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Intersubspecific hybrid sterility is a common form of reproductive isolation in rice (Oryza sativa L.), which significantly hampers the utilization of heterosis between indica and japonica varieties. Here, we elucidated the mechanism of S7, which specially causes Aus-japonica/indica hybrid female sterility, through cytological and genetic analysis, map-based cloning, and transformation experiments. Abnormal positioning of polar nuclei and smaller embryo sac were observed in F 1 compared with male and female parents. Female gametes carrying S7 cp and S7 i were aborted in S7 ai /S7 cp and S7 ai /S7 i , respectively, whereas they were normal in both N22 and Dular possessing a neutral allele, S7 n . S7 was fine mapped to a 139-kb region in the centromere region on chromosome 7, where the recombination was remarkably suppressed due to aggregation of retrotransposons. Among 16 putative open reading frames (ORFs) localized in the mapping region, ORF3 encoding a tetratricopeptide repeat domain containing protein was highly expressed in the pistil. Transformation experiments demonstrated that ORF3 is the candidate gene: downregulated expression of ORF3 restored spikelet fertility and eliminated absolutely preferential transmission of S7 ai in heterozygote S7 ai /S7 cp ; sterility occurred in the transformants Cpslo17-S7 ai . Our results may provide implications for overcoming hybrid embryo sac sterility in intersubspecific hybrid rice and utilization of hybrid heterosis for cultivated rice improvement.KEYWORDS hybrid sterility; female gamete; tetratricopeptide repeat (TPR); transgenic; rice (Oryza sativa L.) H YBRIDIZATION between two different species can lead to a distinct phenotype, which can also be fitter than the parental lineage. However, reproductive isolation maintains the integrity of a species over time, reducing or directly impeding gene flow between individuals of different species (Mayr 1942;Grant 1981;Coyne and Orr 2004;Widmer et al. 2009;Baack et al. 2015). The mechanisms of reproductive isolation were classified into two broad categories: prezygotic and postzygotic isolation mechanisms (Mayr 1963;Levin 1978;Sweigart and Willis 2012;Chen et al. 2014). According to the classical Dobzhansky-Muller model, postzygotic isolation results from a deleterious interaction between functionally diverged genes from the hybridizing species (Dobzhansky 1937;Ting et al. 1998;Barbash et al. 2003;Presgraves et al. 2003;Brideau et al. 2006;Bayes and Malik 2009;Ferree and Barbash 2009;Phadnis and Orr 2009;Tang and Presgraves 2009;White et al. 2011). Genes for hybrid sterility, a common pattern of postzygotic isolation, have been reported in several organisms, including fungi, animals, and plants (Brideau et al. 2006;Lee et al. 2008;Bikard et al. 2009;De Vienne et al. 2009).Major progress has been made in rice and the interspecific and intersubspecific hybrid sterilities are perhaps the best known examples (Chen et al. 2008;Long et al. 2008 Wan and Ikehashi 1995;Zhu et al. 2005;Li et al. 2007;Zhao et al. 2007;Chen et...

Heading date in rice is an important agronomic trait controlled by several genes. In this study, flowering time of variety Dianjingyou 1 (DJY1) was earlier than a near-isogenic line (named NIL) carried chromosome segment from African rice on chromosome 3S, when grown in both long-day (LD) and short-day (SD) conditions. By analyzing a large F2 population from NIL × DJY1, the locus DTH3 (QTL for days to heading on chromosome 3) controlling early heading date in DJY1 was fine mapped to a 64-kb segment which contained only one annotated gene, a MIKC-type MADS-box protein. We detected a 6-bp deletion and a single base substitution in the C-domain by sequencing DTH3 in DJY1 compared with dth3 in NIL, and overexpression of DTH3 caused early flowering in callus. Quantitative real-time PCR revealed that the transcript level of dth3 in NIL was lower than that DTH3 in DJY1 in both LD and SD conditions. The Early heading date 1 (Ehd1) which promotes the RFT1, was up-regulated by DTH3 in both LD and SD conditions. Based on Indel and dCAPs marker analysis, the dth3 allele was only present in African rice accessions. A phylogenetic analysis based on microsatellite genotyping suggested that African rice had a close genetic relationship to O. rufipogon and O. latifolia, and was similar to japonica cultivars. DTH3 affected flowering time and had no significant effect on the main agronomic traits.

