Apomixis in plant reproduction: a novel perspective on an old dilemma

Abstract

Seed is one of the key factors of crop productivity. Therefore, a comprehension of the mechanisms underlying seed formation in cultivated plants is crucial for the quantitative and qualitative progress of agricultural production. In angiosperms, two pathways of reproduction through seed exist: sexual or amphimictic, and asexual or apomictic; the former is largely exploited by seed companies for breeding new varieties, whereas the latter is receiving continuously increasing attention from both scientific and industrial sectors in basic research projects. If apomixis is engineered into sexual crops in a controlled manner, its impact on agriculture will be broad and profound. In fact, apomixis will allow clonal seed production and thus enable efficient and consistent yields of high-quality seeds, fruits, and vegetables at lower costs. The development of apomixis technology is expected to have a revolutionary impact on agricultural and food production by reducing cost and breeding time, and avoiding the complications that are typical of sexual reproduction (e.g., incompatibility barriers) and vegetative propagation (e.g., viral transfer). However, the development of apomixis technology in agriculture requires a deeper knowledge of the mechanisms that regulate reproductive development in plants. This knowledge is a necessary prerequisite to understanding the genetic control of the apomictic process and its deviations from the sexual process. Our molecular understanding of apomixis will be greatly advanced when genes that are specifically or differentially expressed during embryo and embryo sac formation are discovered. In our review, we report the main findings on this subject by examining two approaches: i) analysis of the apomictic process in natural apomictic species to search for genes controlling apomixis and ii) analysis of gene mutations resembling apomixis or its components in species that normally reproduce sexually. In fact, our opinion is that a novel perspective on this old dilemma pertaining to the molecular control of apomixis can emerge from a cross-check among candidate genes in natural apomicts and a high-throughput analysis of sexual mutants.

The Genetic Control of Apomixis: Asexual Seed Formation

“…APOSTART may also be related to programmed cell death and the degeneration of the nonfunctional megaspores, with one of the two isolated copies (APOSTART1) overexpressed in sexual lines relative to apomicts. The Arabidopsis APOSTART1 ortholog is expressed in mature female embryo sacs and developing embryos, and the phenotype of APOSTART1/APOSTART2 double mutants suggests that this gene has a role in embryo and seed development (Barcaccia and Albertini 2013).…”

A parthenogenesis gene of apomict origin elicits embryo formation from unfertilized eggs in a sexual plant

“…In gametophytic apomixis, a chromosomally unreduced embryo sac develops from the megaspore mother cell (diplospory) or from a nearby nucellar cell (apospory) in a process termed apomeiosis. Parthenogenesis, the development of the unreduced, unfertilized egg into an embryo, constitutes the second step of the apomictic process (1,2).…”

Cytogenetic and Embryologic Analyses of the Mechanism of Parthenogenesis Induced by 1BL/1RS Male Sterile Lines in Wheat

“…In wheat, the alloplasmic line 'Salmon' with the 1BL/1RS chromosome in its genome is a suitable material for parthenogenesis research (Barcaccia and Albertini 2013;Curtis and Grossniklaus 2008). It has been reported that the short arm of chromosome 1B from common wheat (Triticum aestivum L.) is substituted by the short arm of chromosome 1R from rye (Secale cereale L.) in 'Salmon', thereby yielding 1BL/1RS translocation wheat.…”