How a tiny bacterium kills ancient olive trees

The fire bacterium (Xylella fastidiosa) threatens the European olive tree population - in the Plant Biotechnology Unit of the University of Natural Resources and Life Sciences Vienna, scientists now want to breed resistant olive varieties.

The spread of Xylella fastidiosa (fire bacterium) can be compared in a certain way to the outbreak of the Covid pandemic. We need to recognise what the Covid pandemic taught us and apply these lessons to all aspects of our lives. How an infection originated from an unknown source, led to an epidemic and eventually, through global spread, to a pandemic. For these reasons, rapid and accurate methods of detecting plant pathogens, enabling their isolation and control, and new breeding strategies are needed.

Burning problem

One such current burning example is provided by the bacterium Xylella fastidiosa (fire bacterium), which infects a broad spectrum of more than 600 plant species from various taxonomic groups. This pathogen causes several economically relevant diseases, including Pierce's disease of grape, oleander leaf spot disease, citrus spot disease, almond leaf spot disease or olive tree death, by causing plants to dry out and eventually die. "Olive Quick Decline Syndrome" (OQDS) is currently one of the most important emerging plant pathogens on a global scale.

Xylella fastidiosa is rated as one of the top ten plant pathogenic bacteria and is listed as a quarantine organism in the EU (Mansfield et al., 2012). The list of host plants for the Xylella fastidiosa pathogen currently already comprises 655 plant species and includes ornamental plants, wild plants and crops. Its path from North and Latin America to Southern Europe is unclear, and a control method has not yet been found. Direct transmission occurs through cicadas. For some time now, the bacterium has also been threatening European olive trees.

Olea europaea

Olive tree plantations occupied 12.8 million hectares in the world in 2020, on which 20.8 million tonnes of olives were harvested. The production area in Europe accounted for about 5.1 million hectares in 2020. Here, about 59.6% of the world's olive crop was harvested in 2020. Spain, Italy and Portugal produced 53.1 % of all olives worldwide. There are over 1000 varieties of olive trees in the Mediterranean alone. The EU is not only the largest producer of olive oil, but also the largest consumer. As the demand for olive oil has steadily increased in northern countries as well, the cultivation of olive trees has expanded considerably. In many regions, the olive tree is the basis of the rural economy.

In Europe, Xylella fastidiosa was isolated in olive trees for the first time in 2013 in southern Italy (Puglia region). Puglia had the misfortune that several factors came together to favour an epidemic, including the almost unbroken cover of olive trees. The bacterial strain found on 8000 ha is most closely related to the pauca subspecies and was probably introduced from Costa Rica with contaminated plant material (coffee) (EFSA Panel on Plant Health, 2015; EFSA, 2016). The infested trees must be dug up and burned, because the European Commission has classified Xylella fastidiosa as one of the most dangerous plant diseases of all. At the beginning of 2015, hundreds of thousands of olive trees on an estimated 230,000 ha had to be felled due to the infestation, and a further rapid spread of olive tree death was feared.

Foam cicadas as carriers

Unfortunately, there is still no cure for Xylella. Therefore, different approaches are being researched, in the short term, for example, by combating the spittlebugs (Philaenus spumarius) as vectors, but in the longer term, of course, by breeding resistant planting material - as is currently being done at BOKU. And as always, the more flavourful varieties are susceptible and the more robust varieties less palatable.

This is where the efforts of BOKU's Plant Biotechnology Unit (Margit Laimer's working group) come in. Within the framework of an international IAEA/FAO research project "Development of Integrated Techniques for Induced Genetic Diversity and Improvement of Vegetatively Propagated and Horticultural Tree Crops", the PBU researchers are working on and responsible for "cell and tissue cultures for resistance breeding in selected olive varieties". Establishing a live collection of source plants in vivo (mother plants).

It must be taken into account that olive plants as potential carriers of a quarantine organism cannot simply be placed or exposed everywhere. The mother plants are placed in the glass house or in the insect-proof saran house of the quarantine station of the PBU, maintained and used as starting material for the establishment of tissue cultures. Establishment of an in vitro collection of Olea europaea L.

Plants maintained under greenhouse and saranhouse conditions provide the starting material for vegetative propagation. The steps for establishing sterile tissue cultures are optimised with the help of the PBU's many years of experience with woody crops. First, axillary shoot cultures and suspension cultures of selected varieties are established, which in turn can be used again as starting material for further trials. Thus, protocols for somatic embryogenesis and the formation of adventitious shoots from adult material will be established for the selected olive varieties.

Improved genotypes

The aim of these efforts will be to generate improved genotypes of olive trees (Olea sp.) after applying appropriate techniques such as gamma and X-ray radiation or CRISPR-Cas, which will subsequently be tested for altered susceptibility to Xylella. Molecular validation of mutants should make it possible to provide a valid recommendation on how to generate, screen and obtain improved genotypes of olives after irradiation.

"Due to the changing climatic conditions and the possible northward spread of the vectors, but especially due to the fact that the different subspecies of the fire bacterium can infect such an incredibly large host plant range, all efforts are needed to avert a threat scenario in Europe," emphasises Margit Laimer, head of the working group. "Every insight we can gain into the interaction between a plant (Olea), the pathogen (Xylella) and the vector (Philaenus) will improve the control strategies we can also develop for other crops, the different subspecies of the bacterium and new potential vectors." The phytosanitary quality of the planting material, but also the breeding of resistant varieties while preserving the factors that determine quality, represent a significant contribution to the control of this disease.

Ao. Univ.Prof. Dr. Margit Laimer
Institute for Molecular Biotechnology
University of Natural Resources and Applied Life Sciences Vienna
01 47654 - 79010