The Problem of Acquired Physiological Immunity in Plants

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Abstract
This paper is a critical analysis of the subject based on a literature review of 200 titles and on the author''s own investigations. The subject is introduced with a delimitation of terminology and a priori analysis of the possibility of demonstrating acquired physiological immunity in plants. It is shown that normal, non-specific antibodies of various types have been repeatedly observed in plants but that their demonstration is complicated by the frequent presence in plants of substances which exert actions simulating those of true antibodies (pseudo-antibodies). Artificial introduction of foreign antigenic materials into the plant has not yet been shown to be a successful method of attack due to the insufficient data submitted, to errors due to uncontrolled or unknown variables, and to the inherent difficulties in applying to plants a technique of demonstration comparable to those of animal serology. On the other hand, the study of host-parasite relationships and of symbiosis has yielded abundant evidence that the living plant opposes the aggressiveness of parasite or of symbiont with parasitic potentialities by reactions of a type fundamentally analogous to the acquired defensive mechanisms of animals. The evidence from parasitism is based on observations of resistance to reinfection, preliminary exps. in plant vaccination and a study of morphological behavior of host in contact with parasite, which lines of evidence all support the thesis that acquired physiological immunity is a true mechanism of defense in plants, although the serological proof has been universally unsuccessful, supposedly because of the absence in plants of a fluid for antibody demonstration comparable to vertebrate blood. Again in symbiosis the thesis is supported by strong evidence yielded by a study of symbiont behavior in vivo, and here certain types of in vitro tests have been successfully applied in confirmation of the immune phenomena in vivo. The evidence from symbiosis is based particularly on careful studies of mycorrhizae and of the root-tubercle bacilli; studies of the graft-symbiosis have been less fruitful. All the evidence thus adduced implies that plants may defend themselves more or less successfully against the injurious effects of foreign bodies through the acquirement of a certain degree of specific immunity following sensitization, but such phenomena in plants differ from those in animals chiefly in the fact that acquired immunity in plants is in general manifested by local, cellular reactions and not by any generalized, humoral immunity such as plays a leading role in animal defense.

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