Crop breeding

How do scientists do traditional plant breeding?

The “science” of plant breeding has only existed for a little more than a century. But, humans have unofficially been selecting for the ‘cream of the crop’ for over 10,000 years. Farmers selected seeds from their prize crops for the following year. Plant traits they might have looked for were crops that yielded the highest, stood the tallest, and overall had the best traits. This practice largely shaped how our food looks today.

sweet potato flower being fertilized by hand
Sweet potato fertilization by hand. Credit Cary Fowler, Global Crop Diversity Trust

Plant breeders are still focused on their number one goal: yield. Population growth and land restrictions mean we have to grow more food with less space. Different plant breeders work on different crops. Each crop has its own set of diseases, insects, and special requirements that make it unique. Additionally, changing weather patterns make drought (or flood) tolerance necessary. Breeders are working in the background to select for the most nutritious, flavorful fruits and vegetables!

Crop breeders would all agree that plant breeding, while technically defined as a science, is equally an art. Breeding involves genetic selection of plants to the benefit of people. But, there are several different styles of plant breeding. This post will cover the more traditional styles.

Classical, or conventional plant breeding, encompasses field methods, physical measurements, and an assortment of breeding designs with a variety of crops in different environments. This is in contrast to molecular plant breeding, which goes all the way to the DNA level. Crop breeders who specialize in one breeding type work with breeders in other areas, as the two types often go hand in hand.

Often, an economically valuable type of crop will be susceptible to a specific pest. To help keep the best traits of this crop, but increase its pest resistance, traditional crop breeders of often look to other plants. Relatives of these crops, either from different countries or from the wild, often carry natural pest resistance.

chestnuts in a basket
American chestnut trees were saved from disease by traditional plant breeding. Credit: Morguefile

Let’s look at an example from the American chestnut tree. The tall American chestnuts once dominated Eastern forests, and were valued for their beautiful wood. But, today’s American chestnuts are susceptible to a disease called chestnut blight. American chestnuts are a relative of the variety called Chinese chestnut, which is native to Asia. Chinese chestnut is resistant against chestnut blight disease. However, Chinese chestnut is a shorter tree that does not have good qualities for timber. Could breeders find a way to mix the good traits of both American and Chinese chestnuts – quality wood and disease resistance?

Yes! Breeders crossed American chestnut with Chinese chestnut, and blight resistance was transferred to the next generation. Referred to as backcrossing, the cultivar is crossed repeatedly ‘back’ to the plants which have inherited resistance. After multiple generations of this, the resulting line reflects primarily the traits of the cultivar parent (American chestnut) plus resistance from the donor relative (Chinese chestnut). Backcrossing was used successfully to transfer chestnut blight resistance from Chinese chestnut to American chestnut trees.

corn plants and corn cobs
Two inbred lines of corn (left and right) and their hybrid (center). Credit: S. Flint-Garcia, USDA

Inbreeding is used by plant breeders to increase the proportion of favorable traits in a plant line. The method and amount of inbreeding varies in the development of inbred lines. By making intentional crosses among close relatives in crops, hidden traits are revealed that were once masked by dominant traits. The best performing plants in that inbred group are selected, and continue to be self-pollinated over time in order to maintain a line with superior traits. Inbreeding is most often used in crops such as corn, to develop special parent lines.

Ultimately, the goal of inbreeding is to develop inbred lines that, when crossed together, have superior traits. In this method of hybrid breeding, it would be impossible to measure all the possible outcomes if every inbred line combination were individually crossed (never mind all the field space and helpers)! Instead, lines are crossed to a single parent, called a ‘tester’. Ultimately, two or more inbred lines may be crossed into a hybrid line. Besides corn, sugar beet and sorghum are also hybrids.

All plants – and animals – have DNA. Mutations in DNA occur naturally, and are the underlying reason for all of our physical differences. Mutation breeding has been studied by scientists for almost 90 years in plants. It has been used to induce mutations associated with favorable traits in plants. Seeds are treated with X-rays, gamma rays, or chemicals in low doses, and then the next generations are measured for the best qualities. One example of this is cotton seed treated with the chemical ethyl-methyl sulfate (EMS) to improve fiber length. Because there is no way to know where a mutation will occur in the genome, this type of breeding requires very large numbers of plants.

Answered by Christine Bradish, AVOCA

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About us: This blog is sponsored and written by members of the American Society of Agronomy and Crop Science Society of America. Our members are researchers and trained, certified, professionals in the areas of growing our world’s food supply, while protecting our environment. We work at universities, government research facilities and private businesses across the United States and the world.

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