Background: Chrysosplenium L. (Saxifragaceae) is a genus of plants widely distributed in Northern Hemisphere and usually found in moist, shaded valleys and mountain slopes. This genus is ideal for studying plant adaptation to low light conditions. Although some progress has been made in the systematics and biogeography of Chrysosplenium, its chloroplast genome evolution remains to be investigated. Results: To fill this gap, we sequenced the chloroplast genomes of six Chrysosplenium species and analyzed their genome structure, GC content, and nucleotide diversity. Moreover, we performed a phylogenetic analysis and calculated non-synonymous (Ka) /synonymous (Ks) substitution ratios using the combined protein-coding genes of 29 species within Saxifragales and two additional species as outgroups, as well as a pair-wise estimation for each gene within Chrysosplenium. Compared with the outgroups in Saxifragaceae, the six Chrysosplenium chloroplast genomes had lower GC contents; they also had conserved boundary regions and gene contents, as only the rpl32 gene was lost in four of the Chrysosplenium chloroplast genomes. Phylogenetic analyses suggested that the Chrysosplenium separated to two major clades (the opposite group and the alternate group). The selection pressure estimation (Ka/Ks ratios) of genes in the Chrysosplenium species showed that matK and ycf2 were subjected to positive selection. Conclusion: This study provides genetic resources for exploring the phylogeny of Chrysosplenium and sheds light on plant adaptation to low light conditions. The lower average GC content and the lacking gene of rpl32 indicated selective pressure in their unique habitats. Different from results previously reported, our selective pressure estimation suggested that the genes related to photosynthesis (such as ycf2) were under positive selection at sites in the coding region.

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.

Changes in environmental temperature affect multiple meiotic processes in flowering plants. Polyploid plants derived from whole-genome duplication (WGD) have enhanced genetic plasticity and tolerance to environmental stress but face challenges in organizing and segregating doubled chromosome sets. In this study, we investigated the impact of increased environmental temperature on male meiosis in autotetraploid Arabidopsis (Arabidopsis thaliana). Under low to mildly increased temperatures (5°C–28°C), irregular chromosome segregation universally occurred in synthetic autotetraploid Columbia-0 (Col-0). Similar meiotic lesions occurred in autotetraploid rice (Oryza sativa L.) and allotetraploid canola (Brassica napus cv Westar), but not in evolutionarily derived hexaploid wheat (Triticum aestivum). At extremely high temperatures, chromosome separation and tetrad formation became severely disordered due to univalent formation caused by the suppression of crossing-over. We found a strong correlation between tetravalent formation and successful chromosome pairing, both of which were negatively correlated with temperature elevation, suggesting that increased temperature interferes with crossing-over predominantly by impacting homolog pairing. We also showed that loading irregularities of axis proteins ASY1 and ASY4 co-localize on the chromosomes of the syn1 mutant and the heat-stressed diploid and autotetraploid Col-0, revealing that heat stress affects the lateral region of synaptonemal complex (SC) by impacting the stability of the chromosome axis. Moreover, we showed that chromosome axis and SC in autotetraploid Col-0 are more sensitive to increased temperature than those in diploid Arabidopsis. Taken together, our data provide evidence suggesting that WGD negatively affects the stability and thermal tolerance of meiotic recombination in newly synthetic autotetraploid Arabidopsis.

Background: Seed germination is a series of ordered physiological and morphogenetic processes and a critical stage in plant life cycle. Tamarix hispida is one of the most salt-tolerant plant species; however, its seed germination has not been analysed using combined transcriptomics and metabolomics. Results: Transcriptomic sequencing and widely targeted metabolomics were used to detect the transcriptional metabolic profiles of T. hispida at different stages of seed germination and young seedling growth. Transcriptomics showed that 46,538 genes were significantly altered throughout the studied development period. Enrichment study revealed that plant hormones, such as auxin, ABA, JA and SA played differential roles at varying stages of seed germination and post-germination. Metabolomics detected 1022 metabolites, with flavonoids accounting for the highest proportion of differential metabolites. Combined analysis indicated that flavonoid biosynthesis in young seedling growth, such as rhoifolin and quercetin, may improve the plant’s adaptative ability to extreme desert environments. Conclusions: The differential regulation of plant hormones and the accumulation of flavonoids may be important for the seed germination survival of T. hispida in response to salt or arid deserts. This study enhanced the understanding of the overall mechanism in seed germination and post-germination. The results provide guidance for the ecological value and young seedling growth of T. hispida.

Chrysosplenium zhouzhiense Hong Liu, a new species from Shaanxi, north-western China, is described and photographed. The new species belongs to Subgen. Gamosplenium Sect. Nephrophylloides Ser. Macrophylla and is most similar to C. macrophyllum and C. zhangjiajieense from which it differs by having a shorter stem, rhizome absent, basal leaf absent, sterile branch arising from the flowering stem and a light yellow flower with longer stamen. A global conservation assessment is performed and classifies C. zhouzhiense as Endangered (EN).

Chrysosplenium, a perennial herb in the family Saxifragaceae, prefers to grow in low light and moist environments and is divided into two sections of Alternifolia and Oppositifolia based on phyllotaxy. Although there has been some progress in the phylogeny of Chrysosplenium over the years, the phylogenetic position of some species is still controversial. In this study, we assembled chloroplast genomes (cp genomes) of 34 Chrysosplenium species and performed comparative genomic and phylogenetic analyses in combination with other cp genomes of previously known Chrysosplenium species, for a total of 44 Chrysosplenium species. The comparative analyses revealed that cp genomes of Chrysosplenium species were more conserved in terms of genome structure, gene content and arrangement, SSRs, and codon preference, but differ in genome size and SC/IR boundaries. Phylogenetic analysis showed that cp genomes effectively improved the phylogenetic support and resolution of Chrysosplenium species and strongly supported Chrysosplenium species as a monophyletic taxon and divided into three branches. The results also showed that the sections of Alternifolia and Oppositifolia were not monophyletic with each other, and that C. microspermum was not clustered with other Chrysosplenium species with alternate leaves, but with C. sedakowii into separate branches. In addition, we identified 10 mutational hotspot regions that could serve as potential DNA barcodes for Chrysosplenium species identification. In contrast to Peltoboykinia, the clpP and ycf2 genes of Chrysosplenium were subjected to positive selection and had multiple significant positive selection sites. We further detected a significant positive selection site on the petG gene between the two sections of Chrysosplenium. These evolutionary characteristics may be related to the growth environment of Chrysosplenium species. This study enriches the cp genomes of Chrysosplenium species and provides a reference for future studies on its evolution and origin.