Understanding Crop Breeding for Disease Resistance

Crop breeding is a crucial process aimed at developing new plant varieties with desirable traits, such as higher yield, improved quality, and importantly, resistance to diseases. Diseases can significantly reduce crop production, making the development of disease-resistant varieties a key focus in agriculture.

The question asks for the correct sequence of steps specifically for breeding crop plants for disease resistance. The steps provided are:

• (A) Testing and release of new varieties
• (B) Hybridization of the selected parents
• (C) Screening germplasm for resistance source
• (D) Selection and evaluation of hybrids

We need to arrange these steps in the correct order based on the provided options.

Let's analyze the sequence given by the chosen correct option:

C → B → A → D

Now, let's detail what each step in this sequence involves:

Screening Germplasm for Resistance Source (C)

The process typically begins with identifying a source of resistance. This involves screening a wide collection of genetic resources, known as germplasm, to find individuals or varieties that naturally possess resistance to the target disease. Scientists expose different plant lines to the disease under controlled conditions to identify which ones show resistance symptoms and which do not. This step is fundamental as you need a source of resistance genes to incorporate into the new variety.

Hybridization of the Selected Parents (B)

Once a source of resistance (identified in step C) is found, it is crossed with a cultivated variety that may be susceptible to the disease but has other desirable traits, such as high yield or good quality. This crossing process is called hybridization. The goal is to combine the resistance genes from the resistant parent with the favorable characteristics of the susceptible parent in the offspring (hybrids).

Testing and Release of New Varieties (A)

Following hybridization (B), the sequence indicates 'Testing and release of new varieties'. This step generally involves evaluating the performance of promising advanced breeding lines under various environmental conditions, including natural disease pressure or artificial inoculation. If the varieties consistently show resistance, high yield, and other desired traits over several years and locations, they are considered for official release to farmers. This is typically one of the final stages in a breeding program.

Selection and Evaluation of Hybrids (D)

According to the provided sequence, this step follows 'Testing and release' (A). 'Selection and evaluation of hybrids' involves growing the progeny (hybrids) resulting from the cross (B). These hybrid plants are carefully examined for disease resistance (often through artificial inoculation) and other important traits. Plants exhibiting the desired combination of resistance and favorable characteristics are selected, and further generations are grown and evaluated to stabilize these traits through processes like self-pollination and re-selection or backcrossing. This rigorous selection process aims to isolate lines that reliably carry the resistance genes and express the desired agricultural traits.

Based on the sequence provided in the correct answer option, the order of steps in breeding crop plants for disease resistance is Screening germplasm for resistance source (C), followed by Hybridization of the selected parents (B), then Testing and release of new varieties (A), and finally Selection and evaluation of hybrids (D).

Revision Table: Disease Resistance Breeding Steps

Additional Information: Plant Breeding Techniques

Disease resistance breeding is a specific application of general plant breeding principles. Other goals of plant breeding include improving yield, nutritional content, adaptability to different environments, and tolerance to stress like drought or salinity.

Methods used in plant breeding include:

• Selection: Choosing plants with desirable traits to be parents for the next generation.
• Hybridization: Crossing two genetically different parents to combine traits.
• Mutation Breeding: Inducing genetic mutations to create new variations.
• Polyploidy Breeding: Altering the number of chromosome sets in a plant.
• Tissue Culture: Growing plants from small pieces of tissue under sterile conditions.
• Genetic Engineering: Directly modifying the genetic material (DNA) of a plant (also known as Genetic Modification or GM).

For disease resistance, breeders often work with genes that confer either qualitative (major gene) or quantitative (minor gene) resistance. Understanding the inheritance of resistance is crucial for designing effective breeding strategies.