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Advanced Technology for Producing Healthy Seeds or Vegetative Materials
C.A. Chang
Dept. of Plant Pathology
Taiwan Agricultural Research Institute
Wu-feng, Taichung 413
Taiwan ROC, 2003-10-01

Abstract

This Bulletins discusses programs for the propagation of disease-free seedlings and other planting materials. The different types of transmissible plant pathogens are discussed: viruses, viroids, phytoplasmas and spiroplasmas, and bacteria. The different techniques of detecting them are discussed and evaluated, in terms of reliability, replicability and cost effectiveness. This is followed by an outline of how to develop a program for pathogen-free planting materials, including understanding the key pathogens and which techniques to adopt for disease diagnosis and pathogen detection. The propagation of pathogen-free mother plants is then discussed, and how to integrate a certification system with the propagation program.

Introduction

Pathogens capable of causing systemic infections on their host plants can usually also be transmitted through vegetative propagation from infected mother plants to the young plants. If all the seedlings or planting materials are infected by this type of pathogen, there may be severe losses of the crop after planting. Infected seedlings or planting materials can easily be transported to new areas, resulting in the invasion and establishment of new diseases. In areas where the pathogens already exist, infected seedlings or planting materials serve as inoculum sources of pathogens that can be transmitted by insects and other vectors, causing outbreaks of disease.

Because these pathogens are always located inside the cells and systemically spread throughout the plant, the diseases they cause are often very difficult to control by conventional measures such as pesticide sprays. Experience over several decades tells us that the most effective way of dealing with this problem is to produce pathogen-free seedlings or planting materials.

In Taiwan, the first propagation system for virus-free planting materials was established in the early 1970s, for the mass production of virus-free seed potatoes. It has been in operation for more than 26 years, and has proved to be effective in controlling potato virus diseases. Since then, more than nine more crops have been included in pathogen-free propagation programs.

According to our experience, the success of a pathogen-free planting material propagation system relies entirely on whether pathogen detection techniques are available or not. Before the 1980s, the most widely used pathogen detection techniques were antibody-based, such as enzyme-linked immuno-sorbent assay (ELISA). These techniques are still widely used in laboratories, and have also been adapted to be used on an industrial scale. The reason for their wide acceptance by industry lies in the fact that their sensitivity and specificity are at an acceptable and reasonable level. Furthermore, the results can be reproduced, while there is a minimum of false positive and negative results. In all, ELISA testing is efficient and cost effective.

However, the invention of DNA-based technologies in the 1980s has driven most research on pathogen detection to take a new direction. Numerous reports have shown that there are many more DNA-based pathogen detection techniques (several thousand times more!) than serological tests such as ELISA. Nevertheless, most of the DNA techniques are so far still confined to laboratory use. More research is needed for them to become usable on a routine, daily basis, for large-scale applications. This paper discusses some widely used serological and DNA-based techniques and protocols.

Different Types of Transmissible Pathogens

Pathogens which can induce systemic infection in their host plants include viruses, viroids, phytoplasmas and some bacteria. As mentioned above, these pathogens can be transmitted from infected mother plants to seedlings, or to planting materials such as scions, cuttings, bulbs, and tubers through vegetative propagation. In fact, vegetative propagation by tissue culture, which has become the dominant method of propagating numerous plant species, is considered to be the most efficient way of multiplying and disseminating planting materials infected with systemic pathogens. There follows a brief introduction to the properties of pathogens that are frequently transmitted in planting materials during vegetative propagation.

Viruses

Viruses are submicroscopic pathogens that cause disease in all known organisms, from bacteria to human beings. They are obligate parasites, requiring the presence of living cells for their multiplication. Recently, viral diseases such as influenza, polio, and AIDS in human beings, and foot-and-mouth disease in cattle and other ruminants, has caused huge economic losses and had a major social impact. There have also been numerous records of plant virus diseases which have caused major economic losses. Plant viruses differ from other plant pathogens, not only in their size and morphology but also in their mode of infection and the way they multiply in host cells and are transmitted to other host plants.

Most plant viruses are very simple, possessing either RNA or DNA as the genome. This is encapsidated in a protein coat. The coat protein and genetic nucleic acid of a virus are synthesized separately in different sites in susceptible cells. At an appropriate time, they are assembled together to form progeny virus particles.

