Effects of Microorganisms on Lignin

Disintegration of plant tissues is a major aspect of pathogenesis. In a large percentage of plant diseases, tissue breakdown is the primary mode of injury and is directly responsible for loss. Consequently, many plant patholo­ gists have studied the enzymatic processes by which plant cell wall constitu­ ents are degraded. Most of these researches have emphasized the microbio­ logical degradation of herbaceous agricultural crops in which pectic materials and cellulose are the major structural components. But plant pathologists must also be concerned with the degradation of the woody tissues of perennial plants. These plants are the predominant vegetation of the earth, and the organisms that degrade their tissues play a major role not only in the loss of valuable crops, but also in the carbon cycle of the earth. In most herbaceous plants pectic materials are the cementing substances between cells. In woody tissues, by contrast, this function is performed by the complex aromatic polymer lignin. Lignin is thus a major component of peren­ nial plants, and as such is probably the second most abundant of all the con­ tinuously cycled organic materials on earth (cellulose is presumably the most abundant). As a major byproduct of pulp manufacture, lignin is also one of the largest industrial waste materials. The biological decomposition of lignin therefore is clearly an important problem, but even so it has not received nearly as much attention as has the degradation of pectic materials and cellulose. Its degradation consequently is only very incompletely understood. The lack of penetrating study reflects partly a lack of persistent interest in a refractory problem. However, it is also clear that until relatively recently the unavailability of lignin prepara­ tions of demonstrated integrity, together with difficulties in working with lignin in chemical and microbiological studies, and especially a lack of under­ standing of the chemical structure and behavior of lignin, have been im­ portant problems impeding progress. These and other problems have earned the respect of scientists who are intimately involved in lignin research, such as 1. A. Pearl, who noted recently (82) that there has been the ... lack of appreciation of so many investigators of the many pitfalls inherent in lig­ nin research and the consequent publication of numerous papers that confuse rather than clarify the state of our knowledge. Unfortunately, this has been just as true for studies of the microbial degradation of lignin as for other