An exonuclease V homologue is expressed predominantly during early megasporogenesis in apomictic Brachiaria brizantha.

MAIN CONCLUSION: An exonuclease V homologue from apomictic Brachiaria brizantha is expressed and localized in nucellar cells at key moments when these cells differentiate to give rise to unreduced gametophytes. Brachiaria is a genus of forage grasses with economical and agricultural importance to Brazil. Brachiaria reproduces by aposporic apomixis, in which unreduced embryo sacs, derived from nucellar cells, other than the megaspore mother cell (MMC), are formed. The unreduced embryo sacs produce an embryo without fertilization resulting in clones of the mother plant. Comparative gene expression analysis in ovaries of sexual and apomictic Brachiaria spp. revealed a sequence from B. brizantha that showed a distinct pattern of expression in ovaries of sexual and apomictic plants. In this work, we describe a gene named BbrizExoV with strong identity to exonuclease V (Exo V) genes from other grasses. Sequence analysis in signal prediction tools showed that BbrizExoV might have dual localization, depending on the translation point. A longer form to the nucleus and a shorter form which would be directed to the chloroplast. This is also the case for monocot sequences analyzed from other species. The long form of BbrizExoV protein localizes to the nucleus of onion epidermal cells. Analysis of ExoV proteins from dicot species, with exception of Arabidopsis thaliana ExoVL protein, showed only one localization. Using a template-based AlphaFold 2 modelling approach the structure of BbrizExoV in complex with metal and ssDNA was predicted based on the holo structure of the human counterpart. Features predicted to define ssDNA binding but a lack of sequence specificity are shared between the human enzyme and BbrizExoV. Expression analyses indicated the precise site and timing of transcript accumulation during ovule development, which coincides with the differentiation of nucelar cells to form the typical aposporic four-celled unreduced gametophyte. A putative function for this protein is proposed based on its homology and expression pattern.

Identification of IPT9 in Brachiaria brizantha (syn. Urochloa brizantha) and expression analyses during ovule development in sexual and apomictic plants.

BACKGROUND: In Brachiaria sexual reproduction, during ovule development, a nucellar cell differentiates into a megaspore mother cell (MMC) that, through meiosis and mitosis, gives rise to a reduced embryo sac. In aposporic apomictic Brachiaria, next to the MMC, other nucellar cells differentiate into aposporic initials that enter mitosis directly forming an unreduced embryo sac. The IPT (isopentenyltransferase) family comprises key genes in the cytokinin (CK) pathway which are expressed in Arabidopsis during ovule development. BbrizIPT9, a B. brizantha (syn. Urochloa brizantha) IPT9 gene, highly similar to genes of other Poaceae plants, also shows similarity with Arabidopsis IPT9, AtIPT9. In this work, we aimed to investigate association of BbrizIPT9 with ovule development in sexual and apomictic plants. METHODS AND RESULTS: RT-qPCR showed higher BbrizIPT9 expression in the ovaries of sexual than in the apomictic B. brizantha. Results of in-situ hybridization showed strong signal of BbrizIPT9 in the MMC of both plants, at the onset of megasporogenesis. By analyzing AtIPT9 knockdown mutants, we verified enlarged nucellar cell, next to the MMC, in a percentage significantly higher than in the wild type, suggesting that knockout of AtIPT9 gene triggered the differentiation of extra MMC-like cells. CONCLUSIONS: Our results indicate that AtIPT9 might be involved in the proper differentiation of a single MMC during ovule development. The expression of a BbrizIPT9, localized in male and female sporocytes, and lower in apomicts than in sexuals, and effect of IPT9 knockout in Arabidopsis, suggest involvement of IPT9 in early ovule development.

Anther development in Brachiaria brizantha (syn. Urochloa brizantha) and perspective for microspore in vitro culture.