Plant viruses differ from other plant pathogens in being unable to liberate themselves from the infected host plant into the environment. Without the aid of vectors such as insects, mites, nematodes or fungi, they are also incapable of introducing themselves into their host plants through the epidermis or natural openings. However, once viruses have successfully infected the host plant, they can easily be transmitted in vegetatively propagated materials. Some plant viruses may also be transmitted from plant to plant by mechanical contact, while others may spread in infected seeds or pollen. Another characteristic of plant virus infection is that virus particles can spread through the plant, and can be detected in tissues or organs that are some distance away from the infection site. This ability to spread throughout the organism is known as systemic infection.

Although some viruses may induce distinct symptoms, most viruses show visible symptoms which are very similar to those caused by other viruses. Therefore, we cannot depend on visual inspection to identify the causal virus. Since plant viruses do not produce any structure outside the infected tissue, and are not liberated from the infected cell, different methods have been developed to detect their presence in infected cells.

Studies of the molecular biological characteristics of viruses have helped researchers to understand the function of genes in the viral genomes, and the strategies adopted by viruses for gene replication and expression of their gene products. Over the past two decades, a tremendous amount of information has been gathered on the sequence and function of viral genes. This information is now widely used for the quick identification of viruses by DNA-based technology.

Viroids

Viroids are the simplest and the most primitive of all the plant pathogens. Numerous plant diseases are induced by viroids, many of them common, such as potato spindle tuber disease and citrus exocortis. Viroids differ from viruses in being composed of only a naked single strand of ribonucleic acid (ssRNA), without the protection of a protein coat. The RNA of viroids is made up of small, molecules either rod like or a closed circle, which are highly base-paired.

Potato spindle tuber viroids have been studied in detail. They possess a serial arrangement of 26 double-stranded segments, interrupted by bulging loops of varying sizes. This arrangement of the RNA molecule gives them a characteristic stability and flexibility. In general, viroids are highly stable against thermal denaturation treatment. They are readily transmissible by mechanical contact or injury, but not by any vectors.

Like viruses, viroids can induce systemic infection in their host plants. Symptoms induced by different viroids can usually be distinguishable, and can thus be useful for diagnosis. However, it is better to use modern techniques for detection and identification, instead of depending on visual inspection alone. Since viroids contain only RNA without a coat protein, serological methods cannot be used to detect or identify them. However, they can be detected by DNA-based technology such as electrophoresis, DNA hybridization or polymerase chain reaction (PCR).

Phytoplasmas and Spiroplasmas

Phytoplasmas used to be called mycoplasma-like organisms (MLOs). They are a special group of plant pathogens, similar to the mycoplasmas known to cause human and animal diseases. Mycoplasmas differ from bacteria in that they lack a cell wall or penicillin-binding sites. They are, therefore, resistant to penicillin, to which bacteria are sensitive. However, mycoplasmas are sensitive to tetracycline antibiotics.

Mycoplasmas have a triple-layered plasma membrane. Because they lack a cell wall, mycoplasma cells have a characteristic lack of rigidity, and appear as pleomophic bodies.

Phytoplasmas, or MLOs, differ from animal mycoplasmas in that they cannot be isolated as pure cultures and grown in an artificial medium. However, because they are similar in morphology, and also in their sensitivity to whip-like tetracycline, these plant-infecting mycoplasmas were called MLOs until the 1990s, when `phytoplasma' was accepted as the official name of this group of pathogens.

There is also a similar, related group of pathogens, the spiroplasmas. These also used to be considered MLOs until about 1970, when it was found that they could be cultured. They were also found to have a spiral movement in artificial medium. Like phytoplasmas and mycoplasmas, spiroplasmas are pleomorphic, lack a cell wall, and are sensitive to tetracycline.

Plant-infecting phytoplasmas and spiroplasmas are both found in the phloem tissue. Although they are known to induce systemic symptoms, they are usually present in a low concentration and are not distributed evenly throughout the plant. Syndromes induced by phytoplasmas and spiroplasmas include yellowing, stunting, phyllody (development of leaves in places where ovules should develop), changes in color to reddish or purple and a bunching of shoots known as witch's broom. Although some viruses may also induce symptoms of yellowing and stunting, the latter three syndromes are generally typical of infection by phytoplasmas.

Neither phytoplasmas not spidoplasmas are transmissible by mechanical contact. They can only be transmitted by vectors, particularly leafhoppers. A few can be transmitted by psyllids.

In the past, the identification of phytoplasma and spiroplasma diseases relied on microscopic observation, and biological techniques such as grafting or use of insects to inoculate susceptible hosts. In the 1980s and 1990s, serological techniques, especially monoclonal antibody technology, were applied successfully. More recently, molecular techniques, including DNA probe hybridization and PCR, have been able to provide a quicker diagnosis.

