.

The chromosome-scale reference genome of safflower (Carthamus tinctorius) provides insights into linoleic acid and flavonoid biosynthesis.

Safflower (Carthamus tinctorius L.), a member of the Asteraceae, is a popular crop due to its high linoleic acid (LA) and flavonoid (such as hydroxysafflor yellow A) contents. Here, we report the first high-quality genome assembly (contig N50 of 21.23 Mb) for the 12 pseudochromosomes of safflower using single-molecule real-time sequencing, Hi-C mapping technologies and a genetic linkage map. Phyloge nomic analysis showed that safflower diverged from artichoke (Cynara cardunculus) and sunflower (Helianthus annuus) approximately 30.7 and 60.5 million years ago, respectively. Comparative genomic analyses revealed that uniquely expanded gene families in safflower were enriched for those predicted to be involved in lipid metabolism and transport and abscisic acid signalling. Notably, the fatty acid desaturase 2 (FAD2) and chalcone synthase (CHS) families, which function in the LA and flavonoid biosynthesis pathways, respectively, were expanded via tandem duplications in safflower. CarFAD2-12 was specifically expressed in seeds and was vital for high-LA content in seeds, while tandemly duplicated CarFAD2 genes were up-regulated in ovaries compared to CarFAD2-12, which indicates regulatory divergence of FAD2 in seeds and ovaries. CarCHS1, CarCHS4 and tandem-duplicated CarCHS5˜CarCHS6, which were up-regulated compared to other CarCHS members at early stages, contribute to the accumulation of major flavonoids in flowers. In addition, our data reveal multiple alternative splicing events in gene families related to fatty acid and flavonoid biosynthesis. Together, these results provide a high-quality reference genome and evolutionary insights into the molecular basis of fatty acid and flavonoid biosynthesis in safflower.

trans-2-Enoyl-CoA Reductase Tecr-Driven Lipid Metabolism in Endothelial Cells Protects against Transcytosis to Maintain Blood-Brain Barrier Homeostasis.

The transport and metabolism of lipids in cerebrovascular endothelial cells (ECs) have been hypothesized to regulate blood-brain barrier (BBB) maturation and homeostasis. Long-chain polyunsaturated fatty acids (LCPUFAs) as the important lipids components of cell membranes are essential for the development and function of BBB, but the direct links of lipid metabolism and ECs barrier function remain to be established. Here, we comprehensively characterize the transcriptomic phenotype of developmental cerebrovascular ECs in single-cell resolution and firstly find that trans-2-enoyl-CoA reductase (Tecr), a very-long-chain fatty acid synthesis, is highly expressed during barriergenesis and decreased after BBB maturation. EC-specific knockout of Tecr compromises angiogenesis due to delayed vascular sprouting. Importantly, EC-specific deletion of Tecr loss restrictive quality of vascular permeability from neonatal stages to adulthood, with high levels of transcytosis, but maintains the vascular tight junctions. Moreover, lipidomic analysis shows that the expression of Tecr in ECs is associated with the containing of omega-3 fatty acids, which directly suppresses caveolae vesicles formation. These results reveal a protective role for Tecr in BBB integrity and suggest that Tecr as a novel therapeutic target in the central nervous system (CNS) diseases associated with BBB dysfunction.

Complete plastid genome sequence of Halimodendron halodendron (Leguminosae).

Halimodendron halodendron (Pall.) Voss. is a deciduous shrub belonging to the genus Halimodendron, Leguminosae, and is mainly distributed in dry areas. This species can be used for saline-alkali soil improvement and sand fixation. The complete plastid genome of H. halodendron first reported here is 129,342 bp in length, and contains 110 genes, including 76 protein coding genes, 30 tRNA genes, and 4 rRNA genes. A total of 105 simple sequence repeats (SSRs) were identified in the chloroplast genome. This information will be useful for study on the evolution and genetic diversity of Halimodendron halodendron in the future.

The complete chloroplast genome of Alhagi sparsifolia Shap. (Leguminosae).

