Breeding of Vegetable, Spice and Tuber Crops

Base analogues and related compounds

• True base analogues are closely related to DNA bases, adenine, guanine, cytosine or thymine and can be incorporated into DNA without affecting its replication. But analogue differs from the normal base in certain substitutes hence its electronic structure is modified and it can be expected that occasional pairing errors will occur at the time of DNA replication after the analogue has been incorporated.

• The most frequently used analogues are 5-bromo-uracil (BU) and 5-bromo-deoxyuridine (BUDR), which are analogues of thymine. Apart from true analogues, it has been found that N-methylated oxypurines have a chromosome effect.The most efficient compounds are 8-ethoxy caffeine (EOC) and 1,3,7,9 tetramethyluric acid (TMU), but their mode of action is still unknown. Maleic hydrazide (MH), an isomer of uracil, induces chromosome breaks in cell and aberrations are localized in heterochromatic regions near the centromere of the chromosomes.

Antibiotics

• Antibiotics such as azaserine, mitomycin C, streptonigrin and actinomycin D have been found to have chromosome breaking properties, but their usefulness are limited.

Alkylating agents

• This is the most important group of mutagenic chemicals for mutation induction in cultivated plants.
• They have one or more reactive alkyl groups which can be transferred to other molecules.
  
• They react with DNA by alkylating the phosphate groups as well as the purine and pyrimidine bases. One should be extremely careful in using them because most are potential carcinogens, such as EI, EMS, MNH and should be used in small units.

Azide

• Azide is an effective mutagen under certain treatment conditions.
  
• It is possible to obtain high mutation frequencies with azide.
  
• Most mutations are gene mutations with some minor frequencies of chromosome aberrations.
  
• It is relatively safe, non-persistent and inexpensive but are also potential carcinogens.