Unlike for plants, in animals especially humans, body mass index (BMI, which is a person’s weight divided by the square of height) is a measure of physical health and pre-disposure to conditions like obesity. BMI does not make sense in plant health because of differences between plant and animal physiological systems.

However, large body size in plants may have some advantages. Apart from controlling a larger proportion of available resources and space within crowded vegetation, what other advantage does a large plant body size offer to an individual plant?

The banana’s plant body architecture

From the botanical point-of-view, the banana plant is a gigantic herb. A plant that is a herb or “herbaceous” is unable to undergo “secondary growth” and cannot form wood during its vegetative development.

The banana plant springs from an underground “true stem”, also called the “corm” or “rhizome”, to form a false stem, also called a “pseudostem” of 2-7 m height. The pseudostem is composed of the basal portions of leaf sheaths and is crowned with a rosette of 10 to 20 oblong to elliptic leaves that sometimes attain a length of 2-4 m and a breadth of 70 cm.

In mature banana plants, true stem emerges at the top of the pseudostem and bends downward to become a bunch of 10 to 300 individual fruits, or fingers, grouped in clusters, or hands, of 3 to 22. The edible part of the bunch is the female. In contrast, the inedible distal part, including the purple-colored cone-shaped end (locally known in some Ugandan dialects as “omukanaana” or “empumumpu”) constitutes the male part of the bunch.

How is the giant banana size an advantage in disease control?

A new study, titled “Xanthomonas campestris pv. musacearum Bacterial Infection Induces Organ-Specific Callose and Hydrogen Peroxide Production in Banana” and led by a team of scientists at the Department of Plant Sciences, Microbiology and Biotechnology at Makerere University in collaboration with the University of California, Davis, USA, shows how the giant banana body size can be used to control banana Xanthomonas wilt (BXW) disease.

According to Prof. Arthur Tugume, the lead scientist of this study and expert in plant pathology, when plants get infected, they respond instantly by implementing different strategies that limit the multiplication and/or mobility of the disease agents (pathogens). “For example, plants rapidly produce reactive oxygen species (ROS) such as hydrogen peroxide, superoxide ions, and hydroxyl ions. These ROS act as rapid messengers in the plant tissues to activate additional responses spreading over the entire plant body. This helps the plant’s distant tissues or organs to be aware and prepare advance defenses against the intruding pathogens”.

Prof. Dinesh-Kumar the project’s research collaborator based at the University of California-Davis, USA and expert in plant biology explains that “disease is a form of stress in plants and plants cannot perform well their biological functions when they are sick since they have to spend a lot of energy fighting against the disease. This is why disease control is important to enable plants grow well and yield high.”

The research indicates that ROS set in motion additional processes to ensure limited impact of disease and pathogens on the plant. For example, Hydrogen peroxide (H₂O₂), which is a ROS, has direct bactericidal, fungicidal or other anti-effects on the pathogens. Also, following H₂O₂ production, a unique plant carbohydrate, named “callose” starts to accumulate in large quantities within plant cells as a means of fortifying plant tissues. Callose differs from the other usual plant carbohydrates such as starch or cellulose because of the way its structures are formed.

Increased production of callose acts as a roadblock to any pathogen e.g., bacteria by limiting bacterial movement that would otherwise allow ease of attack on other tissues or cells at distant locations in the plant. “Although these plant defense responses are rapid, plant organs that are distant from the site of pathogen attack can be instrumental and block progression of bacteria or other pathogens by depositing callose in advance at strategic points” Prof. Tugume explains.

However, Prof. Tugume notes that callose participates also in many other normal developmental processes of plants, and for that reason, there is always some “housekeeping” callose in the plant tissues even without pathogenic infection. “This means that one must be able to accurately and quantitatively distinguish between ‘stress-induced’ and normal ‘housekeeping’ callose”, he adds.

How was the study done?

In this study, the researchers used young (2.5-months old) banana plantlets that had been generated from tissue culture at Kawanda Agricultural Research Institute. They then infected the plantlets with a bacterium called Xanthomonas campestris pv. musacearum (Xcm). This bacterium is the causative agent of banana Xanthomonas Wilt (BXW), the most destructive disease of bananas in East and Central Africa (ECA).

The banana leaves, pseudostems, corms and roots were analyzed for callose and compared with the control plants that had been inoculated with water instead of bacteria. H₂O₂ production was monitored by “DAB staining”, and by “spectrophotometry” while the analysis of callose was done by two methods: staining and visualization of callose using florescence microscopy, and using “Sandwich Enzyme-Linked Immunosorbent Assay” methods.

What did the researchers discover and how can it be used in BXW disease control?

This study revealed that the underground corm tissues assemble the stiffest resistance against BXW by depositing the highest concentrations of callose, while the pseudostem produced the highest quantities of H₂O₂. This is interesting for three (3) main reasons:

1. Firstly, Xcm bacteria often enter through the leaves in regular plantation husbandry; hence, the corm being distant from leaves gives it an anatomical advantage in promoting the ability of lateral plants to escape Xcm infection.
2. Secondly, the corm is an organ of perennation supporting vegetative and perennial continuity of the crop across seasons; hence it is charged in ensuring a disease-free next generation by severely constraining “mother-child transmission” of Xcm bacteria.
3. Thirdly, the control of BXW now becomes easy when farmers are observant to the first aerial disease symptoms because Xcm is strongly constrained by bottlenecks in the pseudostem and corm.

Therefore, at the onset of aerial symptoms, diseased peudostems should immediately be removed by aseptically cutting them off at the corm without interfering with symptomless lateral shoots, which allows continuous food production and disease control to go on simultaneously. This is facilitated by the large size of the banana plant because at the onset of leaf symptoms (2.5 to 5 meters away from the corm), the bacteria have not yet arrived at the base of the pseudostem where the diseased plant can be cut off from the corm. This gives chance to a farmer to eliminate the infected pseudostems early (in 1 to 7 days) since the appearance of leaf symptoms.

This research was part of the PhD studies for Mr. Abubakar S. Mustafa at Makerere University and University of California, Davis. According to Mr. Mustafa, these discoveries make the management of BXW in banana plantations easy as long as the farmers are observant and act fast by removing diseased plants aseptically.

This study has been published by the American Phytopathological Society (APS) in an open access journal, “PhytoFrontiers”, and is freely accessible on https://apsjournals.apsnet.org/doi/full/10.1094/PHYTOFR-11-21-0073-R.

This study was funded by the Bill and Melinda Gates Foundation through the National Agricultural Research Organization (NARO), Uganda. The project had partners including the International Institute of Tropical Agriculture (IITA), the Alliance for Bioversity International and International Centre for Tropical Agriculture (CIAT) and Centre for Agriculture and Bioscience International (CABI).

For more details, contact;

Prof. Arthur Tugume
Lead Scientist
College of Natural Sciences (CoNAS)
Makerere University
Email: arthur.tugume@mak.ac.ug
Tel: +256772514841

Mr. Abubakar S. Mustafa
Co-Author and PhD student on the study
Email: mustafa.abubakar.sadik@gmail.com
Tel: +256702813233

Hasifa Kabejja
Principal Communication Officer
College of Natural Sciences (CoNAS)
Makerere University
Email: pr.cns@mak.ac.ug
Tel: +256774904211