Phosphatidylserine synthase (PSS)-mediated phosphatidylserine (PS) synthesis is crucial for plant development. However, little is known about the contribution of PSS to Na + homeostasis regulation and salt tolerance in plants. Here, we cloned the IbPSS1 gene, which encodes an ortholog of Arabidopsis AtPSS1, from sweet potato (Ipomoea batatas (L.) Lam.). The transient expression of IbPSS1 in Nicotiana benthamiana leaves increased PS abundance. We then established an efficient Agrobacterium rhizogenes-mediated in vivo root transgenic system for sweet potato. Overexpression of IbPSS1 through this system markedly decreased cellular Na + accumulation in salinized transgenic roots (TRs) compared with adventitious roots. The overexpression of IbPSS1 enhanced salt-induced Na + /H + antiport activity and increased plasma membrane (PM) Ca 2+-permeable channel sensitivity to NaCl and H 2 O 2 in the TRs. We confirmed the important role of IbPSS1 in improving salt tolerance in transgenic sweet potato lines obtained from an Agrobacterium tumefaciens-mediated transformation system. Similarly, compared with the wild-type (WT) plants, the transgenic lines presented decreased Na + accumulation, enhanced Na + exclusion, and increased PM Ca 2+-permeable channel sensitivity to NaCl and H 2 O 2 in the roots. Exogenous application of lysophosphatidylserine triggered similar shifts in Na + accumulation and Na + and Ca 2+ fluxes in the salinized roots of WT. Overall, this study provides an efficient and reliable transgenic method for functional genomic studies of sweet potato. Our results revealed that IbPSS1 contributes to the salt tolerance of sweet potato by enabling Na + homeostasis and Na + exclusion in the roots, and the latter process is possibly controlled by PS reinforcing Ca 2+ signaling in the roots.

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The initiation and development of storage roots (SRs) are intricately regulated by a transcriptional regulatory network. One key challenge is to accurately pinpoint the tipping point during the transition from pre-swelling to SRs and to identify the core regulators governing such a critical transition. To solve this problem, we performed a dynamic network biomarker (DNB) analysis of transcriptomic dynamics during root development in Ipomoea batatas (sweet potato). First, our analysis identified stage-specific expression patterns for a significant proportion (>9%) of the sweet potato genes and unraveled the chronology of events that happen at the early and later stages of root development. Then, the results showed that different root developmental stages can be depicted by co-expressed modules of sweet potato genes. Moreover, we identified the key components and transcriptional regulatory network that determine root development. Furthermore, through DNB analysis an early stage, with a root diameter of 3.5 mm, was identified as the critical period of SR swelling initiation, which is consistent with morphological and metabolic changes. In particular, we identified a NAM/ATAF/CUC (NAC) domain transcription factor, IbNAC083, as a core regulator of this initiation in the DNB-associated network. Further analyses and experiments showed that IbNAC083, along with its associated differentially expressed genes, induced dysfunction of metabolism processes, including the biosynthesis of lignin, flavonol and starch, thus leading to the transition to swelling roots.

Hybrid sterility hinders the exploitation of the heterosis displayed by japonica × indica rice hybrids. The variation in pollen semi-sterility observed among hybrids between the japonica recipient cultivar and each of two sets of chromosome segment substitution lines involving introgression from an indica cultivar was due to a factor on chromosome 5 known to harbor the gene S24. S24 was fine mapped to a 42 kb segment by analyzing a large F(2) population bred from the cross S24-NIL × Asominori, while the semi-sterility shown by the F(1) hybrid was ascribable to mitotic failure at the early bicellular pollen stage. Interestingly, two other pollen sterility genes (f5-Du and Sb) map to the same region (Li et al. in Chin Sci Bull 51:675-680, 2006; Wang et al. in Theor Appl Genet 112:382-387, 2006), allowing a search for candidate genes in the 6.4 kb overlap between the three genes. By sequencing the overlapped fragment in wild rice, indica cultivars and japonica cultivars, a protein ankyrin-3 encoded by the ORF2 was identified as the molecular base for S24. A cultivar Dular was found to have a hybrid-sterility-neutral allele, S24-n, in which an insertion of 30 bp was confirmed. Thus, it was possible to add one more case of molecular bases for the hybrid sterility. No gamete abortion is caused on heterozygous maternal genotype with an impaired sequence from the hybrid-sterility-neutral genotype. This result will be useful in understanding of wide compatibility in rice breeding.