miRNA and mRNA studies reveal pollination activates hormonal signaling and increases fruit set in the apomictic tree Zanthoxylum bungeanum Maxim

Background Apomixis is a form of reproduction that does not involve fertilization of female by male gametes but instead produces offspring from the female parent directly. The progeny of apomixis is genotypically identical to the female parent and so maintains any elite traits of the female parent. Apomixis has considerable potential in genetic plant breeding. However, the mechanism of apomictic reproduction remains unclear. Zanthoxylum bungeanum is an apomictic plant. Studies on miRNAs, mRNAs, hormone changes and fertilization process of the pollinated and non-pollinated materials of Zanthoxylum bungeanum allows screening of the important regulatory factors of the apomictic process at both physiological and molecular levels. This information should be of considerable help in understanding the mechanism of apomixis in this species.Results Our results show that Zanthoxylum bungeanum pollen can germinate on the stigma, and that the pollen tube can extend to the ovary wall after two days but that it then degenerates about the fifth day and before reaching the egg cell. So fertilization does not occur. Nevertheless, pollination did increase fruit set from 74.12% (unpollinated) to 89.31% (pollinated). The reproductive mode of the Zanthoxylum bungeanum cultivar ‘Hancheng Dahongpao’ was identified as obligate apomixis. Enrichment analysis of differential genes indicates that pollination activates genes involved in the synthesis of ABA, IAA, GA3 and JA, and that genes related to asexual development, genes involved in flower development, and transcription factors are also activated. RT-qPCR verification shows that the relative expression levels of mRNAs and miRNAs related to hormone signaling pathways and flower development were negatively correlated. Also, that miRNA is an important factor influencing hormone regulation after pollination of this apomictic species. The contents of ABA, IAA, GA3 and JA were all significantly increased following pollination.Conclusions In general, although pollination does not result in fertilization in this species, it does activate mRNAs and miRNAs which serve as signals to activate the anabolic pathways for ABA, IAA, GA3 and JA. Increased levels of these hormones result in increased fruit set.

The potential for ploidy level increases and decreases in Crataegus (Rosaceae, Spiraeoideae, tribe Pyreae)

Unlike their diploid relatives, some triploid and tetraploid Crataegus frequently produce unreduced megagametophytes. In all cases, pollination is required for successful seed set, but in polyploids, endosperm formation can involve fertilization by either one or both sperm. Apomixis, in which the egg develops parthenogenetically, is widely documented in polyploid Crataegus, and as in many other groups with gametophytic apomeiosis, fertilization of unreduced eggs can also occur. Reciprocal pollinations were made between diploids, triploids, and tetraploids belonging to five taxonomic series in the genus to evaluate opportunities for gene flow between ploidy levels. The ploidy levels of embryo and endosperm in mature seeds, estimated from flow-cytometric DNA measurements, indicate the meiotic or apomeiotic origin of the megagametophyte and whether fertilization has occurred. These experiments demonstrated that although some tetraploids maintain near-obligate apomixis when supplied with pollen from diploids, others produced seeds containing embryos ranging from diploid to hexaploid. Allotriploid embryos were produced when a diploid was provided with pollen from tetraploids. A triploid produced tetraploid embryos when pollinated by a diploid and pentaploid embryos when pollinated by a tetraploid. Gametophytic apomixis in Crataegus thus can be facultative or near-obligate and may be implicated in the formation of interserial hybrids.

Cytoembryology of Paspalum chaseanum and Sexual Diploid Biotypes of Two Apomictic Paspalum Species

This study was undertaken to determine the cytology, method of reproduction, and flowering behaviour of three Paspalum species. Paspalum plicatulum has long been considered a segmental allotetraploid that reproduces by obligate apomixis with the type being apospory followed by pseudogamy. Paspalum simplex is an apomictic autotetraploid species, while P. chaseanum is a rare species for which no information regarding cytology and reproduction is available. This investigation concerns diploid cytotypes (2n = 2x = 20) of P. plicatulum and P. simplex that were recently collected in subtropical South America. In addition, two accessions of P. chaseanum were also recorded and both had 2n = 2x = 20 chromosomes. Meiosis showed regular bivalent chromosome pairing. Embryological observations indicated that the three species reproduce sexually at the diploid level. Pollen–pistil interaction following self-pollination suggested the presence of a self-incompatibility system responsible for allogamy. The results indicate that P. plicatulum and P. simplex complexes consist of diploid sexual allogamous cytotypes in addition to the known tetraploid apomictic cytotypes. Diploid strains constitute a source of sexuality for plant improvement. Chromosome doubling will likely produce sexual tetraploids to be used as females in crosses with natural apomictic tetraploid biotypes. Since diploid self-incompatible sexual Paspalum plants usually have apomictic tetraploid co-specific counterparts, the self-incompatible diploid P. chaseanum described in this study warrants further exploration for its apomictic counterpart.

Embryo sac development in some representatives of the tribe Cynodonteae (Poaceae)

Chloris virgata Sw., Cynodon dactylon (L.) Pers., Harpochloa falx (L. f.) Kuntze, and Tragus berteronianus Schult. have a Polygonum type of embryo sac development. Unreduced embryo sacs were found in Eustachys paspaloides (Vahl) Lanza Mattei, Harpochloa falx, and Rendlia altera (Rendle) Chiov. Both facultative and obligate apomixis were observed. The Hieracium type of embryo sac development was observed in the aposporic specimens.