The ins and outs of leaf scald of sugarcane

The seminar disclaimer applies to this post.

The topic of seminar for this week was the bacterial pathogen Xanthomonas albilineans, the causal agent of leaf scald of sugarcane. The speaker was Dr. Philippe Rott, from CIRAD UMR BGPI, Montpellier, France.

Sugarcane is the main source of sugar in the world. Sugarcane is propagated vegetatively from stalk cuttings. The cuttings germinate, tiller (produce new stalks), grow, and mature (final accumulation of sugar). The crop cycle is 3-10 years long. Sugarcane is grown in many countries, but Brazil, India and China are the main producers, the US is #10 on the list.

Leaf scald disease is caused by a bacterium, Xanthomonas albilineans, and the most typical symptom of leaf scald is a white pencil-line streak running parallel along the veins (Comstock and Lentini, 1991, 2005). Over time the leaf becomes necrotic and dies.
Picture credit.

Leaf scald of sugarcane was first described in 1911 in Australia, and is currently controlled to some extent by using resistant sugarcane crosses Saccharum officinarum x S. spontaneum. The genus Xanthomonas contains several major plant pathogens, including X. campestris pv. campestris (black rot of several plants), X. axonopodis pv. citri (citrus canker), X. axonopodis pv. vesicatoria (bacterial spot), and X. oryzae pv. oryzae (bacterial leaf blight of rice).

Some known pathogenicity factors (factors that the pathogen needs to cause disease) in these pathogens are the xanthan gum genes, the LPS (lipopolysaccharides that the bacteria secrete, rpf (regulation of pathogenicity factors) genes, and a type III secretion system (also called a hrp system).

X. albilineans invades the sugarcane xylem. It is an unusual xanthomonad in that is phylogenetically distinct from other plant-pathogenic xanthomonads, based on the sequences of the ITS region and gyrB. Based on PCR and Southern blot experiments, there is no evidence that X. albilineans has a hrp system like other phytopathogenic xanthomonads. The pathogen does produce a toxin, albicidin.

Albicidin is a specific molecule essential for the disease symptoms, and toxin(-) mutants show no symptoms on sugarcane. Albicidin inhibits DNA replication, chloroplasts, prokaryotes (which means it's an antibiotic), and bacteriophages.

In 1993, Dr. Rott joined the lab of Dr. Dean Gabriel in the department of Plant Pathology at the University of Florida as a visiting professor to further study X. albilineans. His goal was to characterize the biosynthetic pathway of albicidin, identify the genes involved, and study the relationship between biosynthesis variability and variability in disease symptoms.

Using Tn5 mutagenesis, he isolated 7100 mutants, 50 of which did not produce albicidin (assayed by the lack of an inhibition ring around the bacterial colonies). He identified 3 genomic regions that were involved in albicidin production. XALB1, a 55.8 kb region, a cluster of 20 predicted open reading frames, XALB2, a 2.9 kb region, predicted to encode a single protein, phosphopantheteinyl transferase (PPT), and XALB3, a 2.6 kb region, predicted to encode a single protein, a heat shock factor, HtpG (Vivien et al., 2007).

The XALB1 region contains genes that are similar to genes that encode non-ribosomal peptide synthases (NRPSs) and polyketide synthases (PKSs) in addition to predicted regulatory, modifying, and resistance genes. Mutations made in some of these genes and subsequent complementation confirmed they were necessary for albicidin synthesis (Royer et al., 2004).

NRPs and PKSs are large, multi-functional enzymes that are responsible for one specific elongation step of adding a peptide or ketide to a growing polypeptide/ketide chain. Certain domains within these proteins determine their substrate specificity (meaning they determine which particular peptide or ketide is added to the chain). The albicidin backbone is made by NRPSs and PKSs, although the complete structure has not been elucidated yet, because albicidin contains some unusual peptide residues.

Dr. Rott then set out to determine whether the diversity of the genes involved in the biosynthesis of albicidin are also involved in the pathogenicity variation observed for X. albilineans associated with different pathotypes, and disease outbreaks. Research found no structural differences in albicidin. There were differences found in the albicidin production levels, but there was no correlation with pathogenicity (Champoiseau et al., 2006). This implies that there is some other factor involved, which is responsible for the observed variation in pathogenicity. This hypothesis is supported by the fact that toxin(-) mutants are still able to colonize the sugarcane stalk, even though they do not produce symptoms.

Dr. Rott's team then set out to sequence the entire genome of X. albilineans. Because this research has not been published yet, he did not give many specifics about the sequence data. He did reveal that X. albilineans turned out even more strange than anticipated, with an unusually high number of NRPS genes, only 3 of which are required for albicidin synthesis.

