Interactions between pseudomonads and cereal leaves

Abstract

ABSTRACT

Interactions between leaves of oats, wheat and barley and phytopathogenic and saprophytic pseudomonads were investigated.

The mechanisms by which successful pathogenesis or host resistance were achieved were examined. Records of symptom development of oat halo blight incited by P. coronafaciens in a susceptible oat cultivar enabled construction of a symptom key which was subsequently used to clarify the reactions of the cereals to artificial inoculation with pseudomonads. Four types of resistance were elucidated on the basis of symptom appearance. These were: (a) delayed development of symptoms as noted for barley and a reportedly resistant oat line Cc4146 in response to P. coronafaciens; (b) a hypersensitive response (HR) induced by plant pathogens isolated from the Gramineae or with cereal pathotypes; (c) a resistant symptom in response to other plant pathogens and in wheat in response to P. coronafaciens and (d) symptomless resistance following inoculation with the saprophyte P. fluorescens. In contrast to other plants (notably tobacco) the HR was not the general response to non- pathogenic bacteria. P. coronafaciens multiplied rapidly in susceptible oat leaves, watersoaking and normosensitive collapse of the infiltrated area occurred when a critical level of 6 2 c. UxlO bacteria/cm leaf had been reached. The HR was marked by a sharp decline in bacterial numbers but in leaves undergoing the resistant symptom and symptomless resistance bacterial numbers remained constant. Live bacteria were required during an induction period preceding a latent phase and subsequent hypersensitive or normosensitive (susceptible) collapse. Induction times were generally longer in cereals than in broad bean, French bean or tobacco plants and varied between bacteria when the range of plants were compared. Tissue collapse during hypersensitive or susceptible interactions was prevented by prior infiltration with heat-killed cells of the HR eliciting P. coronafaciens var. atropurpúrea and both live and heat-killed cells of P. tabaci (resistant reaction) and P. fluorescens (symptomless). P. coronafaciens (live or heat-killed) was not effective in preventing tissue collapse. Prior infiltration of oat leaves with heat-killed cells of P. coronafaciens var. atropurpúrea prevented or delayed electrolyte leakage from tissues undergoing normosensitive or hypersensitive collapse respectively. The protective effect of heat-killed P. coronafaciens var. atropurpúrea was neither light dependent nor systemic. Ultrastructural studies on the interactions revealed that extensive membrane damage and cell collapse coincided with electrolyte leakage during hypersensitive and normosensitive collapse. Little cell collapse was observed during the resistant S*j*fh*a« of P. tabaci or following inoculation with P. fluorescens. In vivo the pathogen P, coronafaciens was associated with an extracellular matrix, polysaccharide in nature, which may play a role in pathogenesis. By contrast, non-pathogenic bacteria were not associated with extracellular polysaccharide but were often attached to mesophyll cell walls by stalks or enveloping fibrils apparently emanating from host cell walls. The degree of attachment did not increase significantly beyond 12h except for the saprophyte P. fluorescens. Attachment rarely exceeded 30% except in the case of P. fluorescens (51.5%) at 24h after inoculation. The numbers of bacteria attached to host walls were not in amounts likely to contribute to resistance per se. Entrapment in polysaccharide released from host walls during injection of sterile distilled water suspensions (physical entrapment) of pathogenic and non-pathogenic bacteria within intercellular spaces was also recorded. Extracts from oat leaves previously inoculated with P. coronafaciens, P. coronafaciens var. atropurpúrea, P. tabaci or sterile distilled water alone were examined for antimicrobial activity. Antifungal and antibacterial activity was confined largely to ethyl acetate phases of extracts when tested by a variety of bioassay techniques. Antimicrobial activity was found in all extracts and no consistent pattern emerged. The results are discussed on the basis of the presence of both induced and constitutive' inhibition in infected leaves.