Molecular tools for achieving Apomixis in rice

• CSIRO Plant Industry, Australia
• CSIRO Plant Industry, Australia

Apomixis is asexual reproduction, which in plants can produce seed that gives rise to plants genetically identical to the maternal parent. Apomixis fixes hybrid vigour, because it avoids aspects of the sexual reproductive processes, such as meiotic recombination, that lead to genetic variation. The economic and agronomic advantage that apomixis could bring to crop plants worldwide is well recognised.
Apomixis does not occur naturally in rice, but if the trait could be introduced it would simplify hybrid seed production in rice. Farmers' costs would be reduced and a higher productivity maintained per acre, as they could save seed from their harvest year after year and maintain hybrid seed yield advantage.
To achieve apomictic rice scientists needed first to identify, isolate and modify genes controlling apomixis and apply them in ways that would capture the yield advantage of hybrid rice for farmers and consumers. This project was part of a 15-year program with a 3-phase goal, designed 'To enhance food security in Asian countries through development of a synthetic form of apomixis that allows cheaper, more flexible production of high-yielding hybrid rice'.

The aims sof this project were to isolate the necessary molecular tools for use in the development of apomixis in rice.

The major research program, of which this project is but one component, comprises three phases. The work of this project involved collaboration between IRRI and CSIRO Australia. CSIRO sought genes from non-rice species (Arabidopsis and Hieracium) that enable the initiation of apomictic development. IRRI aimed to provide rice-specific tools to enable apomictic gene expression and the induction of apomictic pathways in rice. Genetic information from both sources was combined during the course of the research, in the search for a series of genes tailored for apomictic induction and ready for initial testing in rice.
The later two phases of the overarchng research program concern the development of these tools, the detailed manipulation of the rice plant with these tools to achieve apomictic rice and the integration of apomixis into hybrid rice programs.

A genetic screen in Arabidopsis thaliana isolated three FIS (fertilisation-independent seed) mutants and the corresponding genes. These mutants showed autonomous endospermy (a necessary trait for apomictic seed development). FIS gene orthologues (genes in different species that derive from a common ancestor) were identified in rice and the technique of RNA interference was used to disrupt their function. The transformants also displayed autonomous endospermy, but the endosperms were shrunken. Some form of parthenogenesis may be needed to allow autonomous production of full-size endosperms.
The CSIRO scientists introduced fusions between FIS promoters and GUS reporter genes into wild-type and mutant Arabidopsis and into sexual and apomictic Hieracium. These actions led to the discovery of (i) similarities and differences between the sexual and apomictic pathways and (ii) points in the sexual pathway responsive to genetic modification of ovary structure. These results showed the basic feasibility of using molecular genetics to switch from sexuality to apomixis.
Rice orthologues were identified for Arabidopsis genes known to regulate embryogenesis, such as leafy cotyldedon1 (LEC1). Fusions between promoter and reporter genes were introduced into rice and led to the expression of seed-specific genes in leaves, consistent with partial induction of autonomous embryony (which is the creation of an embryo without fertilisation - a step towards apomixis).
At IRRI, fusions between GUS reporter gene and the promoter of OsAsp1, encoding a novel aspartate proteinase expressed in young zygotic embryos, led to a form of parthenogenesis that allows rice ovaries to expand in the absence of fertilisation.