Brachiaria, a genus from the Poaceae family, is largely cultivated as forage in Brazil. Among the most cultivated varieties of Brachiaria spp., B. brizantha cv. Marandu (syn. Urochloa brizantha) is of great agronomical importance due to the large areas cultivated with this species. This cultivar is apomictic and tetraploid. Sexual diploid genotype is available for this species. The difference in levels of ploidy among sexual and apomictic plants contributes to hindering Brachiaria breeding programs. The induction of haploids and double haploids is of great interest for the generation of new genotypes with potential use in intraspecific crosses. A key factor for the success of this technique is identifying adequate microspore developmental stages for efficient embryogenesis induction. Knowledge of the morphological changes during microsporogenesis and microgametogenesis and sporophytic tissues composing the anther is critical for identifying the stages in which microspores present a higher potential for embryogenic callus and somatic embryo through in vitro culture. In this work, morphological markers were associated with anther and pollen grain developmental stages, through histological analysis. Anther development was divided into 11 stages using morphological and cytological characteristics, from anther with archesporial cells to anther dehiscence. The morphological characteristics of each stage are presented. In addition, the response of stage 8 anthers to in vitro culture indicates microspores initiating somatic embryogenic pathway.

SERK genes identification and expression analysis during somatic embryogenesis and sporogenesis of sexual and apomictic Brachiaria brizantha (Syn. Urochloa brizantha).

MAIN CONCLUSION: In Brachiaria brizantha BbrizSERK1, BbrizSERK2 and BbrizSERK3 were identified. SERK expression marks somatic embryogenesis, sexual MMC, and sexual and apomictic PMC. BbrizSERK3 might have a regulatory role in reproductive development. Somatic embryogenesis receptor-like kinase (SERK) consists of plasma membrane receptor genes that have been characterized in various species, associated with several aspects of plant development, including reproduction. SERK genes are involved in anther development and in early embryo development in sexual and asexual seed formation. To comprehend the complexity of the SERK genes and their function in Brachiaria reproduction, we performed a homology-based search in a genomic database of a sexual B. brizantha and identified sequences of three SERK genes, BbrizSERK1, BbrizSERK2, and BbrizSERK3. RNASeq data showed equivalent abundance of BbrizSERK1 and BbrizSERK2 transcripts in ovaries at early megasporogenesis of sexuals and apomicts, while BbrizSERK3 transcripts were more abundant in ovaries of sexuals than in apomicts. BbrizSERK3 results in three coding sequences due to alternative splicing, among them Variant 1 results in a protein with all the predicted domains of a SERK. BbrizSERK transcripts were detected in male reproductive tissues of both sexual and apomictic plants, suggesting a role in controlling anther development. BbrizSERK transcripts were detected early in ovule development, in the integuments, and in the megaspore mother cell of the sexual plant, but not in the cells that give rise to apomictic embryo sacs, suggesting a role in female reproductive development of sexuals. This paper provides evidences that SERK genes plays a role in the onset and establishment of somatic embryogenesis and in the reproductive development of B. brizantha and suggests a distinct role of BbrizSERK in apomixis initiation.

Genetic transformation of Brachiaria brizantha cv. Marandu by biolistics.

Brachiaria brizantha is a forage grass well adapted to tropical areas and cultivated in millions of hectares in Brazil. The apomictic mode of reproduction in this species, in addition to differences in ploidy between sexual and apomictic plants, impairs crossbreeding. The development of a methodology to transform apomictic cultivars will provide an option to introduce agronomic important traits to B. brizantha cv. Marandu. In addition, it will open the possibility to study in vivo the function of candidate genes involved in the apomictic reproduction. The objective of this work was to evaluate peeled seeds, isolated embryo from mature seeds, embryogenic calluses and embryogenic cell suspensions, as target explant for genetic transformation via biolistics. Plasmids bearing the marker genes gus and hptII under the control of the rice actin 1 promoter (pAct1-Os) or the maize ubiquitin 1 promoter (pUbi1Zm) were used. All the target-explants used were suitable for transient gene expression after bombardment, showing gus expression and resistance to hygromycin. Using embryogenic calluses and cell suspensions as target tissues, transgenic plants were regenerated and transgenes detected.

Somatic Embryogenesis and Plant Regeneration of Brachiaria brizantha.