Dr. Rott concluded by summarizing that X. albilineans
1) is an unusual xanthomonad that does not have high similarity in genome sequence to other published xanthomonad genomes;
2) produces no xanthan gum;
3) does not have a hrp system;
4) has a fairly small genome compared to other xanthomonads;
5) is phylogenetically distinct from other xanthomonads;
6) appears to specialize in the production of secondary metabolites, which can enter the plant cell, using NRPSs and PKSs.

References:

Champoiseau, P., Daugrois, J.-H., Girard, J.-C., Royer, M. and Rott, P.C. (2006) Variation in albicidin biosynthesis genes and in pathogenicity of Xanthomonas albilineans, the sugarcane leaf scald pathogen. Phytopathology 96(1):33-45.

Comstock, J.C. and Lentini, R.S. (1991, 2005) Sugarcane Leaf Scald Disease. Document SS-AGR-201, Agronomy Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. First printed March 1991. Revised March 2005. Online at: http://edis.ifas.ufl.edu/SC001. Last accessed: March 26, 2008.

Royer, M., Costet, L., Vivien, E., Bes, M., Cousin, A., Damais, A., Pieretti, I., Savin, A., Megessier, S., Viard, M., Frutos, R., Gabriel, D.W., and Rott, P.C. (2004) Albicidin pathotoxin produced by Xanthomonas albilineans is encoded by three large PKS and NRPS genes present in a gene cluster also containing several putative modifying, regulatory, and resistance genes. Mol. Plant-Microbe Interact. 17(4)414-427.

Vivien, E., Pitorre, D., Cociancich, S., Pieretti, I, Gabriel, D.W., Rott, P.C., and Royer, M. (2007) Heterologous production of albicidin: a promising approach to overproducing and characterizing this potent inhibitor of DNA gyrase. Antimicrob Agents Chemother. 51(4): 1549–1552.

Late-season vine decline of melons

The seminar disclaimer applies to this post.

This week our departmental seminar was given by none other than the current President of the profession organization for plant pathologists, the American Phytopathological Society (APS), Dr. Raymond D. Martyn. Dr. Martyn is a homegrown graduate, who did his PhD research on biological control of waterhyacinths. He then moved to Texas A&M, where he was a professor for 20 years, and his research focused on soil-borne diseases of melon. He then moved to Purdue University as Head of the Botany and Plant Pathology Department.

Dr. Martyn is no longer involved in active research but this has given him plenty of time to digest the work he did over the years.

During the past 20-25 years a dramatic increase in late-season vine decline (LSVD) of melons has been observed. This involves all members of the cucurbit family of plants, for example watermelon, squash, and muskmelon. LSVD is a generic term used to describe a set of symptoms that include the sudden death of cucurbit foliage about 2-3 weeks before harvest. This period is critical for the accumulation of sugars during the final ripening stages of the melons, so the vine death results in major losses.

There is no single cause for LSVD, nothing that correlates exclusively with vine decline. Sometimes there are pathogens involved, other times not. Vine decline has reportedly been associated with presence of fungi, viruses, bacteria, application of herbicides, and more. The fungus Monosporascus cannonballus, is one of the major fungal pathogens found to be associated with LSVD.

M. cannonballus is a fungus that forms fruiting bodies called perithecia on the roots of the melons. These perithecia have structures in them called asci (singular: ascus). Fungi belonging to the ascomycota typically have asci with 8 ascospores, M. cannonballus is different in that each ascus only has one ascospore, but this spore contains 8 nuclei (it's a called a multinucleate ascospore). Sometimes 16, but mostly 8. Looking at the picture below, it's easy to see where the fungus gets its name from, the perithecium is almost perfectly round and smooth. The ascospores have very thick walls, and are hard to germinate: they are dormant spores. M. cannonballus was first reported in 1970 as a saprophyte (not a pathogen) on canteloupe, in 1983 it was first reported as a plant pathogen in Israel, and in 1989 it was first reported within the US by guess who... Dr. Ray Martyn.

Picture credit

In the greenhouse, M. cannonballus can infect many different plant species. When a fungus can infect many species, it is said to have a "wide host range." However, M. cannonballus is only a problem on watermelon and muskmelon in the field, so for practical purposes M. cannonballus has a narrow host range. Dr. Martyn studied potential biocontrol methods. He looked at ways in which the disease can be managed using organisms. In this case, M. cannonballus changed in culture, and became hypovirulent, meaning they caused a lot less disease than the original fungus. He investigated whether he could decrease the damage caused in the field by adding the hypovirulent fungus, which would "mate" with the disease-causing fungus, resulting in hypovirulent offspring.

