Study advances understanding of witch’s broom, the disease that devastated Brazil’s cocoa crop
September 15, 2021
By José Tadeu Arantes | Agência FAPESP – The worst crisis ever suffered by Brazil’s growers of cocoa trees was due to a severe outbreak of witch’s broom disease, caused by the fungus Moniliophthora perniciosa. The symptoms include deformity, rotting and death of the affected parts of cocoa trees, which sprout bundles of stunted twigs resembling old brooms. Endemic to the Amazon, in 1989 the disease spread to the south of Bahia, a state in the Northeast region. Brazil was then the world’s second-largest grower of the crop, producing more than 400,000 metric tons of cocoa beans per year in the mid-1980s. As a result of the outbreak, its annual harvest had fallen to about 100,000 tons by 2000.
The impact was enormous. Falling incomes and rising debt wiped out many cocoa farms, bankrupting growers and driving up unemployment. For the natural environment, it was a disaster, as Brazilian cocoa trees are grown in the shade of Atlantic Rainforest remnants and have traditionally been an important incentive for conservation of the biome.
Several initiatives have been undertaken since then, but the fungus has yet to be eradicated. Healthy trees can be seen alongside trees blighted by the disease. Bahia is no longer Brazil’s foremost cocoa-producing state, having fallen behind Pará, and Brazil ranks seventh worldwide, with a national crop of 250,000 tons in 2020.
A study conducted in Brazil by the University of São Paulo’s Center for Nuclear Energy in Agriculture (CENA-USP) and Luiz de Queiroz College of Agriculture (ESALQ-USP), with local and foreign partners, has brought to light important new information about the process of infection by M. perniciosa. The findings are reported in an article in the journal New Phytologist.
“We demonstrate for the first time that the fungus synthesizes cytokinin and that this hormone directly or indirectly alters the plant’s hormone balance, leading to excessive growth of infected tissue. What happens to this tissue is comparable to cancer. It modifies the metabolism and drains the plant’s energy, competing with fruit production and root growth. That’s why witch’s broom disease weakens the trees and reduces cocoa yields,” said agricultural engineer Antônio Figueira, a professor at CENA-USP and principal investigator for the study.
“Until now, rotten fruit was considered the main cause of cocoa crop losses. The hormone cytokinin is known to induce the formation of metabolic sinks in plants, and this appears to be the pathogen’s strategy. It’s possible that the fungus uses cytokinin to promote an increase in biomass in roots and other tissues it infects. When these infected tissues die, it opportunistically uses the dry matter to grow and reproduce.”
This new knowledge directly affects strategies for managing the disease, which until now have failed to take into account the extent to which the fungus inhibits root growth. On the other hand, the removal of infected branches, termed phytosanitary pruning, is still recommended, and should indeed be intensified to minimize the occurrence of metabolic sinks.
As Figueira explained, the cocoa tree (Theobroma cacao) is an evergreen that blooms and bears fruit the whole year round but begins to produce pods and beans only at age five or six, and this hinders genetic analysis. Its physiology is also hard to study because of its size. These are the reasons why little progress has been made in research on the disease using the cocoa tree directly.
“To get around these difficulties, in this latest study we used a small, fast-growing species that’s also susceptible to infection by the same fungus: the Micro-Tom tomato,” he said.
The Micro-Tom, a dwarf variety of tomato (Solanum lycopersicum), is widely used as a laboratory model in botanical research, for reasons analogous to those that justify the use of mice in studies of human biology and medical research. The plant is only 15 centimeters in height and has a 90-day life cycle from seed to seed.
“Lázaro Peres, professor at ESALQ-USP and a co-author of the article, has conducted a great deal of research using the Micro-Tom. He has a collection of strains with mutant genes and transgenic lines, including several mutants with alterations for hormone synthesis or perception,” Figueira said.
“Because the witch’s broom fungus infects tomato, and there was a suspicion that in doing so it caused a hormone imbalance, we investigated a series of mutants with hormone synthesis or perception genes to see if any didn’t display symptoms or were more susceptible to the fungus. What we found was that the cytokinin-deficient mutant didn’t manifest symptoms, suggesting the hormone’s involvement in the appearance of symptoms.
“We confirmed this involvement in several ways, by quantifying the cytokinins in infected tissue and in fungal mycelium, applying synthetic hormones to simulate symptoms of the disease, using synthetic inhibitors of hormone perception to reverse the symptoms, and using a transgenic line that expresses a reporter gene directed by a cytokinin-induced promoter. We also analyzed, via sequencing, the expression of cytokinin signaling marker genes. All assays confirmed the hormone’s role in the pathogenesis of witch’s broom.”
The article “Moniliophthora perniciosa, the causal agent of witches’ broom disease of cacao, interferes with cytokinin metabolism during infection of Micro-Tom tomato and promotes symptom development” can be retrieved from: nph.onlinelibrary.wiley.com/doi/10.1111/nph.17386.
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