The genus Brachiaria (Trin.) Griseb. belongs to the family Poaceae, order Poales, class Monocotyledonae. In Brachiaria brizantha (Hochst. ex A. Rich.) Stapf., embryogenic callus can be induced from seeds from apomictic plants, which results in high frequency somatic embryo development and plant regeneration. We report here a detailed protocol for callus induction from apomictic seed; followed by in vitro morphogenesis (somatic embryo and bud differentiation), plant regeneration, and acclimatization in the greenhouse. Important details regarding the positioning of seeds for callus induction and precautions to avoid endophytic contamination and the occurrence of albino plants are presented.

Toward in vitro fertilization in Brachiaria spp.

Brachiaria are forage grasses widely cultivated in tropical areas. In vitro pollination was applied to accessions of Brachiaria spp. by placing pollen of non-dehiscent anthers on a solid medium near isolated ovaries. Viability and in vitro germination were tested in order to establish good conditions for pollen development. Comparing sexual to apomictic plants, apomictic pollen has more abortion after meiosis during the microspore stage and a lower viability and, of both types, only some plants have sufficient germination in a high sugar concentration. Using in vitro pollination with the sexual plant, the pollen tube penetrates into the nucellus and micropyle, but the embryo sac degenerates and collapses. In the apomictic B. decumbens, in vitro pollination leads to the transfer of the sperm nuclei into the egg cell and the central cell. The results are discussed according to normal fertilization and barriers in sexual and apomictic plants.

Physical mapping of rDNA genes corroborates allopolyploid origin in apomictic Brachiaria brizantha.

Brachiaria (Trin.) Griseb belongs to the family Poaceae, and within the genus, apomixis or sexuality is present in different accessions of the same species. The majority of Brachiaria species are polyploid and apomictic, making strategies for crop improvement by breeding very intricate. In spite of the high frequency of apomictic polyploids, the relationship of polyploidy and hybridization with apomixis in Brachiaria is still unclear. Further analysis requires detailed knowledge regarding the genomic composition of the polyploids. The present work introduces the use of fluorescent in situ hybridization (FISH) into cytogenetic analysis of Brachiaria. Physical mapping of heterologous rDNA sequences, associated with conventional karyotyping of the B. brizantha diploid sexual (BRA 002747) and the tetraploid apomictic (BRA000591) accessions, provided evidence of the latter being of allopolyploid origin. Based on our results and on previous knowledge on apomixis in B. brizantha, we suggest that the origin of apomixis was probably a consequence of hybridization.

Identification of differentially expressed cDNA sequences in ovaries of sexual and apomictic plants of Brachiaria brizantha.

The isolation of genes associated with apomixis would improve understanding of the molecular mechanism of this mode of reproduction in plants as well as open the possibility of transfer of apomixis to sexual plants, enabling cloning of crops through seeds. Brachiaria brizantha is a highly apomictic grass species with 274 tetraploid apomicts accessions and only one diploid sexual. In this study we have compared gene expression in ovaries at megasporogenesis and megagametogenesis of sexual and apomictic accessions of B. brizantha by differential display (DD-PCR), with 60 primer combinations. Specificity of 65 cloned fragments, checked by reverse northern blot analysis, showed that 11 clones were differentially expressed, 6 in apomictic ovaries, 2 in sexual and 3 in apomictic and sexual, but at different stages. Of the 6 sequences isolated that were preferentially expressed in the apomictic accession: one sequence was from ovaries at megasporogenesis stage; three were from megagametogenesis stage; two were from both stages. Of the two sequences isolated from the sexual accessions, one showed expression in ovaries at megagametogenesis, while the other sequence was shown to be specific to both stages. Three sequences were from megasporogenesis stage in apomicts but were also detected at megagametogenesis in sexual plants. Sequence analysis showed that 5 of the 11 clones had no apparent homologues in the protein database. Some of the clones identified as apomictic-specific shared homology with known genes enabling their functional annotation. The relationships of these functions to the generation of the apomictic trait are discussed.