Dr. Martyn authored an article for the APS Education website on Monosporascus root rot and late-season vine decline of melons.

A number of other pathogens were studied as possible causes of late-season vine decline, but all turned out to be fairly weak pathogens, that only caused damage when the plant was very young. In LSVD, the plant is mature, and much stronger, and these pathogens were not responsible for the disease symptoms. In addition, Monosporascus is not the cause of LSVD everywhere in the world. It is reported only in areas that are fairly warm.

Since LSVD is a fairly recent syndrome, Dr. Martyn considered the changes in melon production in the past 25 years. These are:
1. Melon production switched from open-pollinated plots to the use of hybrid melon cultures.
2. Instead of direct seeding into the ground, current production techniques almost exclusively use transplants (seedlings are grown in small trays, and transferred into the ground when they are older).
3. Planting used to be in bare ground, and irrigation systems used furrows, currently plastic mulch and drip irrigation are used.
4. Plants are planted much closer together than in the past.

Dr. Martyn proposed during the seminar that these changes in melon culture practices result in compromised and restricted root systems on the plants. When melons are seeded directly into the ground, the initial root that is formed when a seed germinates, the tap root, continues to grow deep into the soil, as deep as 6-8 feet. A large number of secondary and tertiary roots form along the length of the tap root, allowing the plant to extract water and nutrients from deep in the soil. When seed is germinated in little seedling trays, the tap root grows to meet the bottom of the tray and starts growing in a circle. Once the plant is transplanted into soil, the taproot never recovers, and dies off. Secondary roots that are formed after transplanting do not grow deeply into the soil, but remain about 4" or so below the ground level, and grow essentially horizontally. This drastically limits the availability of water and nutrients to the root system, because the roots do not reach deep enough into the soil.

The compromised root systems have a hard time keeping up with the demand for water of the mature plant, especially during the latter stages, when fruits need to accumulate sugars. Minor pathogens and additional stress, for example lack of water, cause rapid death of the plant in the final stages of fruit development, when the plant needs the resources the most.

Dr. Martyn investigated using cone-tainers, which are very long containers to start seedlings, and they allow for much more growth of the tap root before transplanting, and are so long that the tap root does not reach the bottom before transplanting occurs. After the seedling is planted in soil, the tap root can continue to develop.
Picture credit

In conclusion, LSVD correlates with cultural changes, resulting in vulnerable root systems that cannot handle multiple stresses.

Weekly seminar

The department of Plant Pathology at the University of Florida offers weekly seminars in the Spring and Fall semesters. I attend most of these seminars, and take notes. In the "weekly seminar" series I will summarize my notes on this blog.

It is important to note that the normal disclaimer applies to these posts (see top of the blog page). In addition, these posts are exclusively based on my notes, which are the result of my personal interpretation of what the speaker says. The speaker does not read or comment on my post before publishing, so what I write is not in any way endorsed or sanctioned by the speaker.

Be that as it may, most of the time, the topics addressed during the seminars are current, interesting, and a great source for discussion.

Plants get sick too

Welcome to The Hypersensitive Response! This blog is intended to provide information about the basics of plant pathology, the history of plant pathology, plant diseases, plant disease diagnosis, disease management, current topics, recently published papers, and much more.

For those of you who have never thought about it, plants are organisms that suffer stresses just like animals/humans do. Because of the nature of plants, the stresses they endure are in many ways different from those endured by animals, but they are remarkably similar too, as future posts will point out.

Plant stresses can be broadly divided in abiotic and biotic stresses. Abiotic stresses are those that result from the environment that the plant is in. For example, a plant cannot simply walk off to a cooler area when it is too hot, and low and high temperature can cause stress. Plants derive their mineral nutrition (mainly) through their root system, so availability of nutrients (either too much or not enough) can cause stress to a plant. Humidity, physical wounding, too much water or not enough, all are causes of abiotic stress.

Biotic stresses are those caused by other organisms. The term "organisms" is used broadly here, and includes viruses (which are not considered living organisms), bacteria, fungi, nematodes, insects, and even other plants.

The field of plant pathology mainly focuses on diseases caused by organisms, however, often interactions with abiotic factors are also considered. For example, a plant may normally not be attacked by a certain pathogen, but is a target whent he plant is already weakened by an excess water. There often are interactions between abiotic and biotic factors to plant disease, and hopefully these will be discussed extensively on this blog.

If you want to contribute as a guest author, please contact me at the e-mail address provided in the sidebar. I will gladly turn this blog into a group blog, and I invite everyone that has something to write on plant pathology to become a one-time guest author, or a regular contributor. The more people, the broader the scope, and the more interesting the blog becomes.

Welcome and enjoy!