Background Spinach (Spinacia oleracea L.) is a dioecious species with an XY sex chromosome system, but its Y chromosome has not been fully characterized. Our knowledge about the history of its domestication and improvement remains limited. Results A high-quality YY genome of spinach is assembled into 952 Mb in six pseudo-chromosomes. By a combination of genetic mapping, Genome-Wide Association Studies, and genomic analysis, we characterize a 17.42-Mb sex determination region (SDR) on chromosome 1. The sex chromosomes of spinach evolved when an insertion containing sex determination genes occurred, followed by a large genomic inversion about 1.98 Mya. A subsequent burst of SDR-specific repeats (0.1–0.15 Mya) explains the large size of this SDR. We identify a Y-specific gene, NRT1/PTR 6.4 which resides in this insertion, as a strong candidate for the sex determination or differentiation factor. Resequencing of 112 spinach genomes reveals a severe domestication bottleneck approximately 10.87 Kya, which dates the domestication of spinach 7000 years earlier than the archeological record. We demonstrate that a strong selection signal associated with internode elongation and leaf area expansion is associated with domestication of edibility traits in spinach. We find that several strong genomic introgressions from the wild species Spinacia turkestanica and Spinacia tetrandra harbor desirable alleles of genes related to downy mildew resistance, frost resistance, leaf morphology, and flowering-time shift, which likely contribute to spinach improvement. Conclusions Analysis of the YY genome uncovers evolutionary forces shaping nascent sex chromosome evolution in spinach. Our findings provide novel insights about the domestication and improvement of spinach.

Single-stranded DNA-binding proteins (SSBs) are assumed to involve in DNA replication, DNA repairmen, and gene transcription. Here, we provide the direct evidence on the functionality of an Arabidopsis SSB, WHIRLY1, by using loss- or gain-of-function lines. We show that WHIRLY1 binding to the promoter of WRKY53 represses the enrichment of H3K4me3, but enhances the enrichment of H3K9ac at the region contained WHIRLY1-binding sequences and TATA box or the translation start region of WRKY53, coincided with a recruitment of RNAPII. In vitro ChIP assays confirm that WHIRLY1 inhibits H3K4me3 enrichment at the preinitiation complex formation stage, while promotes H3K9ac enrichment and RNAPII recruitment at the elongation stage, consequently affecting the transcription of WRKY53. These results further explore the molecular actions underlying SSB-mediated gene transcription through epigenetic regulation in plant senescence.

Plastid-nucleus-located WHIRLY1 protein plays a role in regulating leaf senescence and is believed to associate with the increase of reactive oxygen species delivered from redox state of the photosynthetic electron transport chain. In order to make sure whether WHIRLY1 plays a role in photosynthesis, in this study, the performances of photosynthesis were detected in Arabidopsis whirly1 knockout (kowhy1) and plastid localized WHIRLY1 overexpression (oepWHY1) plants. Loss of WHIRLY1 leads to a higher photochemical quantum yield of photosystem I Y(I) and electron transport rate (ETR) and a lower non-photochemical quenching (NPQ) involved in the thermal dissipation of excitation energy of chlorophyll fluorescence than the wild type. Further analyses showed that WHIRLY1 interacts with Light-harvesting protein complex I (LHCA1) and affects the expression of genes encoding photosystem I (PSI) and light harvest complexes (LHCI). Moreover, loss of WHIRLY1 decreases chloroplast NAD(P)H dehydrogenase-like complex (NDH) activity and the accumulation of NDH supercomplex. Several genes encoding the PSI-NDH complexes are also up-regulated in kowhy1 and the whirly1whirly3 double mutant (ko1/3) but steady in oepWHY1 plants. However, under high light conditions (800 μmol m−2 s−1), both kowhy1 and ko1/3 plants show lower ETR than wild-type which are contrary to that under normal light condition. Moreover, the expression of several PSI-NDH encoding genes and ERF109 which is related to jasmonate (JA) response varied in kowhy1 under different light conditions. These results indicate that WHIRLY1 is involved in the alteration of ETR by affecting the activities of PSI and supercomplex formation of PSI with LHCI or NDH and may acting as a communicator between the plastids and the nucleus.

Leaf senescence, either as a natural stage of development or as an induced process under stress conditions, incorporates multiple intricate signaling pathways. At the cellular level, retrograde signals have been considered as important players during the initiation and progression of senescence in both animals and plants. The plant-specific single-strand DNA-binding protein WHIRLY1 (WHY1), a repressor of leaf natural senescence, is dually located in both nucleus and plastids. Despite many years of studies, the myth about its dual location and the underlying functional implications remain elusive. Here, we provide further evidence in Arabidopsis showing that alteration in WHY1 allocation between the nucleus and chloroplast causes perturbation in H2O2 homeostasis, resulting in adverse plant senescence phenotypes. The knockout of WHY1 increased H2O2 content at 37 days post-germination, coincident with an early leaf senescence phenotype, which can be rescued by ectopic expression of the nuclear isoform (nWHY1), but not by the plastid isoform (pWHY1). Instead, accumulated pWHY1 greatly provoked H2O2 in cells. On the other hand, exogenous H2O2 treatment induced a substantial plastid accumulation of WHY1 proteins and at the same time reduced the nuclear isoforms. This H2O2-induced loss of nucleus WHY1 isoform was accompanied by enhanced enrichments of histone H3 lysine 9 acetylation (H3K9ac) and recruitment of RNA polymerase II (RNAP II) globally, and specifically at the promoter of the senescence-related transcription factor WRKY53, which in turn activated WRKY53 transcription and led to a senescence phenotype. Thus, the distribution of WHY1 organelle isoforms and the feedback of H2O2 intervene in a circularly integrated regulatory network during plant senescence in Arabidopsis.

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