Phloem-and xylem-restricted plant pathogenic bacteria

Abstract

1. Introduction: an overview This review concerns plant pathogenic bacteria, which are strictly restricted either to the sieve tubes in the phloem or to the vessels in the xylem. These bacteria are endogenous as opposed to exogenous bacteria such as species of Erwinia , Pseudomonas , Ralstonia or Xanthomonas , which colonize the apoplast (intercellular spaces) of plant tissues, even though some of the exogenous bacteria, such as Xylophilus ampelina , can also induce vascular infections. Due to their vascular habitat, the endogenous bacteria have a systemic distribution throughout the plant, they are transmitted from plant to plant by graft inoculation, and most of them are vectored by insects which feed in the phloem (leafhoppers, psyllids), or the xylem (sharpshooters). Because of these virus-like properties, the diseases caused by endogenous bacteria have long been taken for virus diseases. The principal conducting cells of the phloem are the sieve tube elements [37]. These elements are joined end to end into sieve tubes, and are associated with parenchymatic, nucleated cells, the companion cells, an important role of which is to load sucrose into the sieve tubes. The sieve tube elements are living cells, which become enucleated at maturity. The sieve plates are lateral wall areas between two adjacent sieve elements. The sieve plates are clustered with pores, resembling giant plasmodesmata and interconnecting two adjacent sieve elements through their cytoplasms. The diameter of a pore ranges from a fraction of a micron (m )t o 15m and more, and is large enough to allow passage of sieve tube-restricted bacteria. The principal conducting elements of the xylem are tracheids and vessel members [37]. Both are dead cells, contain no cytoplasm, and have lignified secondary walls. The vessel elements are joined end to end into vessels, and the adjoining ends have open perforation plates. These openings allow relatively unimpeded longitudinal spread of the xylem-restricted bacteria within