Sporophytic ovule tissues modulate the initiation and progression of apomixis in Hieracium

Apomixis in Hieracium subgenus Pilosella initiates in ovules when sporophytic cells termed aposporous initial (AI) cells enlarge near sexual cells undergoing meiosis. AI cells displace the sexual structures and divide by mitosis to form unreduced embryo sac(s) without meiosis (apomeiosis) that initiate fertilization-independent embryo and endosperm development. In some Hieracium subgenus Pilosella species, these events are controlled by the dominant LOSS OF APOMEIOSIS (LOA) and LOSS OF PARTHENOGENESIS (LOP) loci. In H. praealtum and H. piloselloides, which both contain the same core LOA locus, the timing and frequency of AI cell formation is altered in derived mutants exhibiting abnormal funiculus growth and in transgenic plants expressing rolB which alters cellular sensitivity to auxin. The impact on apomictic and sexual reproduction was examined here when a chimeric RNAse gene was targeted to the funiculus and basal portions of the ovule, and also when polar auxin transport was inhibited during ovule development following N-1-naphthylphthalamic acid (NPA) application. Both treatments led to ovule deformity in the funiculus and distal parts of the ovule and LOA-dependent alterations in the timing, position, and frequency of AI cell formation. In the case of NPA treatment, this correlated with increased expression of DR5:GFP in the ovule, which marks the accumulation of the plant hormone auxin. Our results show that sporophytic information potentiated by funiculus growth and polar auxin transport influences ovule development, the initiation of apomixis, and the progression of embryo sac development in Hieracium. Signals associated with ovule pattern formation and auxin distribution or perception may influence the capacity of sporophytic ovule cells to respond to LOA.Matthew R. Tucker, Takashi Okada, Susan D. Johnson, Fumio Takaiwa, and Anna M. G. Koltuno

A Genome-Wide Survey of Imprinted Genes in Rice Seeds Reveals Imprinting Primarily Occurs in the Endosperm

Genomic imprinting causes the expression of an allele depending on its parental origin. In plants, most imprinted genes have been identified in Arabidopsis endosperm, a transient structure consumed by the embryo during seed formation. We identified imprinted genes in rice seed where both the endosperm and embryo are present at seed maturity. RNA was extracted from embryos and endosperm of seeds obtained from reciprocal crosses between two subspecies Nipponbare (Japonica rice) and 93-11 (Indica rice). Sequenced reads from cDNA libraries were aligned to their respective parental genomes using single-nucleotide polymorphisms (SNPs). Reads across SNPs enabled derivation of parental expression bias ratios. A continuum of parental expression bias states was observed. Statistical analyses indicated 262 candidate imprinted loci in the endosperm and three in the embryo (168 genic and 97 non-genic). Fifty-six of the 67 loci investigated were confirmed to be imprinted in the seed. Imprinted loci are not clustered in the rice genome as found in mammals. All of these imprinted loci were expressed in the endosperm, and one of these was also imprinted in the embryo, confirming that in both rice and Arabidopsis imprinted expression is primarily confined to the endosperm. Some rice imprinted genes were also expressed in vegetative tissues, indicating that they have additional roles in plant growth. Comparison of candidate imprinted genes found in rice with imprinted candidate loci obtained from genome-wide surveys of imprinted genes in Arabidopsis to date shows a low degree of conservation, suggesting that imprinting has evolved independently in eudicots and monocots

The LOSS OF APOMEIOSIS (LOA) locus in Hieracium praealtum can function independently of the associated large-scale repetitive chromosomal structure

