Health

An ancient herbarium has revealed the first secrets of the evolutionary history of a serious bacterial disease of citrus

Results & impact 30 July 2021

For the first time, a research team has just proved that plant pathogenic bacteria can be studied from ancient specimens conserved in natural history herbaria. In a scientific paper published on 29 July in the journal PLOS Pathogens, scientists from CIRAD, from the French Muséum national d’Histoire naturelle (MNHN) and from the Mauritius Herbarium retraced the evolutionary history of a bacterium infecting citrus in the Indian Ocean, comparing an ancient genome to those of its modern counterparts. This work highlighted the merits of century old herbaria for understanding how crop pathogenic bacteria have emerged and evolved in the past 300 years.

Researchers have studied tens of ancient herbaria worldwide, to begin retracing the evolutionary history of a bacterial pathogen that affects citrus. Here, the Smithsonian Institution in the United States © L. Gagnevin, CIRAD
Researchers have studied tens of ancient herbaria worldwide, to begin retracing the evolutionary history of a bacterial pathogen that affects citrus. Here, the Smithsonian Institution in the United States © L. Gagnevin, CIRAD

A first for a plant pathogenic bacterium. A team of scientists led by CIRAD recently demonstrated that the DNA of “ancient” bacteria can be isolated and sequenced from plants preserved in herbaria. They sequenced the genome of the bacterium Xanthomonas citri, responsible for citrus bacterial canker, from leaves collected in 1937 and conserved at the Mauritius Herbarium. This historical genome was then compared to 150 modern genomes of the same pathogen, in order to understand how it was introduced and spread throughout the islands of the South-West Indian Ocean. “A detailed analysis of the bacterium’s phylogeny revealed that it was probably introduced in Mauritius” explains Lionel Gagnevin, a plant pathologist at CIRAD in Montpellier, who worked on the study with Nathalie Becker, a molecular biologist at MNHN and Adrien Rieux, a population genomics specialist at CIRAD, both researchers working on the island of Réunion.

A pathogen originating from Asia, just like citrus plants

The bacterium probably emerged in Asia, the area where citrus plants – its exclusive hosts – appeared and were domesticated. From there, citrus canker disease spread to the Americas, Africa and Oceania in the late 19th or early 20th centuries. In the Indian Ocean, the disease was initially reported in 1917 on the islands of Mauritius and Rodrigues, then in the 1960s on the island of Réunion, in the Comoros and the Seychelles.

“We found a citrus specimen apparently infected with canker, collected in 1937 by Reginald E. Vaughan at Phoenix, Mauritius. This is the oldest specimen of its kind in our collections, and has been preserved under strictly controlled moisture and temperature conditions” said Claudia Baider, curator of the Mauritius Herbarium.

Sheet with a diseased citron sample dating from 1937, in the Mauritius Herbarium © A. Rieux, CIRAD

Hundreds of specimens from the 18th and 19th centuries ready to be analysed

Natural history museums worldwide still offer hundreds of specimens to be explored. Many infected samples have already been collected by the team, corresponding to other regions of the world, other time periods (notably the 18th and 19th centuries), and other citrus species. Their analysis should shed light on the global history of this major citrus disease. “The potential of museum herbarium resources is colossal and will help in understanding how plant pathogens emerge, particularly in relation to recent changes in agriculture” points out Lionel Gagnevin, who has explored several of these museums. Especially as DNA analysis techniques are evolving rapidly and offer some exciting new possibilities.