Plant Tissue Culture and Plant Biotechnology
This section covers plant tissue culture and its applications. The exam tests totipotency, explant preparation, media, hormone balance, micropropagation, and somaclonal variation.
Core rules
Plant cells are totipotent: a single cell or explant can regenerate a whole plant under suitable conditions. The explant is surface-sterilized and placed on a sterile medium, usually Murashige and Skoog (MS) medium with sucrose, vitamins, and plant hormones. The auxin-to-cytokinin ratio controls development: high auxin favours roots, high cytokinin favours shoots, and an intermediate ratio favours callus. Callus is an unorganised mass of dividing cells. Somatic embryogenesis produces embryos from somatic cells. Anther culture yields haploids, which can be doubled to homozygous diploids. Protoplast fusion creates somatic hybrids. Micropropagated plants need hardening before field transfer.
Worked example
A nodal explant is placed on MS medium with 2 mg/L BAP and 0.5 mg/L IAA. The high cytokinin-to-auxin ratio favours shoot multiplication. After shoots form, they are transferred to MS medium with NAA, an auxin, to induce rooting. If one explant produces five shoots per subculture and all survive, four subcultures give 5^4 = 625 shoots. The reasoning is exponential multiplication, not a linear count.
Traps that cost marks
Do not say every explant regenerates easily; genotype and hormone balance matter. Do not reverse the hormone rule: cytokinin promotes shoots, auxin promotes roots. Callus is not an embryo. Contamination and poor hardening are common practical failures. Somaclonal variation is a source of variation, not a guarantee of identical clones.