Seed abortion in the sexual counterpart of Brachiaria brizantha apomicts (Poaceae)

Abstract

Gametophytic organization, fertilization and reproductive success are described for the fertile diploid Brachiaria brizantha accession BRA-002747 which is being raised for use in Brachiaria breeding programs, as well as to understand and control of apomixis in this genus. The current paper reports on reproductive biology and analysis of seed set in field experiments during three consecutive years. Unsuccessful seed production in this plant is believed to correlate with early inbreeding depression, based on the reproductive features analyzed. Caryopsis development was observed using differential interference contrast microscopy with seed set determined by the number of self-and open-pollinated pistils that fully developed into viable seeds. Developing and mature female and male gametophytes were observed in the context of flower phenology, morphology and anthesis patterns. Pollen viability was determined by acetocarmine staining and by observation of germination in vivo, which was also used to observe pollen tube/pistil interaction. Although normal development was observed in floral structures, anthesis and gametophytes, seed set was low, with 2 and 6% in self-and open-pollination, respectively, producing seed. Variations observed in the female organs, such as the presence of a hermaphrodite flower in 50% of the inferior floscules and the presence of multiple embryo sacs of the Polygonum type within the same ovule in 15% of the pistils, are not related to low fertility. The majority of pollen grains are viable, in spite of the reduced number of pollen tubes within the style and ovary carpel, and a developing caryopsis was observed in 70% of self-pollinated pistils, indicating successful double fertilization from 2 days after anthesis (DAA). Nevertheless, abortion gradually increased from 2 until 7 DAA and remains elevated until 12 DAA, when caryopsis maturity is achieved. These data confirm low seed set in this accession and indicate that low fertility is not a consequence of abnormalities, either in the floral or gamete structures, or pollen tube rejection, but most likely a consequence of inbreeding depression.

Somatic embryogenesis and organogenesis in apomictic and sexual Brachiaria brizantha

“…Marandu is a fertile tetraploid (2n = 4x = 36) (Valle and Savidan 1996;Araujo et al 2000), pseudogamic plant (Ngendahayo 1988;Alves et al 2001). Among B. brizantha genotypes, a sexual diploid (2n = 2x = 18) was found to produce very few viable seeds (Araujo et al 2007). Due to the general asexual mode of reproduction, B. brizantha genetic improvement is restricted to selection of superior cultivars from natural variability (Euclides et al 2008) or use of the apomictic plant as pollen donor in interspecific hybridization (Valle et al 2009).…”

Toward in vitro fertilization in Brachiaria spp.

“…Thus, both the mechanical barrier and self-incompatibility can be overcome by in vitro pollination. Inbreeding depression (Araujo et al 2007) is also present in Brachiaria. Although some seed set was observed in sexual plants, apomictic plants have no self-incompatibility and good seed set, which could be reasons for more successful in vitro pollination in the apomicts than the sexual plants.…”

“…Observations of ovaries of sexual B. decumbens with remnants of endosperm nuclei and degeneration of other embryo sac cells, point to the likely occurrence of fertilization. Araujo et al (2007) showed that in vivo pollination of the sexual B. brizantha can produce fertilized products, and that embryo and endosperm developed; however, the abortion of the caryopsis starting near anthesis suggested the occurrence of inbreeding depression. In sexual Brachiaria 48 HAP, most of the embryo sacs collapsed.…”

“…Ngendahayo (1988) noted that using a high number of pollen grains resulted in only a low percentage of pollen tubes reach the ovary. In the sexual B. brizantha, one pollen tube and rarely two were found at the micropyle of the ovary (Araujo et al 2007). In the sexual B. decumbens, only one in five pistils had a pollen tube inside the ovule 24 h after pollination.…”

“…In the apomictic Brachiaria, several apospores develop into embryo sacs of the Panicum type (Ngendahayo 1988;do Valle 1990;Dusi and Willemse 1999b;Araujo et al 2000). In sexual plants, the fusion of the sperm nucleus with the egg cell nucleus initiates the zygote, while the other sperm cell nucleus fuses with the diploid central cell nucleus and leads to the development of the endosperm (Alves et al 2001;Araujo et al 2007). …”