Yuxiao Wang

Quercus texana seeds are recalcitrant and thus highly sensitive to desiccation, which makes storage difficult. For practical seed handling, it is important to define their safe water content and to understand how water is distributed during dehydration. The present study utilized magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR) technologies to investigate the migration and phases of water, respectively, revealing the underlying reasons for the recalcitrance of Q. texana seeds. The water content of fresh Q. texana seeds was found to be 39.6% and the germination percentage was 93.3%. As the water content decreased, the germination percentage decreased continuously, reaching 0% at a water content of 13.0%. At 20.0% water content, the germination percentage was 71.7%. MRI showed that water was primarily stored in the embryo axis and cotyledon center in fresh Q. texana seeds. Water loss occurs in the following order during seed dehydration: embryo axis, cotyledon center, cotyledon periphery, and cotyledon end. However, water in the radicle region persisted until seed water content decreased to 15.0%, at which point no signal was detected. The NMR T₂ relaxation spectrum indicated the presence of bound water (T₂₁ = 0.01–5.44 ms) and free water (T₂₂ = 7.19–1401.93 ms) in the seeds. During the dehydration process, most of the water was lost as free water, and the T₂₂ shifted to longer times. Concurrently, the bound water shifted to shorter T₂₁ times. Overall, for practical purposes, seed water should be maintained at or above 20.0%. MRI further showed that water loss from the radicle plays a decisive role in the decline of seed germination, and that protecting the region of radicle and the cupule scar can effectively retard water loss. Furthermore, the bound-water content is positively correlated with seed germination.

Ilex rotunda Thunb. stands as a representative tree species in subtropical evergreen broad-leaved forests, widely distributed across southeast Asia. This species holds significant value in forestry due to its ecological resilience and adaptability. Although researchers have conducted in-depth research on the plastid genome (plastome) of I. rotunda, the mitochondrial genome (mitogenome) of this species has remained undocumented. In the present study, we successfully sequenced and assembled the I. rotunda mitogenome. The mitogenome has a circular structure and is 567,552 bp in total length, with a GC content of 45.47%. The composition of the mitogenome encompasses 40 protein-coding genes, along with 3 rRNA genes and 19 tRNA genes. Notably, the mitogenome exhibits a universal distribution of repetitive sequences, but the total length of repeats contributes to a relatively small proportion (4%) of the whole mitogenome, suggesting that repeats do not serve as the primary cause of the amplification of the Ilex mitogenomes. Collinear analysis indicates that the I. rotunda mitogenome is very conservative within Aquifoliales species. Additionally, our research identified 51 fragments of plastid genomic DNA, which have migrated from the plastome into the mitogenome, with five genes from the plastome remaining intact. Eventually, the phylogenetic analyses based on the plastomes and mitogenomes of 36 angiosperms determine the Aquifoliales to be the basal group in the campanulids. This study establishes the bedrock for prospective investigations in molecular breeding research.

The plant mitochondrial genome (mitogenome) has a complex organization and carries genetic traits of value in exploiting genetic resources. In this study, the Ilex macrocarpa mitogenome was sequenced and assembled to understand the genetic diversity and phylogenetic relationship. The mitogenome has a cyclic molecular structure and is 539,461 bp long with a GC content of 45.53%. It contained 39 protein-coding proteins, 19 tRNA genes, and 3 rRNA genes. The 146 simple sequence repeats and 591 dispersed repeat sequences were identified in the mitogenome. Codon usage analysis revealed a preference for codons ending in A/T. A total of 517 C to U RNA editing sites were predicted, with nad4 and ccmB being edited most frequently (39 times). Phylogenetic analysis via mitochondrial protein-coding genes showed that the Aquifoliales order contains sister plants to Lamiales, Solanales, and Gentianales. In addition, the conflicts between chloroplast and mitochondrial phylogeny were also observed. This study provides a theoretical framework for understanding the evolution, classification, and identification of the Ilex genus, which lays an important foundation for future molecular breeding investigations.

Background: Acer truncatum (purpleblow maple) is a woody tree species that produces seeds with high levels of valuable fatty acids (especially nervonic acid). The species is admired as a landscape plant with high developmental prospects and scientific research value. The A. truncatum chloroplast genome has recently been reported; however, the mitochondrial genome (mitogenome) is still unexplored. Results: We characterized the A. truncatum mitogenome, which was assembled using reads from PacBio and Illumina sequencing platforms, performed a comparative analysis against different species of Acer. The circular mitogenome of A. truncatum has a length of 791,052 bp, with a base composition of 27.11% A, 27.21% T, 22.79% G, and 22.89% C. The A. truncatum mitogenome contains 62 genes, including 35 protein-coding genes, 23 tRNA genes and 4 rRNA genes. We also examined codon usage, sequence repeats, RNA editing and selective pressure in the A. truncatum mitogenome. To determine the evolutionary and taxonomic status of A. truncatum, we conducted a phylogenetic analysis based on the mitogenomes of A. truncatum and 25 other taxa. In addition, the gene migration from chloroplast and nuclear genomes to the mitogenome were analyzed. Finally, we developed a novel NAD1 intron indel marker for distinguishing several Acer species. Conclusions: In this study, we assembled and annotated the mitogenome of A. truncatum, a woody oil-tree species producing nervonic acid. The results of our analyses provide comprehensive information on the A. truncatum mitogenome, which would facilitate evolutionary research and molecular barcoding in Acer.