40 SUBCELLULAR PARTICLES 



chloroform-soluble products distinct troni lignin were formed; and in the latter 

 system, which also contains peroxidase, lignin-like products were formed only 

 on addition of hydrogen peroxide. It was later found, however, that some bryo- 

 phytes possessing normally little or no lignin can form lignin-like products from 

 eugenol in the absence of peroxide (31 ); these observations do not eliminate 

 conclusively the peroxidases, as the formation of hydrogen peroxide via phenol 

 autoxidation is well known (22, 38). 



More than 30 compounds have been tested in various systems as lignin pre- 

 cursors. These substances have ranged from simple phenols such as catechol 

 and pyrogallol through phenols with sidechains of varying length, degree of 

 unsaturation, and oxidation level (hydrocarbon, alcohol, aldehyde, ketone, etc.). 

 The more complex phenols include eugenol, isoeugenol, caEEeic acid, ferulic acid, 

 cinnamic acids, vanillin, flavonoids, etc. (29); hydroxycinnamyl alcohols, coniferyl 

 alcohol (8, 9); and, in long-term studies, the labeled compounds shikimic acid, 

 sinapic acid and tyrosine (5). Structural requirements are least rigid when lignin 

 formation is followed in long-term experiments with still growing plants, and it 

 is to the model systems which form lignin within minutes, making use of high 

 peroxide and precursor levels, that one must turn for a more definitive picture 

 of ultimate or penultimate precursors. Using tissue slices from embryonic bean 

 hypocotyl, pea root tips, and Elodea stem, as well as segments of celery vascular 

 strands, it was established that the lignin precursor possessed a phenylpropane 

 (Cg-Cs) carbon skeleton with a free hydroxyl group para to the sidechain. 

 Blockage of the OH in this position by methylation or glucosylation renders 

 otherwise suitable molecules inert as precursors. Suitable sidechains occurred as 

 unsaturated hydrocarbons with or without oxygen functions. Eugenol, its isomer, 

 isoeugenol, the hydroxylated derivative of the latter, coniferyl alcohol, and 

 /7-hydroxypropiophenone, with a beta keto group in the side chain, are repre- 

 sentative precursor molecules. It is of interest that isoeugenol in some systems 

 (with celery vascular tissue, for example) is a far poorer precursor than eugenol, 

 yielding one-seventh as much lignin. 



Inhibitory precursor analogs include those substances which form little or no 

 lignin alone, but are capable of blocking the formation of lignin from suitable 

 precursors without inhibiting peroxidase (36). Examples of such substances are 

 phenyl-2-propanone, cinnamaldehyde, and 3-(chloropropenyl-) benzene. 



Although the conversion of eugenol to a lignin-like substance in plant tissues 

 was undeniable, more compelling evidence for its relation to native lignin was 

 required than that afiforded by color and solubility tests alone. Isolated synthetic 

 lignins were therefore compared with native lignins from various plant groups 

 for common physical and chemical properties, including ultraviolet spectra 

 (fig. 2), elementary composition, as well as solubility and color reactions 

 (table i). Of the synthetic lignins tabulated, the product isolated from celery 



