Hybrid Wheat Breeding: A way forward

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With the expected growth in world population over 9 billion by 2050, the Food and Agriculture Organization of the United States is eying on an increase of about 60% in food grains by that year. It seems to be an ambitious target probably because of two reasons: there are serious concerns about the viability of existing production systems and the sustainability of current growth rates in crop production and the expected climatic changes are supposed to have a severe negative impact on the agricultural production. Wheat is grown most extensively worldwide with a promise to meet almost 20% of our calorie and protein requirements. To meet the global challenge of increasing agriculture productivity @ 60% till 2050 the rate of grains need to be 1.6% rather than the current gain of 1%. However improvements in the disease resistance and stress tolerance will hardly meet this challenge. Instead we have to shift the breeding systems of wheat and other food crops.

Incorporation of new genetic and genomic technologies (CRISPR/Cas9 etc.) in wheat is quite difficult because of complex genome interplay (an allohexaploid with genome size 50 times larger than rice). One of the promising option for achieving the significant yield boost across diverse environment is via hybrid breeding. Hybrid breeding offers couple of advantages firstly it provide a yield boost of 10% and yield stability secondly hybrid seed production system will draw the investment from both the public and private sectors. However competitiveness of hybrid wheat seed system with line varieties will depend on the cost effectiveness of the system.

Lowering the hybrid seed production cost depends on the reliable and inexpensive system which forces outcrossing. Wheat male sterility and restoration system were reported a long ago in 1960s but seemed cost ineffective and depleted. There are three types of male sterile systems i.e. chemical based, cytoplasmic male sterile system and non-conditional nuclear-encoded recessive male steriles (ms) system. The value of the recessive male sterile was first recognized in 1972 with the introduction of the XYZ system. The system was mainly focused on reducing the cost associated with propagation of pure stands of male sterile lines by cytogenetic chromosomal manipulation. This system further provided advantages of broadening the parental lines choice ignoring the negative alloplasmic and cytoplasmic yield penalties and alleviating the problems associated with incomplete fertility restoration.

A cost-effective and flexible hybridization platform that uses a recessive male steriles is Seed Production Technology (SPT). The SPT has already been developed in rice and wheat but yet awaited in wheat. SPT platform overcome a lot of problems associated with large scale production of male steriles for use as female parent in hybrid breeding. SPT uses a maintainer line solely for the propagation of non-GM homozygous recessive male-sterile parents; therefore, F1 hybrids provided to farmers are considered to be non-GM. However development of SPT in wheat requires the identification of appropriate non-conditional, nuclear encoded recessive male sterile. These types of mutants are rare to identify in the polyploids due to genetic redundancies. Many of our food crops are polypolids i.e. wheat, oats, potato, sweet potato, peanut, sugarcane, cotton, kiwifruit, strawberry, and plums. For example to date only 10 nuclear encoded recessive male sterile has been identified in wheat as compared to 108 in barely (a diploid).

Polyploidy not only makes it difficult to find suitable male sterile mutations but also complicates deploying mutants since multiple mutations would be needed to deal with genetic redundancy and this increases breeding costs and population sizes needed for introgression of each additional mutation. So the effective mutant will be one that is control by single locus. There are only two locus out of ten ms1 and ms5 located on chromosome 4BS and 3AL respectively. The first mutant ms1 was observed in Australian wheats in late 1950s, later on it was also observed in the EMS treated wheat population in 1976. However, even for ms1, the variability between backgrounds and mutant alleles, and problems with male sterility penetrance were reported. In order to address these problems, it was necessary to identify the gene underlying the Ms1 locus and explain its function.

However a couple of week ago a paper was published in Nature in which the authors described the identification of the Ms1 gene sequence (TaMs1) by map-based cloning and demonstrated its function in male fertility by complementation of the ms1d mutant. TaMs1 encodes a glycosylphosphatidylinositol (GPI)-anchored lipid transfer protein, which is necessary for pollen exine development. From a traditional breeding perspective, the molecular identity of the TaMs1 gene sequence now allows the development of germplasm-specific markers for fast-tracking ms1 introgression into diverse female-inbred parental lines. Moreover, complementation studies demonstrate that ms1 is unique and contrasts with other reported wheat ms mutant alleles in that ms1 behaves as a single-mutant locus in hexaploid wheat and a single copy of Ms1 restores fertility. Observed variation in penetrance of sterility between these ms alleles, understanding the relationship of these mutations to pollen production, as well as optimizing ms1 as a system for the production of a hybrid seed is now possible through the adoption of advanced breeding technologies such as gene editing. Further, once highly penetrant ms1 alleles are identified, rather than introgression through conventional backcrossing, this new variant allele could be rapidly introduced into the most elite genetics by directly editing TaMs1. The identification of the Ms1 gene sequence represents a key step towards developing a robust hybridization platform in wheat similar to the maize SPT and will be a positive shift in wheat breeding to achieve the set goals.

This article is collectively authored by Rahil Shahzad1, Shakra Jamil1, Iqra Ghafoor2-1. Agricultural Biotechnology Research Institute, Ayub Agricultural Research Institute Faisalabad 2. Wheat Research Institute, Ayub Agricultural Research Institute Faisalabad.


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