(a) Describe the causes and sources of disease resistance in plants. (20 marks) (b) Explain the functions and structure of…

(a) Causes and sources of disease resistance in plants (20 marks)

Disease resistance is the ability of a plant to prevent or limit the growth and damage caused by a pathogen. Its causes lie in the host's structure and chemistry, and in the way the host and pathogen interact.

Causes of resistance:

• Structural (pre-formed) barriers: a thick cuticle and wax, thick-walled epidermis, few or narrow stomata and lenticels, and lignified or suberised tissue, which stop entry. Hairs and closed flower structure can also reduce infection.
• Biochemical (pre-formed) defences: phenolics, saponins, tannins, glucosinolates and antimicrobial proteins present before attack, and the lack of a compound the pathogen needs (a receptor or nutrient), which gives non-host resistance.
• Induced defences: after recognition, plants build papillae, tyloses and gums, produce a hypersensitive response (HR) with localised cell death, generate reactive oxygen species, and form phytoalexins and pathogenesis-related (PR) proteins such as chitinases and glucanases. Systemic acquired resistance (SAR) operates through salicylic acid.
• Genetic basis: resistance is either vertical (race-specific, monogenic, governed by R genes and the gene-for-gene relationship, effective but breaking down when the pathogen changes) or horizontal (race-non-specific, polygenic, partial but more durable).
• Escape and tolerance: early maturity or altered sowing time helps the crop avoid the disease period, and tolerant plants yield well despite infection.

Sources of resistance:

1. Cultivated varieties and landraces, the easiest source to use.
2. Wild relatives and related species, which carry many resistance genes, for example the stem-rust resistance in wild wheat relatives and grassy-stunt resistance from Oryza nivara.
3. Germplasm collections, gene banks and other breeders' lines, screened under natural or artificial epidemics.
4. Induced mutations, where mutation breeding creates resistant plants.
5. Genetic engineering and genome editing: transgenes (for example chitinase genes or virus coat-protein genes), RNA interference, and edited susceptibility (S) genes.
6. Somaclonal variation and in-vitro selection against pathogen toxins.

These sources are used in breeding through backcrossing and pyramiding several genes, to give varieties with durable resistance.

(b) Structure and functions of mitochondria (20 marks)

Mitochondria are double-membrane organelles, about 0.5 to 1 µm wide and up to several µm long, found in all eukaryotic cells. They contain their own circular DNA and 70S ribosomes, divide by fission, and are inherited maternally in most crops, which is why they are called semi-autonomous organelles.

Structure, as shown in the diagram:

• The outer membrane is smooth and permeable to small molecules through porins.
• The inner membrane is selectively permeable and folded into cristae, which greatly increase its area. It carries the electron transport chain and the ATP synthase (F₀F₁) particles, and it contains cardiolipin.
• The intermembrane space lies between the two membranes; protons accumulate here to form a gradient.
• The matrix, enclosed by the inner membrane, holds the enzymes of the Krebs (TCA) cycle and β-oxidation of fatty acids, mitochondrial DNA, ribosomes, and calcium and phosphate granules.

Functions:

1. ATP production (the powerhouse of the cell): pyruvate enters the matrix and passes through the Krebs cycle; NADH and FADH₂ pass electrons through the inner-membrane chain, and ATP synthase uses the proton gradient for oxidative phosphorylation.
2. Fatty acid oxidation and part of amino acid metabolism.
3. Photorespiration in plants, where mitochondria convert glycine to serine, and the alternative oxidase pathway, which gives heat, for example in thermogenic flowers such as Arum.
4. Synthesis of some vitamins, iron-sulphur clusters and steroid precursors.
5. Regulation of calcium and of programmed cell death, through the release of cytochrome c.
6. Cytoplasmic male sterility in crops such as maize and rice, which is encoded by mitochondrial genes and is used in hybrid seed production.

(c) Importance of molecular markers in plant improvement (10 marks)

A molecular marker is a DNA sequence at a known location that shows variation between individuals and is inherited, so it can be used to follow a nearby gene. Examples are RFLP, RAPD, AFLP, SSR (microsatellites), SNPs and ISSR. They are unaffected by environment, appear at any growth stage, and are numerous.

Importance:

• Marker-assisted selection (MAS): plants are selected by a linked marker, instead of by the trait, which is useful for traits that are recessive, expressed late or hard to score, such as disease and quality traits, and saves time and cost.
• Marker-assisted backcrossing and gene pyramiding: a target gene is transferred while the rest of the genome of the recurrent parent is recovered faster, and several genes are combined in one variety, for example the bacterial blight genes in rice (Improved Samba Mahsuri).
• Gene mapping and QTL analysis: linkage maps place the genes for quantitative traits and help in their cloning.
• Germplasm characterisation and diversity: markers identify duplicates, measure genetic distance and guide the choice of parents.
• Variety identification and seed purity: DNA fingerprinting confirms hybrids and protects breeders' rights.
• Genomic selection and speeding up breeding: marker information is used to predict breeding value, and shortens the breeding cycle.