BSPP Presidential Meeting 1997

Session X - Controlling the problem

Inducing host resistance to pathogens
Prof. John Mansfield
Biological Sciences Department, Wye College, University of London, Wye, Ashford, Kent, TN25 5AH

This paper will include discussion of what we now know about mechanisms of disease resistance in plants and how this knowledge has been, and might be, used to develop new control strategies. The major gaps in our understanding of plant-pathogen interactions will also be considered.

Established features of the plant's defence are antimicrobial compounds either phytoanticipins (pre-formed) or phytoalexins (induced). It is perhaps surprising that the rich variety of chemical structures found in plants has not produced a useful fungicide. Molecular genetics has confirmed the role of secondary metabolites in plant-microbe interactions and provides routes to engineer new forms of resistance. The introduction of novel phytoalexins and modification of structures to enhance antimicrobial activity are becoming more attainable as the complexity of biosynthetic pathways becomes unravelled. Activation of local accumulation of phytoalexin is, in some plants, followed by induction of systemic acquired resistance (SAR) in distant plant parts. The expression of SAR is characterized by increased speed of response in the protected tissue. Such a potentiation towards resistance has been linked to accumulation of salicylic acid. Compounds which might activate SAR or enhance natural defence responses have potential in crop protection but not all plants respond in the same way. For example dichloroisonicotinic acid, an effective inducer of SAR in tobacco and Arabidopsis, surprisingly can cause increased susceptibility to downy mildew in lettuce.

SAR and other defence responses are associated with the hypersensitive reaction (HR) at infection sites. The recognition processes leading to the HR are closely linked to gene-for-gene interactions between pathogens and their hosts. There has been remarkable progress in cloning genes for resistance to a range of pathogens including bacteria, fungi, nematodes and viruses. The emerging theme so far is that the proteins encoded by resistance genes are structurally related. Introduction of cloned genes into previously susceptible plants confers resistance. A notable success has been use of the Xa21 gene to engineer resistance to bacterial blight of rice, but how durable will the introduced resistance be to rapidly evolving pathogens? Understanding the signal transduction pathways that activate the HR requires characterization of both the resistance gene in the host and the avirulence (avr) gene in the pathogen. In fungi, such 'matching pairs' are only available for Cladosporium fulvum. Compared with the bacterial pathogens, the avr genes from fungi are poorly understood especially amongst the obligate parasites, the rusts and mildews which remain of major economic importance. Cloning genes on the basis of mapping molecular markers should allow potential avr genes to be isolated from the obligate parasites. Recent results with several bacterial genes for example avrPphB and avrPphE from Pseudomonas syringae pv. phaseolicola has demonstrated that their expression within plant cells leads to the HR i.e. the encoded Avr proteins act as the elicitors of the plant's response. Using the approach of expression in the plant may remove the stumbling block of transforming obligate fungal parasites which would have been necessary to confirm their function.

Understanding the delivery of Avr proteins from bacteria has led to the discovery of the key determinant of pathogenicity the type III secretion system. Analysis of how fungal avr genes function might lead to the discovery of similar fundamental processes in fungi which lead to the establishment of obligate parasitism. New targets for chemotherapeutic intervention should emerge from these basic studies.

In the preface to his book on Physiological Plant Pathology, R.K.S. Wood wrote in 1967,
".... most plants resist infection and colonization by most bacteria and fungi. They are naturally in the state that we still seek to reproduce by the use of fungicides that have for the most part been discovered .... by empirical methods". Although, 30 years on, this statement remains a useful focus for further studies, our increased understanding of resistance has revealed several direct routes and new avenues for the development of disease control strategies.

GMOs - a boon or a major risk?
Dr Philip J Dale
Cambridge Laboratory, John Innes Centre, Colney Lane, Norwich NR4 7UH, UK

Modern methods of genetic modification (GM) present opportunities to improve our crops, but also challenges to manage the technology carefully and responsibly. It is important to assess the potential impact of GM crops within the context of conventional plant breeding. Many of the issues raised by the widespread use of GM crops are familiar to the traditional plant breeder. However, some are different, and as geneticists, breeders, pathologists and agriculturalists, we need to take account of this. Because we can introduce genes into our crops from viruses, bacteria, plants, animals, humans, and even make genes synthetically in the laboratory, there is international agreement that a risk assessment should be carried out to determine their possible impact on human health, the environment and on food. This involves asking a series of questions about the modified crop plant and in generating new data, where necessary.

GM methods provide us with very important opportunities both to understand the ways in which plants defend themselves against diseases and in the design of new kinds of resistance mechanisms, some of which are likely to be more robust than those available through traditional breeding. There are also a number of challenges that genetic modification presents, including developing new agricultural strategies for their management; the extent to which their use should be governed by regulation, market forces and codes of practice; and how they can benefit developing countries.

The role of sanitation in suppressing inoculum
Mr David J. Yarham
Croxton Cereal pathology, Fulmodeston, Fakenham, Norfolk NR21 0NP

The aim of any disease control strategy is to delay for as long as possible the epidemic development of the pathogen. This can be achieved either by slowing the rate of increase of the pathogen on the host or by reducing the initial level of inoculum available for infection. The interaction of these two approaches has been expressed mathematically be Van der Plank in the equation:

230 I_a
t = . log
r I_b

where "t" is the delay in the development of an epidemic achieved by reducing the initial level of inoculum ("I_a") to a lower level ("I_b") when the rate of increase of the pathogen during the delay period is "r".

Any strategy aimed at reducing "r" will be assisted by a reduction in the value of "I_b". Conversely, the benefit derived from reducing "I_b" will be lessened if the value of "r" is high. Obviously, if either "I_b" or "r" can be reduced to zero the value of "t" will be increased to infinity and complete control of the pathogen will have been achieved.

In protected horticulture strict hygiene can so completely eliminate inoculum of some obligate parasites that there is no need for the use of chemicals to control them. In agricultural practice elimination of indigenous pathogens is seldom a feasible option, and for some the rate of increase is so rapid as almost to obviate the benefits of inoculum reduction. In many situations, however, a reduction in the initial level of inoculum can greatly augment the use of other methods of delaying epidemic development and can thus form a vital component of a disease control strategy.

Are fungicides the ultimate answer to disease control?
Mr Andy Leadbeater
Novartis Crop Protection, CH4002 Basle, Switzerland

The use of chemical fungicides is routine practice in agriculture and horticulture throughout the world as a measure to provide protection against yield and quality reducing plant diseases. The need for fungicides should however always be questioned, as they are only a part of the integrated management of crops and are almost the final step after consideration of agronomic good practice to reduce the occurrence and effects of pathogens.

New technologies bring new opportunities for disease control. These technologies include new classes of conventional fungicides such as anilinopyrimidines or strobilurins, transgenic crops offering disease resistance, and utilisation of plant natural defence mechanisms through Systemic Acquired Resistance (SAR). These have now reached the stage where practical products are available, for example in SAR, the plant activator acibenzolar-s-methyl ("Bion"). We therefore have several new possible approaches to plant disease control to really provide alternatives to the conventional fungicide one. Chemical fungicides are increasingly selected for yield and quality effects rather than purely disease control.

The current status of these new technologies can be reviewed with their possible impact on future management strategies. A major issue with new technology such as transgenic crops is acceptance by the public and in the market place, this could be a real barrier to advances being made.

Whilst fungicides are certainly not the ultimate answer to disease control, they equally certainly have their place and will continue to do so, providing effective and flexible disease control and yield and quality improvements, whilst also in themselves protecting the new technology (and being protected) in terms of durability of control and resistance management.