Accelerating Crop Development with Improved Haploid Fertility

Food and nutritional security face major challenges in coming decades, including increasing populations and climate change threats. Plant breeders and researchers are developing crops that can better withstand climate stresses, produce larger yield or contain higher levels of essential nutrients – but it takes plant breeders an average of 10 years to develop a new crop. Current technologies cannot develop crops fast enough to keep pace with population and climatic changes.

New breeding tools like doubled haploid technology can speed up development of hybrid crops, which are created by crossbreeding varieties of plants to promote desired traits such as increased yield or pathogen resistance. Crops tend to have two sets of chromosomes, one set from a female parent and one set from a male parent. Researchers can create versions of plant cells with only half the amount DNA, called haploids. These cells have only one set of chromosomes from one parent, similar to an egg or sperm cells, for example. However, the plants that develop from these haploid cells are unable to reproduce. To solve this problem, the plants are treated with chemicals early in the growth process to double the chromosomes and allow them to breed, creating consistent inbred lines of crops, which are used as parents for a hybrid crop.

Unfortunately, the haploid breeding technique has limits that restrict the effectiveness of doubled haploid technology to meet food demands. Only a small percentage of the crops grown using the technique are haploid plants with one set of chromosomes; most still have two sets of chromosomes. Further, haploid male flowers are usually sterile unless they undergo expensive and time-consuming treatment with toxic chemicals. These barriers severely limit the number of hybrid crops breeders can generate and provide to growers.

While studying accelerated plant breeding in a project funded in part by FFAR, Iowa State University (ISU) research scientist Dr. Siddique Aboobucker uncovered a process that has significant potential to efficiently and cheaply restore fertility to male haploid plants.

Aboobucker determined that a primary cause of sterility occurs during meiosis, a type of cell division during reproduction. In diploid plants – plants with two sets of chromosomes – chromosomes are evenly distributed during meiosis. In haploid plants, having a single set of chromosomes causes uneven distribution, resulting in an unequal number of chromosomes in the dividing cells that ultimately leads to infertility.

Aboobucker postulated that a genetic mutation called parallel spindle mutants, which can be engineered in plants, may hold the key to restoring fertility. These mutations cause spindle structures in cells, which assist with meiosis, to arrange chromosomes in parallels, allowing chromosomes to be equally distributed between the dividing cells and reproduction to proceed successfully.

With lab partners Dr. Thomas Lübberstedt, president of the National Association of Plant Breeders, former director of ISU’s R.F. Baker Center for Plant Breeding and a FFAR grantee, and Liming Zhou, an ISU agronomy graduate student, Aboobucker put the idea to test in Arabidopsis thaliana, a plant that breeding researchers often use as an experimental precursor to commonly consumed crops. By engineering parallel spindle mutations in haploid versions of the plant, the researchers were able to grow haploid plants that could reproduce.

“Haploid male sterility is a bottleneck in doubled haploid breeding technology,” said Aboobucker. “The reason behind the sterility issue became clear to me, from my reading, that it is the error in haploid meiosis, and if this error can be corrected then male fertility can be restored to haploid plants. I accidentally, by God’s grace, stumbled upon two research articles describing mutations that change the orientation of spindle fibers in meiosis. This led me to dream of a hypothesis that changing the orientation can restore male fertility to haploids and then we did the experiments.”

This breakthrough has major implications for crop breeding research and techniques. According to Aboobucker, “This discovery holds tremendous potential in that it can increase the efficiency of the doubled haploid process and facilitate its adoption in resource-poor settings. Also, this mechanism can be extended to doubled haploid technology in other crops of agricultural and economic importance, since the genes are conserved across the plant kingdom.”

Aboobucker’s next steps include trying this process with haploid corn varieties. By increasing the success of doubled haploid breeding and fertility, breeders will be able to provide growers with a larger number of crops created to promote more desired agricultural traits such as higher yield and increased resilience in a shorter time.