316 THE BIOCHEMISTRY OF B VITAMINS 



The B Vitamin Requirements of Green Plants 



Because the major source of the B vitamins for the animal kingdom 

 ultimately comes from green plants, and because green plants are so 

 frequently referred to as autotrophic, and even further because of the 

 widespread culture of plants in mineral salts-water solutions, it is cus- 

 tomary to think of them as having no B vitamin requirements. It is true 

 that the green plant as a whole will grow without an apparent exogenous 

 supply of B vitamins or other organic nutrients. It is also true, however, 

 that in many cases the plant may be deriving considerable nourishment 

 from symbiotic microorganisms. It also seems reasonably well established 

 that many green plants may not be able to synthesize sufficient B vita- 

 mins for optimum growth. It has been frequently observed (and also 

 denied) that thiamine, at least, frequently causes more luxuriant growth 

 of some plants, when added to the nutriment. Added riboflavin has been 

 reported to be of benefit to eggplant cultured in a synthetic medium, 41 

 and it seems likely that other specific cases may arise involving others 

 of the B vitamins. 



Green plant embryos, young roots, cuttings, and pollen grains are, 

 however, heterotrophic, and all are dependent upon supplies of at least 

 some of the B vitamins from stored foods or other portions of the plant. 

 While only a limited amount of information is available, it seems that 

 thiamine and riboflavin 42 > 43 have pronounced effects on the germination 

 of pollen grains. Biotin is highly active in stimulating the growth of roots 

 on cuttings, 44, 182 but in this case its activity has been likened to that of a 

 hormone. Indeed biotin functions in an auxin-like fashion in roots, in 

 which it is concentrated in the tips (and also in the tips of coleoptiles) . 45 

 Biotin is synthesized by all roots, and even excreted in considerable 

 amounts. 40 Its effect on root formation in peas, for instance, is quite 

 marked, inducing a 100 per cent increase in root formation on pea cut- 

 tings. 44 The pea embryo, however, cannot synthesize biotin, and in this 

 case the function of biotin is more likely associated with the metabolic 

 activities of the embryo than with its differentiation. Biotin shares with 

 auxin a and b the structural feature of a five-membered ring joined to 

 a valeric acid derivative, and may well owe its auxin-like activity to this 

 resemblance. 



The roots and embryos of green plants have been more extensively 

 studied, but plant tissue culture techniques are generally so recent in 

 origin as to have supplied little information. Excised roots have been 

 grown in apparently sterile media, and show a wide variety of organic nu- 

 tritional requirements. White has found some nine amino acids to be re- 

 quired by excised tomato roots — histidine, phenylalanine, lysine, leucine, 



