no W. GORDON WHALEY 



control of thiamin, pyridoxine, or niacin metabolism. In intact plants, it is 

 likely that the green parts supply these substances to their own tissues and to 

 the roots, in amounts satisfactory for growth and development. The root 

 tissue responses, however, are definitely heterotic in certain instances, and 

 these mechanisms merit examination. 



It seems pertinent to explore the role of these B vitamins in growth and 

 development. Thiamin appears to be a metabolic requirement for all types of 

 cells. Its metabolic activity apparently revolves around a role in enzyme 

 systems. Thiamin pyrophosphate is the co-enzyme of the enzyme pyruvate 

 carboxylase (Lohmann and Schuster, 1937). The enzyme carboxylase occurs 

 in many plant tissues. The possible biochemical basis of thiamin action in 

 plants has been set forth in some detail by Bonner and Wildman (1946), 

 Vennesland and Felsher (1946), and Bonner and Bonner (1948). It is assumed 

 that thiamin represents a step in the development of co-carboxylase which is 

 active in one or more of the decarboxylating enzyme systems of the respira- 

 tory mechanism. 



Pyridoxine also has an enzymatic role, apparently being important for its 

 conversion to pyridoxal phosphate, which is a co-enzyme of one or more of 

 the reactions in the nitrogen metabolism of the plant (Bonner and Bonner, 

 1948). As a co-enzyme active in nitrogen metabolism reactions, pyridoxine 

 may be of extreme importance in amino acid-protein building, and hence 

 active in conditioning fundamental growth activities. 



Similarly, niacin activity is enzymatic in character. Niacin appears to be 

 involved as a constituent of the nucleotide cozymase, and possibly of tri- 

 phosphopyridine nucleotide. Cozymase is a co-enzyme for a whole series of 

 dehydrogenase enzymes, including alcohol dehydrogenase, malic dehydrog- 

 enase, and glutamic dehydrogenase (Bonner and Bonner, 1948). 



The genetic background of thiamin, pyridoxine, and niacin metabolism is 

 thus a genetic background concerned with basic components of the plant's 

 enzyme systems. Heterosis, which rests upon recombinations concerned with 

 thiamin, pyridoxine, or niacin metabolism, quite obviously rests upon recom- 

 binations which are concerned with the acceleration, inhibition, or blocking 

 of specific stages or developed substances in the basic enzyme system. 



A considerable amount of supporting evidence for the involvement of such 

 fundamental enzyme and other growth substance activities in the develop- 

 ment of heterosis has been coming for some time from the work on Neuro- 

 spora. In many heterocaryons of Neurospora, increased growth responses 

 directly suggestive of heterosis have been observed. In a number of instances 

 (Beadle and Coonradt, 1944), the growth responses depend upon the two 

 types of nuclei in the heterocaryon — each carrying wild type alleles of de- 

 leterious mutant genes carried by the other nucleus. Such instances represent 

 essentially the same situation as the recombination of favorable dominant 

 alleles in normally diploid organisms. 



