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Plants belonging to the genus Taraxacum are widespread all over the world, which contain rubber-producing and non-rubber-producing species. However, the genomic basis underlying natural rubber (NR) biosynthesis still needs more investigation. Here, we presented high-quality genome assemblies of rubber-producing T. kok-saghyz TK1151 and non-rubber-producing T. mongolicum TM5. Comparative analyses uncovered a large number of genetic variations, including inversions, translocations, presence/absence variations, as well as considerable protein divergences between the two species. Two gene duplication events were found in these two Taraxacum species, including one common ancestral whole-genome triplication and one subsequent round of gene amplification. In genomes of both TK1151 and TM5, we identified the genes encoding for each step in the NR biosynthesis pathway and found that the SRPP and CPT gene families have experienced a more obvious expansion in TK1151 compared to TM5. This study will have large-ranging implications for the mechanism of NR biosynthesis and genetic improvement of NR-producing crops.

In flowering plants, the transition from mitosis to meiosis is the precondition for gametogenesis, which is the most crucial event during sexual reproduction. Here, we report an intriguing mechanism whereby germ cells and surrounding somatic cells cooperatively involve in the meiotic switch during anther development in rice (Oryza sativa). In double mutants with loss function of both leptotene chromosome establishment- and somatic cell layer differentiation-associated genes, chromosome morphology in the reproductive cells remains the same as that in somatic cells, and sporogenous cells fail to differentiate into pollen mother cells. OsSPOROCYTELESS and MICROSPORELESS1, two pivotal genes involved in meiosis entry, are prominently downregulated in anthers of plants with mutations in both MULTIPLE SPOROCYTE1 and LEPTOTENE 1. In addition, the transcription of redox-related genes is also affected. Therefore, germ cells and the surrounding somatic cells collaboratively participate in meiosis initiation in rice.

Replication protein A (RPA), a single-stranded DNA-binding protein, plays essential role in homologous recombination. However, because deletion of RPA causes embryonic lethality in mammals, the exact function of RPA in meiosis remains unclear. In this study, we generated an rpa1a mutant using CRISPR/Cas9 technology and explored its function in rice (Oryza sativa) meiosis. In rpa1a, 12 bivalents were formed at metaphase I, just like in wild-type, but chromosome fragmentations were consistently observed at anaphase I. Fluorescence in situ hybridization assays indicated that these fragmentations were due to the failure of the recombination intermediates to resolve. Importantly, the mutant had a highly elevated chiasma number, and loss of RPA1a could completely restore the 12 bivalent formations in the zmm (for ZIP1-4, MSH4/5, and MER3) mutant background. Protein–protein interaction assays showed that RPA1a formed a complex with the methyl methansulfonate and UV sensitive 81 (and the Fanconi anemia complementation group M–Bloom syndrome protein homologs (RECQ4A)–Topoisomerase3α–RecQ-mediated genome instability 1 complex to regulate chiasma formation and processing of the recombination intermediates. Thus, our data establish a pivotal role for RPA1a in promoting the accurate resolution of recombination intermediates and in limiting redundant chiasma formation during rice meiosis.

During meiosis, the number of double-strand breaks (DSBs) far exceeds the final number of crossovers (COs). Therefore, to identify proteins involved in determining which of these DSBs repaired into COs is critical in understanding the mechanism of CO control. Across species, HEI10-related proteins play important roles in CO formation. Here, through screening for HEI10-interacting proteins via a yeast two-hybrid system, we identify a CO protein HEI10 Interaction Protein 1 (HEIP1) in rice. HEIP1 colocalizes with HEI10 in a dynamic fashion along the meiotic chromosomes and specially localizes onto crossover sites from late pachytene to diplotene. Between these two proteins, HEI10 is required for the loading of HEIP1, but not vice versa. Moreover, mutations of the HEIP1 gene cause the severe reduction of chiasma frequency, whereas early homologous recombination processes are not disturbed and synapsis proceeds normally. HEIP1 interacts directly with ZIP4 and MSH5. In addition, the loading of HEIP1 depends on ZIP4, but not on MER3, MSH4, or MSH5. Together, our results suggest that HEIP1 may be a member of the ZMM group and acts as a key element regulating CO formation.

The phloem-unloading pathway of sucrose and mechanism of sugar-to-oil transition are still unknown in Camellia oleifera Fruit. Here, transmission electronic microscopy (TEM) and confocal laser-scanning microscopy (CLSM) were used to observe the cellular structure of vascular bundles and symplastic tracer, carboxyfluorescein (CF), transport in phloem zone. The results showed that sucrose was transported via symplast system in the early and late phases, whereas apoplast system exerted the function in middle stage. Moreover, enzymatic assays showed that acid invertase had a higher activity at the transition stage during the whole fruit development. The cell wall bound invertase (CWI) activity reached the highest at the middle stage of fruit development and the switch in phloem-unloading coincided with fruit developmental phase change and oil accumulation. Correlation analysis showed that the oil accumulation was significantly negatively correlated with content of soluble sugar at P

Abnormal meiotic chromosome behavior and pollen variation in two intersectional hybrids between Populus simonii Carr. (section Tacamahaca Spach) and Populus euphratica Oliv. (section Turanga Bge.) were investigated by the squashed technique and indirect immunofluorescence. Both hybrids produced abnormal flower buds characterized by a primary bud with an accessary bud at frequencies of 39.0 and 17.8 %, respectively. Chromosome pairing analysis at late diakinesis showed average configurations 20.31 I + 7.04 II + 1.42 III + 0.04 IV in hybrid “Xhu-DK” and 19.18 I + 7.16 II + 1.45 III + 0.04 IV in hybrid “Xhu-TL”, indicating high divergence between their parent species. Meiotic abnormalities, including precocious chromosome migration, lagging chromosomes, chromosome bridges, and micronuclei, were observed at high frequency, resulting in imbalanced chromosome segregation and chromosome elimination and caused high gamete sterility. Some micronuclei triggered formation of minispindles in Metaphase II, which were responsible for the formation of microcytes. Fused and tripolar spindles in the second meiotic division developed into dyads and triads, respectively, leading to viable first meiotic division restitution (FDR) type unreduced pollen formation. However, Xhu-TL was completely sterile because the high frequency of precocious cytokinesis during the second division prevented unreduced pollen formation though spindle fusion. Xhu-DK produced 29.6 % viable pollen grains. The diameter of viable pollen ranged from 19.6 to 64.9 μm, suggesting that Xhu-DK might be capable of producing unreduced pollen or pollen with a higher ploidy level. The strategy of using the distant hybrids for chromosome introgression and polyploidy breeding in Populus L. was considered.