Characterization of ASGR-BABY BOOM-like (ASGR-BBML), a candidate apomixis gene identified in Pennisetum squamulatum and Cenchrus ciliaris.

Summary

Apomixis is a developmental process that leads to the clonal propagation of the maternal plant through seed. We have identified and initiated the characterization the ASGR-BBML gene. We believe this gene may play a role in apomictic embryo development. Our long term goal is to use biotechnology to transfer the genes required for apomixis into crop plants.

Situation

Apomixis is a naturally occurring process of asexual reproduction that results in offspring which are genetically identical to the mother plant. In nature, multiple forms of apomixis exist. Gametophytic apomixis is characterized by apomeiosis, the formation of unreduced embryo sacs derived from nucellar cells of the ovary, and by parthenogenesis, the development of the unreduced egg into an embryo. If apomixis could be harnessed via genetic engineering or introgression, it would have a major economic impact for agricultural crops. Breeding programs could transmit apomixis through the paternal gametes and then subsequently fix hybrid genotypes/vigor through clonal propagation of the maternal seed. Since current crop species are not apomictic, we study gametophytic apomixis in P. squamulatum and C. ciliaris, members the grass family closely related to the crop plant P. glaucum (pearl millet).

Response

In both species, apomixis is transmitted by a physically large chromosomal region, the apospory-specific genomic region (ASGR). Through low coverage sequencing of the ASGR, we identified the ASGR-BBML gene. BBM-like genes have been identified in many plant species and are thought to help regulate embryogenesis. All apomictic Pennisetum and Cenchrus species tested carry a transcribed ASGR-BBML gene. The ASGR-BBML gene was not identified in any sexual Pennisetum and Cenchrus species. The ASGR-BBML gene is expressed in anther, pollen, root, embryogenic callus and in ovaries undergoing parthenogenic embryo development. Expression of ASGR-BBML in P. squamulatum precedes the expression of LEC, a gene required for early embryogenesis. Transgenic lines have been produced to help gain a further understanding of the function of ASGR-BBML in apomixis. Cellular expression of ASGR-BBML has been identified using transgenic lines containing a GUS reporter gene fused with the ASGR-BBML promoter. Staining of the GUS reporter gene is seen in the egg and/or egg/synergid cell complex of both sexual and apomictic ovaries. In sexual ovaries, GUS expression is seen prior to pollination in the egg and/or egg/synergid cell complex of the female gametophyte, enhanced in newly fertilized zygotes and remains active as the embryo develops. No GUS staining occurs in the central or antipodal cells of the female gametophyte or during endosperm development. Apomictic ovaries show similar staining in the gametophyte and the developing embryo. Transgenic lines have been produced which almost completely knock down the expression of the ASGR-BBML gene. These lines do not produce seed with increased ploidy levels as would be expected if the knock down of ASGR-BBML only affected the start of parthenogenesis. Instead, we are seeing no parthenogenesis/embryo development in the knock down lines. Finally, sexual tetraploid transgenic lines were produced which contain the ASGR-BBML under its native promoter. Three independent lines were found to show embryo development two days after pollen shed, though without fertilization as based on the identification of polar nuclei in the sexual embryo sac. Embryo rescued offspring from one line are haploid in genome size.

Impact

The cumulative results of the ASGR-BBML transformed plants strongly suggest that ASGR-BBML controls parthenogenic embryo development in apomixis in pearl millet. We need to continue evaluation of tetraploid/haploid sexual lines to make sure the ASGR-BBML transgene and the parthenogenesis phenotype always segregate together. Furthermore, we would like to test this construct in other crop plants to see if the ASGR-BBML transgene will have a similar effect in other species.

State Issue

S.E.T. - Science, Engineering, and Technology

Details

• Year: 2013
• Geographic Scope: International
• County: Tift
• Program Areas:
  • Agriculture & Natural Resources