Apomixis or asexual seed formation in Hieracium praealtum (Asteraceae) is controlled by two independent dominant loci. One of these, the LOSS OF APOMEIOSIS (LOA) locus, controls apomixis initiation, mitotic embryo sac formation (apospory) and suppression of the sexual pathway. The LOA locus is found near the end of a hemizygous chromosome surrounded by extensive repeats extending along the chromosome arm. Similar apomixis-carrying chromosome structures have been found in some apomictic grasses, suggesting that the extensive repetitive sequences may be functionally relevant to apomixis. Fluorescence in situ hybridization (FISH) was used to examine chromosomes of apomeiosis deletion mutants and rare recombinants in the critical LOA region arising from a cross between sexual Hieracium pilosella and apomictic H. praealtum. The combined analyses of aposporous and nonaposporous recombinant progeny and chromosomal karyotypes were used to determine that the functional LOA locus can be genetically separated from the very extensive repeat regions found on the LOA-carrying chromosome. The large-scale repetitive sequences associated with the LOA locus in H. praealtum are not essential for apospory or suppression of sexual megasporogenesis (female meiosis).Yoshiko Kotani, Steven T. Henderson, Go Suzuki, Susan D. Johnson, Takashi Okada, Hayley Siddons, Yasuhiko Mukai and Anna M. G. Koltuno

A detached leaf assay for testing transient gene expression and gene editing in cowpea (Vigna unguiculata [L.] Walp.)

Background: The legume cowpea (Vigna unguiculata L.) is extensively grown in sub-Saharan Africa. Cowpea, like many legumes has proved recalcitrant to plant transformation. A rapid transient leaf assay was developed for testing gene expression and editing constructs prior to stable cowpea transformation, to accelerate cowpea and legume crop improvement. Results: Attempts to develop a transient protoplast system for cowpea were unsuccessful. Leaflets from plants 3-4 weeks post-germination were age selected to establish a rapid Agrobacterium (Agro) infiltration-mediated transient system for efficacy testing of gene expression and CRISPR/Cas9 gene editing constructs. In planta, Agro-infiltration of leaflets with fluorescent expression constructs, resulted in necrosis. By contrast, Agro-infiltration of detached leaflets with an Arabidopsis (At) ubiquitin3 promoter:ZsGreen construct, followed by culture on solid nutrient medium resulted in fluorescence in over 48% of leaf cells. Expression efficiency was leaf age-dependent. Three cowpea meiosis genes were identified for CRISPR/Cas9 gene-editing, with the forward aim of meiosis-knock out for asexual seed induction in cowpea. Constructs were designed and tested containing candidate gene-specific guide RNAs, expressed using either the cowpea or Arabidopsis U6 promoters with Cas9 expression directed by either the Arabidopsis 40S ribosomal protein or parsley ubiquitin4-2 promoters. Leaflets were infiltrated with test gene-editing constructs and analytical methods developed to identify gene-specific mutations. A construct that produced mutations predicted to induce functional knockout of in the VuSPO11-1 meiosis gene was tested for efficacy in primary transgenic cowpea plants using a previously established stable transformation protocol. Vuspo11-1 mutants were identified, that cytologically phenocopied spo11-1 mutants previously characterized in Arabidopsis, and rice. Importantly, a biallelic male and female sterile mutant was identified in primary transgenics, exhibiting the expected defects in 100% of examined male and female meiocytes. Conclusion: The transient, detached cowpea leaf assay, and supporting analytical methods developed, provide a rapid and reproducible means for testing gene expression constructs, and constructs for inducing mutagenesis in genes involved in both vegetative and reproductive developmental programs. The method and tested editing constructs and components have potential application for a range of crop legumes

Sexual and Apomictic Reproduction in Hieracium subgenus Pilosella Are Closely Interrelated Developmental Pathways

Seed formation in flowering plants requires meiosis of the megaspore mother cell (MMC) inside the ovule, selection of a megaspore that undergoes mitosis to form an embryo sac, and double fertilization to initiate embryo and endosperm formation. During apomixis, or asexual seed formation, in Hieracium ovules, a somatic aposporous initial (AI) cell divides to form a structurally variable aposporous embryo sac and embryo. This entire process, including endosperm development, is fertilization independent. Introduction of reproductive tissue marker genes into sexual and apomictic Hieracium showed that AI cells do not express a MMC marker. Spatial and temporal gene expression patterns of other introduced genes were conserved commencing with the first nuclear division of the AI cell in apomicts and the mitotic initiation of embryo sac formation in sexual plants. Conservation in expression patterns also occurred during embryo and endosperm development, indicating that sexuality and apomixis are interrelated pathways that share regulatory components. The induction of a modified sexual reproduction program in AI cells may enable the manifestation of apomixis in Hieracium