Alhagi sparsifolia Shap. is a perennial herbaceous plant belonging to the genus Alhagi, Leguminosae. This species is of high nutritional, medicinal and ecological values. The complete chloroplast genome was 128,418 bp and lost an IR (inverted repeat) region. Further annotation revealed the chloroplast genome contains 108 genes, including 75 protein coding genes, 29 tRNA genes, and 4 rRNA genes. A total of 103 simple sequence repeats (SSRs) were identified in the chloroplast genome. This chloroplast genome resource will be useful for study on the evolution and genetic diversity of A. sparsifolia in the future.

Genome-Wide Identification, Characterization, and Regulation of RWP-RK Gene Family in the Nitrogen-Fixing Clade.

RWP-RK is a plant-specific family of transcription factors, involved in nitrate response, gametogenesis, and nodulation. However, genome-wide characterization, phylogeny, and the regulation of RWP-RK genes in the nodulating and non-nodulating plant species of nitrogen-fixing clade (NFC) are widely unknown. Therefore, we identified a total of 292 RWP-RKs, including 278 RWP-RKs from 25 NFC species and 14 RWP-RKs from the outgroup, Arabidopsis thaliana. We classified the 292 RWP-RKs in two subfamilies: the NIN-like proteins (NLPs) and the RWP-RK domain proteins (RKDs). The transcriptome and phylogenetic analysis of RWP-RKs suggested that, compared to RKD genes, the NLP genes were just upregulated in nitrate response and nodulation. Moreover, nodule-specific NLP genes of some nodulating NFC species may have a common ancestor (OG0002084) with AtNLP genes in A. thaliana. Further, co-expression networks of A.thaliana under N-starvation and N-supplementation conditions revealed that there is a higher correlation between expression of AtNLP genes and symbiotic genes during N-starvation. In P. vulgaris, we confirmed that N-starvation stimulated nodulation by regulating expression of PvNLP2, closely related to AtNLP6 and AtNLP7 with another common origin (OG0004041). Taken together, we concluded that different origins of the NLP genes involved in both N-starvation response and specific expression of nodulation would contribute to the evolution of nodulation in NFC plant species. Our results shed light on the phylogenetic relationships of NLP genes and their differential regulation in nitrate response of A. thaliana and nodulation of NFC.

Comprehensive Identification and Expression Profiling of Circular RNAs During Nodule Development in Phaseolus vulgaris.

Symbiotic nitrogen fixation by legume nodules provides an abundant nitrogen source for plants, and understanding this process is key for developing green agriculture. Circular RNA (circRNA), a type of endogenous RNA produced by reverse splicing of mRNA precursors, plays important regulatory roles in plants at the transcriptional and post-transcriptional levels. However, the relationship between circRNAs and legume-rhizobium is unknown. Here, we performed comprehensive identification and expression profiling of circRNAs during nodulation in common bean (Phaseolus vulgaris) compared to uninoculated roots of corresponding ages by constructing circRNA-seq and mRNA-seq libraries. We identified 8,842 high-confident circRNAs, 3,448 of which were specifically produced during symbiosis, with the highest number at the nitrogen-fixing stage. Significantly, more circRNAs were derived from exons than from intergenic regions or introns in all samples. The lengths and GC contents of the circRNAs were similar in roots and nodules. However, circRNAs showed specific spatiotemporal expression patterns during nodule and root development. GO and other functional annotation of parental genes of differentially expressed circRNAs indicated their potential involvement in different biological processes. The expression of major circRNAs during symbiosis is independent of parental genes' expression to a certain degree, while expression of the remaining minor circRNAs showed positive correlation to parental genes. Functional annotation of the targeted mRNAs in the circRNA-miRNA-mRNA network showed that circRNAs may be involved in transmembrane transport and positive regulation of kinase activity during nodulation and nitrogen fixation as miRNA sponges. Our comprehensive analysis of the expression profile of circRNAs and their potential functions suggests that circRNAs may function as new post-transcriptional regulators in legume-rhizobium symbiosis.