The effects of selected nitrogen compounds on the growth of plant tissue cultures

Carrot phloem explants, growing and dividing rapidly in a nutrient medium containing coconut milk, synthesize a protein which, like collagen in animals, is rich in hydroxyproline and quite inert metabolically. Hydroxyproline strongly inhibits the growth of such tissues, the l forms of both allohydroxyproline and hydroxyproline being more inhibitory than the d forms. Chemical alteration of the hydroxyproline molecule markedly reduces this inhibitory effect, i.e. the inhibition is quite specific. The inhibition by hydroxy- l-proline is reversed by enzymic casein hydrolysate and only by l-proline of the eight l-amino acids tried. Among twelve inhibitors demonstrated, six are structurally related to proline, and like hydroxy- l-proline, are much less inhibitory in the presence of proline. Of the other six inhibitors, azaserine is not antagonized by glutamine, hydroxy- l-lysine is antagonized by l-lysine, 5-hydroxytryptophan is antagonized by tryptophan and canavanine by arginine. No antagonist is yet known for the most powerful inhibitor of all, gramicidin-S, sulphate. In addition to the twelve inhibitors, six partial inhibitors were found and sixty substances which were tested showed no growth inhibition. These results are interpreted in terms of the effects of the substances in question on protein metabolism.