BSPP Presidential Meeting 1999

Biotic Interactions in Plant-pathogen Associations

Session III - Interactions with prokaryotes

Plant interactions with endophytic bacteria
Johannes Hallmann.
Institut fr Pflanzenkrankheiten, Bonn University, Nussallee 9, D-53115 Bonn, Germany. 

The past years were associated with an increasing interest in endophytic bacteria especially those providing economical interesting features such as plant growth promotion and stimulation of plant defence mechanisms. But what do we really know about plant/endophyte interactions, especially with regard to plant pathology? Endophytic bacteria are defined as bacteria that reside within living plant tissues without doing substantative harm to the plant. The plant/endophyte association expresses a very close interaction were the plant provides nutrients and residency for the bacteria which in exchange can improve plant growth and health. From the evolutionary point of view it can only be speculated if endophytic bacteria originally derived from plant pathogens which lost virulence in favor of an extended growth period within the plant or if endophytic bacteria are potential pathogens just unable to express disease-specific genes. Well known are those plant/endophyte associations based on nitrogen fixation as in the case of the Rhizobia/legume complex or some free-living bacteria of various grass species. However, the plant pathologist probably favours a third group of endophytic bacteria, those which improve plant health. Major traits of how endophytic bacteria can affect plant health include: 1) direct antagonism or niche exclusion of the pathogen, 2) induction of systemic resistance, and 3) increasing plant tolerance towards biotic stresses. Besides these obvious effects, plants are often colonized by endophytic bacteria not showing any effect at all. The bacteria form either a neutral association with the plant or they reside latent until becoming active in later stages of plant development. Over the past years we gained a fairly good understanding of culturable endophytic bacteria but our knowledge regarding non-culturable bacteria is still very limited. What makes a bacterium an endophyte and why are plant defense mechanisms not activated? Why does the plant tolerates these intruders? These are all questions not yet been answered sufficiently and in the future will hopefully help to understand plant/endophyte interactions. With respect to the natures complexity this paper can only focus on some of the plant/endophyte interactions of which the following will be discussed in detail: recognition, specificity, localization, biocontrol potential and mechanisms. Besides own work, this paper will also review the recent literature to provide a more general view of plant/endophyte interactions. In addition, provocative as well as speculative questions will be raised, especially in those areas not yet covered by the literature. The papers main emphasis is to stimulate discussion as well as research interest in this exciting and still unexplored research area. Maybe not to far from now endophytic bacteria will be applied to improve plant health in a way N-fixing bacteria are currently used to enhance plant growth.

Molecular mechanisms underlying GroEL-mediated retention of plant viruses in their insect vectors.
J.F.J.M. van den Heuvel*, S.A. Hogenhout, V. Ziegler-Graff, K. Richards, S. Morin, H. Czosnek, and F. van der Wilk
*Plant Research International, P.O. Box 9060, 6700 GW Wageningen, The Netherlands.
: ; in collaboration with IBMP-CNRS, Strasbourg, France and Hebrew University, Jerusalem, Israel

Affinity for GroEL homologues is a common characteristic of viruses from the family Luteoviridae (genera Luteovirus, Polerovirus and Enamovirus) and has recently also been demonstrated for Tomato yellow leaf curl virus (genus Begomovirus; Geminiviridae). The GroEL homologues are produced by endosymbiotic bacteria of their vector aphids or whiteflies (Bemisia tabaci), and are released into the insect's hemolymph. In vivo interference in the virus-GroEL interaction coincides with reduced capsid integrity and loss of infectivity. Using a full-length luteovirus cDNA clone it has been demonstrated that the readthrough domain (RTD) of the minor luteovirus capsid protein, which is present on the surface of a virus particle, contains the determinants for GroEL-binding. The virus attachment site of GroEL is located in the equatorial domain and not in the apical domain which is generally involved in polypeptide binding and folding. Here we report on the key amino acids in the RTD and in Buchnera GroEL implicated in recognition and attachment.

Are chitinolytic rhizosphere bacteria really beneficial to plants?
Wietse de Boer & J.A. van Veen
Netherlands Institute of Ecology, Centre for Terrestrial Ecology, Department of Plant-Micro-organism Interactions, P.O. Box 40, 6666 ZG Heteren, The Netherlands.

