Microbial Penetration and Immunization of Uncongenial Host Plants

Abstract

To initiate a parasitic disease the propagules of the infectious agent must contact, in an apt environment, the appropriate anatomical region of an appropriate host. Until recently phytopathologists have been mainly in­ terested in the study of the events aroused by the fulfilment of such condi­ tions, overlooking what happens when the propagules of a pathogen fall on plants not belonging to its natural circle of hosts, or when a normally non­ pathogenic microorganism comes in contact with a plant. Only lately has greater attention been paid to this second type of combination. The root system of a plant is permanently in contact with a multitude of microorganisms. But the quantity of nonpathogenic contaminants of both resident or casual types (sensu Leben, 73), on the aerial surface of a plant is generally higher than the quantity of pathogens. At least in theory it must be assumed that the avirulent microorganisms can induce physiologi­ cal changes in the plant and thus variations in its reactivity towards subse­ quent infection by specific pathogens. Besides the direct antagonistic activ­ ity (140) and the physical and chemical alterations that are produced espe­ cially in the soil environment, this is another way through which micmflora may affect the establishment of a parasitic process. The relations between plants and avirulent germs, and the influence of the latter on the likelihood of subsequent by normal pathogens, can be studied better in the laboratory, where they can be mimed and mag­ nified under controlled conditions, than in nature where they cannot. A great deal of progress has been made in the similar field of plant virology where investigations carried out with combinations of different viruses and virus strains have led to evidence and to codifying the complex phenomena known as acquired immunity following recovery, cross protection, lo­ calized and systemic interference (108). This paper is primarily concerned with the immunizing effect that the infections with avirulent fungi and bacteria are capable of inducing in plants. We shall therefore examine (a) whether nonpathogens can pene­ trate and induce morphophysiological responses ill the plant, (b) whether