Growth and Development of Horticultural Crops (1+1)

Discovery and chemical nature

• The discovery of kinetin is comparatively more recent. Its credit goes to Miller et al. (1950) who were working in Prof. Skoog’s lab at the University of Wisconsin on the growth of tobacco pith callus in culture and wanted it to grow indefinitely.
  
• They added various growth substances, nutrients, vitamins etc, into the culture medium but failed till they noticed an old bottle of DNA kept for several years in their lab.
  
• They added the contents of that bottle to the culture medium and observed that the tobacco pith callus could grow for longer periods. They obtained similar results with Yeast extract.
  
• But they did not get positive results with fresh DNA and thought the active substance to be some degradation product of DNA.
  
• They isolated this substance by autoclaving (heating under pressure) the DNA which had been stored for long. It could easily be precipitated by silver salts and was soluble in 90% alcohol, indicating that possibly it was a purine compound.
  
• Later on, they identified it as 6-furfurylaminopurine. Because of its specific effect on cytokinesis (i.e.,) cell division, it was called as kinetin.
• Although kinetin has profound influences in inducing cell division, still it has not been isolated from any plant.
  
• But, certain substances which show kinetin like activity have in fact been isolated from a variety of higher plants.
  
• These substances are collectively called as cytokinins.
  
• There is now sufficient evidence to show that cytokinins do occur in plants and regulate growth and hence, they are also considered as natural plant growth hormones.
  
Some of the very important and commonly known naturally occurring cytokinins are as follows.
Zeatin

• Zeatin is the most abundant and widely distributed natural cytokinin in higher plants and in some bacteria. Although this cytokinin was known earlier but it was obtained in pure crystalline form in 1963 by Letham from immature corn grains and named as Zeatin.
  
• It was identified as 6-(4-hydroxy-3-methylbut-trans-2enyl) amino purine by Letham et al. (1964) and was synthesized by Shaw and Wilson (1964).
1. Zeatin exhibits strong kinetin like activity in stimulating plant cell to divide in presence of auxin in culture media.
2. Zeatin resembles kinetin in molecular structure because both are adenine or amino purine derivatives.
3. Zeatin is remarkably more active than any other cytokinin probably because of the presence of a highly reactive allylic-OH group in its side chain.
Other natural cytokinins

• Apart from zeatin, some other substituted amino purines have been isolated from higher plants and some bacteria which are also considered as natural cytokinins. These are di-hydrozeatin (DZ) and N6-(∆2 - isopentenyl) adenine (or ip) which differ from Zeatin in nature of their side chain.