The observations of mycolysis by chitinolytic bacteria have stimulated research on possible application of these bacteria for biocontrol purposes. The focus has been on rhizosphere bacteria as they should be adapted to the environment where plant-pathogenic fungi infect roots. Chitinolytic rhizosphere isolates, showing in vitro antifungal effects have therefore been tested for their ability to protect plants against infection by plant pathogens. In several cases these strains reduced disease symptoms significantly under controlled greenhouse conditions. However, application of such strains under field conditions has been far less successful, and it is not at all clear that mycolytic activities should be expected under field conditions. Before this issue can be resolved information is needed about the ecological function of bacterial chitinases including environmental conditions that might promote chitinase production and mycolytic activity.

In this paper it will be argued that chitinases of rhizosphere bacteria are most likely involved in mycoparasitism and defence against lysis by fungi. Obviously, mycoparasitic growth of chitinolytic rhizosphere bacteria could be an important mechanism to control plant-pathogenic fungi. However, chitinase production in soil bacteria, including potential biocontrol strains, is repressed by small organic substrates like sugars and amino acids. This indicates that mycolytic activity does only occur when no other growth substrates are available i.e. under starvation conditions. Hence, the release of organic compounds by the root is expected to repress mycolytic activities of most chitinolytic rhizosphere bacteria. This does, however, not suggest that the search for biocontrol strains is futile, but indicates that knowledge of chitinase expression and repression must be taken into account during this process.

A potential negative effect of chitinolytic bacteria may be exterted on mycorrhizal development. In fact, the starvation conditions in the bulk soil, into which mycorrhizal hyphae are extending from the root, should promote mycolytic activities. Therefore, study of interactions between chitinolytic strains and mycorrhizal fungi should be part of biocontrol studies.

Garrett Memorial Lecture

Aphid transmission of potyviruses: the complexities of a seemingly simple process.
Thomas P. Pirone

Department of Plant Pathology, University of Kentucky, Lexington KY 40502, US. 

Potyviruses are transmitted by aphid vectors in the "non-persistent" manner. These viruses can be acquired in brief (less than 1 minute) probes into epidermal cells and can be inoculated just as rapidly; the ability to be transmitted is lost in a matter of minutes to hours, hence the term non-persistent. The rapidity of the transmission process and the fact that these viruses can also be transmitted "mechanically", by manual inoculation, initially led to the concept that transmission occurred as the result of contamination and decontamination of the stylets, although this did not account for several other features associated with this type of transmission.

The availability of mutant potyviruses that have lost the property of aphid transmissibility but retain mechanical transmissibility, combined with the tools of molecular biology, has allowed the dissection of the viral components of the transmission process and the analysis of their functions. In order to be aphid-transmissible, virions must have a transmission-competent coat protein and the virus must produce a functional non-structural protein "helper component". Current evidence indicates that the helper component acts as a "bridge", binding to the virion and to components of the stylet food canal, allowing virions to be retained at a site(s) from which they can subsequently be inoculated. At the molecular level, a three amino acid sequence "DAG" or an equivalent motif near the N-terminus of the coat protein is required for virion binding to the helper component. In the helper component, a "PTK" motif is involved in binding to the virion coat protein and a K or R in the first position of a highly conserved "KITC" motif is required for interaction with the stylets.

Electron microscopy of immunogold-labeled sections of stylets, and autoradiography of radioactively labelled virions within the stylets show that for transmission to occur the helper component-virion complex must be retained in the stylet food canal, most probably near the tip of the stylets. The processes involved in inoculation are not known with certainty but inoculation could occur as the result of egestion of food canal contents or by salivation, if virions are retained in the joint salivary/food canal near the tip of the stylets.

The relative ability of specific helper components to interact with the stylets of particular aphid species can account for some examples of differences in vector specificity/efficiency.

Aphid species that are unable to transmit certain potyviruses are able to do so if they acquire the helper component of a potyvirus that they are able to transmit. Hypotheses to explain this phenomenon include differential ability of specific helper components to interact with stylets and differential effects of salivas on helper component retention or activity.