Embryo rescue for plant breeding

Did you know that embryo rescue is a technique that could help to introduce desired genes in a crop/variety without any genetic modification? Hold on there! Even when it looks like the ‘Holy Grail’ for crop improvement, it has its own limitations.

Let us take a look at how embryo rescue has shown to be a reliable ally in plant breeding over the years. Well, it all goes back to plant reproduction.

Reproduction

Reproduction is the main feature of life on earth. By means of this biological process, the continuity of all species can be ensured. When it comes to plants, they can create offspring through either sexual or asexual reproduction. While asexually produced organisms are a copy in all sense identical to the parent, sexual reproduction requires the genetic material of male and female sex cells, commonly known as ‘gametes’, to produce an offspring: an ‘embryo’.

How is the embryo formed in plants?

The reproduction of flowering plants is called ‘pollination’. Male sex organs of flowers are called ‘stamen’ while female sex organs are called ‘pistils’. ‘Pollen’ which contains the male gametes needs to be moved from ‘anthers’ to the ‘stigma’ (a part of the pistils). When the pollen reaches the stigma, one of the male gametes fertilizes the ‘ovule’ while the other fertilizes the embryo sac to form the ‘endosperm’. The ‘zygote’ (fertilized ovule), through a series of well-defined developmental stages, forms an embryo. The endosperm forms and nourishes the embryo during development or germination.

Plants can either ‘self-pollinate’ or ‘cross-pollinate’:

• Self-pollination occurs when the pollen of a plant fertilizes its own stigma.  
• Cross-pollination happens when the pollen travels from anthers to the stigma of a flower from another plant. The main feature of cross-pollination is that it promotes ‘genetic diversity’.

Some plants have features that prevent self-pollination, such as pollen and ovules that develop at different times and hence no pollination can happen within the same plant.

Seed: plant kingdom babies

The complete reproduction process finishes with the formation of the seeds. Inside the seeds, little embryos grow and mature as well as the endosperm. The endosperm is rich in carbohydrates, proteins, and lipids, basically the perfect womb for a growing baby. In ‘dicotyledonous’ (also called dicots), plants with two cotyledons (the first two leaves of a seedling), the endosperm is consumed during embryo development. While in ‘monocotyledonous’ (also called monocots), plants with one cotyledon, the endosperm remains intact until it is consumed at the germination of the seeds.

Now that we have reviewed all of the reproduction concepts, you can see that the pathways for embryo formation are anything but simple.

What is embryo rescue then?

The growth of an embryo is regulated by many physical and chemical factors. Disruptions in these factors can result in abnormalities and, in the worst-case scenario, abortion of an embryo. By employing plant tissue culture, fertile hybrid plants can be grown from immature embryos in a carefully devised culture media. This technique can save weak, immature and hybrid, or sometimes inviable embryos.

Embryo rescue can be achieved through embryo culture, ovule culture, or ovary culture. All those embryos unable to survive naturally, have a second chance by culturing them in vitro. But why is it important for breeders?

Breeders want to breed

When it comes to successful breeding programs, the production of multiple and fertile seeds is the core for obtaining hybrids/varieties. In order to achieve success, reproductive compatibility among the plant parents plays a key role. Additionally, time and resources are highly important as well.

Let us discuss the most common reasons why breeders find embryo rescue appealing:

• Propagation of rare plants: Some plant species play historical, ecological, economical, and ornamental roles in many parts of the world. But unfortunately, the seed viability and germination rates are very low in nature (For instance: persian cypress, macapuno, butterfly weed, etc.). By using embryo rescue, many more seedlings can germinate successfully, and genetic diversity can be preserved.
• Seedless production: The cost of developing new seedless cultivars is high, and the success rate is low. Table grapes are a prime example. To develop new seedless plants with superior features, a seeded cultivar (the female) is typically crossed with a seedless cultivar (the male). However, the presence of seedless hybrids among the progeny is rare as seeds are aborted at an early stage of growth. However, a significant number of offspring can be genetically assessed in the field by growing the rescued ovule under in vitro conditions.
• Shortening the breeding cycles: Plant breeders are always looking for new ways of shortening the duration of breeding cycles. Seed maturation in some plants can take around 50 to 60% of their life cycle duration (such as: sunflowers, lentils, and soybean). By culturing immature embryos under in vitro conditions, the duration of the breeding cycle can be significantly reduced.
• Genetically ‘distant hybrids’: Distant hybrids are the result of crossing two different species. Those species can come from a different family or a higher-ranking taxon. This method can break species limits and lead to the development of improved varieties.

Why do breeders need to do such complex work?

Well, sometimes economically important crops need to improve their traits in order to survive difficult conditions or plagues. Many of these traits are regulated by different genes which are not naturally available in the domesticated commercial varieties. However, those genes could be available in some wild/non-domesticated species. The process of introducing those new genes to breeding programs is called ‘gene introgression’.

The most valuable outcomes of this type of cross are an increase in genetic variation and combining the biological characteristics of existing species.

Distant hybridization combines the strengths of parents and gives advantages to the hybrid, such as, growth, survival rates, disease resistance, and so on. Even when the outcomes are highly valuable, the survival of the offspring is very low. Embryos are naturally aborted in the early stages of growth. The embryo rescue and in vitro culture have become a great way to allow these embryos to develop and evolve.

After reviewing how embryo rescue is applied to plant breeding, there is no doubt how powerful and important this technique is to speed up breeding programs and break natural barriers.

For more interesting articles on different aspects of plant sciences, keep checking this space.

By Nataly Sánchez Del Río | 17 -05-2022

References:

• Bermejo, C., Gatti, I., & Cointry, E. (2016). In vitro embryo culture to shorten the breeding cycle in lentil (Lens culinaris Medik). Plant Cell, Tissue and Organ Culture (PCTOC), 127(3), 585–590. doi:10.1007/s11240-016-1065-7
• Bhojwani, S.S. and Dantu, P.K. (2013) Plant Tissue Culture: An Introductory Text. Springer, London. http://dx.doi.org/10.1007/978-81-322-1026-9
• Chen, J., Luo, M., Li, S., Tao, M., Ye, X., Duan, W., Zhang, C., Qin, Q., Xiao, J., & Liu, S. (2018). A comparative study of distant hybridization in plants and animals. Science China. Life sciences, 61(3), 285–309. https://doi.org/10.1007/s11427-017-9094-2
• Karimi, H. R., Janghorban, K., Raqamy, M., & Farahmand, H. (2018). In vitro propagation of some old Persian cypress accessions (Cupressus sempervirens L.) by embryo culture. Physiology and molecular biology of plants: an international journal of functional plant biology, 24(6), 1285–1294. https://doi.org/10.1007/s12298-018-0598-0
• Lewis, M., Chappell, M., Zhang, D., & Maynard, R. (2020). Development of an Embryo Rescue Protocol for Butterfly Weed, HortTechnology hortte, 30(1), 31-37. Retrieved Jan 23, 2022, from https://journals.ashs.org/horttech/view/journals/horttech/30/1/article-p31.xml
• Mondal, B., Chaturvedi, S. K., Das, A., Kumar, Y., Yadav, A., Sewak, S., & Singh, N. P. (2020). Embryo rescue and chromosomal manipulations. Chickpea: Crop Wild Relatives for Enhancing Genetic Gains, 95–130. doi:10.1016/b978-0-12-818299-4.00005-1 
• Shen, X., Gmitter, F. G., & Grosser, J. W. (2010). Immature Embryo Rescue and Culture. Plant Embryo Culture, 75–92. doi:10.1007/978-1-61737-988-8_7