Endophytic bacteria are ubiquitous in most plant species, residing latently or actively colonizing plant tissues locally as well as systemically Several definitions have been proposed for endophyt

Bacterial endophytes in agricultural crops

Plant growth-promoting bacterial endophytes.

Most healthy naturally propagated plants grown in field or potting soils are colonized by communities of endophytic bacteria, embracing a wide variety of species and genera. These bacteria form nonpathogenic relationships with their hosts: some beneficial, some neutral, and some detrimental. Such associations can increase plant growth and hasten development or improve resistance to environmental stress. Endophytic bacteria have been implicated in supplying biologically fixed nitrogen in non-legumes, and these associations can increase the nitrogen economy of a crop, reducing the requirement for N fertilizers. Bacterial endophytes have also been shown to prevent disease development through endophyte-mediated de novo synthesis of structural compounds and fungitoxic metabolites. Such induced protection responses have been linked to certain forms of systemic acquired (disease) resistance. Certain crop sequences have been shown to favor the build-up of specific plant growth-promoting bacterial endophyte popula...

Bacterial Endophytes: Potential Role in Developing Sustainable Systems of Crop Production

Most plant-pathogenic fungi gain access into their host by penetration of unwounded tissue Some pathogens such as rusts invade the host via stomata (Hoch and Staples, 1987 and Chapter 2), whereas others penetrate the intact leaf surface without the requirement of natural openings (Aist, 1976; Emmett, 1975; Kunoh, 1984) The latter type of direct penetration encounters the plant cuticle, a noncellular hydrophobic structure covering the layer of epidermal cells The cuticle thus serves as the first surface barrier that the pathogen has to breach There is little evidence for the mere physical strength of the plant cuticle as a major factor in plant defense against pathogens In some cases, the thickness of plant cuticles has been correlated with an increased passive resistance against fungal attack This correlation, however, appears to be an exception rather than the rule (Martin, 1964) Furthermore, the cuticle has not been considered to play a major role in the active defense mechanisms of disease-resistant cultivars There is good evidence that the cuticle is penetrated by the attacking pathogen before the sequential steps of disease development are halted by the active defense reactions of the challenged plant Recent examples for this lack of cuticle involvement in cultivar resistance are the host—pathogen interactions of Venturia ivaequalis—apple (Valsangiacomo and Gessler, 1988) or Phytophthora infestans—potato (Gees and Hohl, 1987) The breaching of the cuticle can also be accomplished in many interactions of pathogens with nonhost plants (Heath, 1987)

The Plant Cuticle

Since the early 1980s, various strains of the gram-negative bacterial pathogen Pseudomonas syringae have been used as models for understanding plant-bacterial interactions. In 1991, a P. syringae pathovar tomato (Pst) strain, DC3000, was reported to infect not only its natural host tomato but also Arabidopsis in the laboratory, a finding that spurred intensive efforts in the subsequent two decades to characterize the molecular mechanisms by which this strain causes disease in plants. Genomic analysis shows that Pst DC3000 carries a large repertoire of potential virulence factors, including proteinaceous effectors that are secreted through the type III secretion system and a polyketide phytotoxin called coronatine, which structurally mimics the plant hormone jasmonate (JA). Study of Pst DC3000 pathogenesis has not only provided several conceptual advances in understanding how a bacterial pathogen employs type III effectors to suppress plant immune responses and promote disease susceptibility but has also f...

Pseudomonas syringae pv. tomato DC3000: A Model Pathogen for Probing Disease Susceptibility and Hormone Signaling in Plants

Successful infection of a host plant by a pathogen involves the movement of the pathogen toward the host, attachment of the pathogen to the plant surface, penetration of the host by the pathogen, and the proliferation of the pathogen inside the host immediately following entrance. Thus, the motility of plant­ pathogenic bacteria (63), as well as the aerotaxis and chemotaxis of bacteria toward plant exudates and leachates (10,21,65), helps bacteria land on host plants. Electric charges (45) and surface structures of the plant and bacteria (29, 42, 74) determine the recognition and adsorption between host and bacteria. Bacterial proliferation following entrance aids establishment of bacteria in infection courts. For a general account of these phases of the infection process, the reader is referred to reviews by Goodman (27, 28) and Billing (7). This review focuses primarily on spatial relationships and ul­ trastructural interactions between bacteria and host tissues at sites of infection with special emphasis on scanning electron microscopy (SEM) and transmis­ sion electron microscopy (TEM) of the movement of bacteria from the outside to the inside of a plant tissue.

Jeng-sheng Huang

Papers

Ultrastructure of bacterial penetration in plants.