Bacteria

Bacteria are prokaryotic microorganisms (i.e. they are cells that lack a membrane). Most of them are strictly saprophytic (i.e. they feed on dead organisms). They often help to decompose organic wastes in the environment. Around 1600 bacterial species are known to be pathogenic to plants. Apart from two species of Streptomyces, all plant-pathogenic bacteria are single rod-shaped cells. Most of the plant- pathogenic bacteria are motile (can move by themselves). They use flagellae to propel their bodies. These flagellae may be present singly, in groups, or distributed evenly over the cell surface. With the help of water, bacteria can move into host tissue through sites of injuries and through natural openings such as stomata. Most plant pathogenic bacteria induce local rather than systemic infection.

However, there are some specially evolved bacteria that can infect xylem or phloem tissue and become distributed throughout the plant, in a rather similar way to the systemic infection caused by viruses. In the early days, it was very difficult to culture plant pathogenic bacteria in artificial medium. For this reason, they were wrongly identified as mycoplasmas or MLO diseases for quite a long time. In the 1990s, it was found that most of them could be cultured in specially designed medium. They are now officially known as fastidious xylem- or phloem-limited bacteria, because they are found only in the xylem and phloem.

Bacteria belonging to these two groups give rise to diverse but usually species-specific symptoms in their host plants. Symptom inspection in the field is widely used for diagnosis. However, because of the low concentration of these bacteria in host tissue and the difficulty of culturing them, it is better to use serological and DNA-based techniques for quicker detection and identification.

Developing a Program for Pathogen-Free Planting Materials

The most effective way of controlling diseases that can be transmitted from mother plants to vegetatively propagated seedlings or planting materials is to develop a propagation system which ensures that all, or nearly all, of the plants produced are free of pathogens.

Understanding the Key Pathogens

The first step in developing a propagation system for disease-free materials is to get a complete picture of the pathogens that may attack the crop. To accomplish this needs several years of disease surveys, or consultation with experts. The key pathogens i.e. those which are most widely distributed or those which are responsible for major reductions in yield and/or quality, should be identified and targeted. Their biological properties, disease ecology, modes of transmission, methods of field diagnosis and keys for identification should be fully understood in as much detail as possible. This step is crucial for developing a sound structure for a propagation program that is effective in producing plants free from infection by key pathogens.

Some crops may be infected by numerous pathogens. In such cases, it may be very difficult to cover all of these, because of the cost. In real life situations, choices should be make to select key pathogens as the target to be eliminated from the propagation system.

Techniques for Disease Diagnosis and Pathogen Detection

The second step in developing a propagation program for healthy planting materials is to develop techniques for detecting the key pathogens. These techniques should be accurate, sensitive, and efficient enough to allow detection as early as possible, so the pathogens can be eliminated from the propagation system. Numerous techniques or diagnostic kits are described in the literature for the detection of plant pathogens and some are even available or the commercial market.

There are four main types of detection techniques.

  • Biological inoculation;
  • Microscopic observation;
  • Serological detection; and
  • DNA-based technology.

Of these four, biological inoculation is the most traditional, but is still a very accurate way of detecting pathogens. In this method, a susceptible plant host is deliberately infected with the pathogen. Biological inoculation requires a good system of different hosts which can be inoculated and observed for specific symptom expression. However, this technique requires space in a greenhouse and skilled labor to maintain the different hosts. It may take several days, or even several weeks, for symptom expression, which is usually too slow for commercial producers.

Nevertheless, biological inoculation reveals the live virus residing in the examined tissue. In contrast, the other three techniques may detect inactive viruses. This may give misleading results for certification. Therefore, biological inoculation is still used, in combination with the other techniques, in the certification of nuclear or mother stock plants. In this situation, there are usually relatively few plants to examine.

Like biological inoculation, microscopic observation requires experienced, well-trained staff and sophisticated facilities for processing. Again, it is best adopted to uses where the sample size is small rather than to large-scaled and routine inspections. Microscopic observation is therefore used mainly government institutes or specific diagnostic companies, rather than seedling producers.

It is the serological and DNA-based techniques, especially the former, which are currently widely accepted by the industry as the most efficient, dependable, reproducible and cost effective method of pathogen detection.

There have been many improvements over the past two decades in serological techniques. For example, enzyme-linked immuno-sorbent assay (ELISA) has been modified into a semi-automated system that can process thousands of samples in one day.

