The Leaf from the Inside and the Outside: A Microbe’s Perspective

Abstract

A microbial spore that arrives by air, rain splash, vector, or brush contact on any plant surface, like a pollen grain but unlike most seeds, has probably not been selected in the form in which it is spread for sustained viability. Therefore, on the exposed, desiccating, radiation-prone, potentially crowded, suppurating, vibrating surface of a leaf, a spore’s effective life may be very short. The spore must germinate and, in many cases, then penetrate the leaf. The requirements of this development are rigid. It may grow epiphytically, but may be required to penetrate or be forced, as in bacteria or viruses, through layers of waxes, cutin, and sometimes polyisoprene, and the pectocellulosic wall, which may occasionally be lignified. Only then may it be able to reach the softer walls, i.e., those lacking the encrusting substances listed above, and the more sheltered, more nitrogen-rich interior. (Only the smallest traces of proteins have been found in the waxes isolated from plant surfaces [Hallam and Juniper, unpublished; see Section 2.4].) All permeable plant surfaces, whether external as in glands or nonwaxy surfaces, or internal as in uncutinized walls, are leaky and nutrient-rich (Tukey, 1970) and will more readily be colonized (Lippincott et al., 1977; Juniper and Jeffree, 1983). Within the leaf or other tissue, although physically safer, the growing hyphal tip, virus population, or bacterial colony may be impeded by induced physical or chemical barriers.