There have also been tremendous improvements in DNA-based techniques in recent years. However, these are still used mainly by specialized laboratories with highly trained experts. Although DNA-based techniques are generally accepted as being more sensitive and specific for pathogen detection than the serological ones, they have some defects. These include the higher cost, the difficulty of reproducing results, and the possibility of false positives and cross-contamination between samples. These problems need to be solved before DNA-based techniques can be used as the routine basis of diagnosis.

Selection and Maintenance of Pathogen-Free Nuclear Stock

The third step in the development of healthy plant propagation program is to select plants which are pathogen-free, and true to type with regard to the horticultural characteristics of the species or cultivar. Once selected, these plants should be protected from reinfection by pathogens. This is the time to decide which detection technique to use as the preliminary test to screen individual plants to make sure that they are free from infection by key pathogens. Because the number of plants for testing at this stage is usually large, techniques suitable for large-scale testing with acceptable sensitivity and cost effectiveness, such as ELISA, are desirable.

Once of this qualifying screening is completed, those selected individuals free from key pathogens should be collected and maintained in a vector-proof house for true to type selection and advanced pathogen testing. At this stage, since the number of plants for inspection should be reasonably small, more sensitive or accurate pathogen detection methods, even if they are more expensive in time and labor, can be used. To be certain that the selected individual plants are free of all suspected pathogens, a combination of several techniques, including biological inoculation, microscopic observation, and serological and DNA-based methods are usually applied, either at the same time or in sequence. The cost, including the labor cost, is not usually the key concern at this stage of selection. It is accuracy which is the crucial point to be considered.

Furthermore, the selected mother stock plants should be carefully protected in order to prevent pathogens from entering their environment. Pathogen detection procedures should be conducted periodically to ensure that they remain free of pathogens. Every stock plant should be individually labeled, and kept well spaced from other plants to prevent mechanical contact. If there are insect vectors involved in the dissemination of pathogens, periodic pesticide applications should be made part of the management system. If re-infection is ever suspected, the suspect plant should be removed immediately from the area.

If no pathogen-free plants are available after extensive screening, this would indicate that the incidence of the pathogens concerned is already very high in the field. In this case, technology to eliminate pathogens from the mother stock plants should be used. Meristem tissue culture, thermo-inactivation treatment and many other treatments can be used for this purpose.

Propagation of Pathogen-Free Mother Plants

After you have established a dependable system for the maintenance of pathogen-free nuclear stock or mother plants, the next step is to consider how to mass propagate these stock plants into seedlings or planting materials for growers. One crucial point to be considered is the amplification factor. The reproduction rate of planting materials must be greater than the infection rate with the disease in the field. If the reproduction rate of pathogen-free planting materials is lower than the spread of disease in the field, the control effect of this strategy will not be acceptable to growers. Even more important, the reproduction method should be efficient, cost effective and acceptable to the industry. It cannot be considered only from the scientific point of view.

In the past, there were frequent cases of propagation methods being developed by researchers which were not accepted by commercial people, because of poor cost effectiveness and profitability. Tissue culture, for example, is a highly efficient method of reproducing planting materials. However, the cost, the high mutation rate and the possibility of re-contamination with pathogens may also be high. Where this is the case, a tissue culture system is unlikely to be acceptable to the industry or to growers.

Integration of Certification Program with the Propagation System

The experience of the Netherlands over several decades has shown that success in the use of healthy planting materials to control virus diseases relies at least partly on a successful certification system. This will help to give growers and importers confidence in the quality and pathogen-free status of the planting materials, so that they are willing to grow them.

The fact is, the more pathogen-free planting materials are grown, the fewer the diseased plants which will exist in the fields, as they will be replaced by healthy ones. Consequently, outbreaks of disease will be suppressed.

If the government or any university could be responsible for inspecting the planting material propagation process, to guarantee the health of the plants and issue certificates, growers will probably be more than happy to grow these certified products, and abandon those may be infected.

Most of those who use certified planting materials should be rewarded with better crops and less damage from disease. In fact, this system benefits not only growers but also propagators, who are more competitive if they produce certified planting materials.

However, most developed countries find that it is not easy to operate a certification system, not to mention developing ones. It takes good financial resources, trained manpower, good academic research and political support to carry out a certification system properly. Nevertheless, the experience of the Netherlands and of many other countries indicates that integrating the certification process with the propagation of pathogen-free planting materials will be the best guarantee of a successful disease control strategy.

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