1. Perceiving pathogens, a key area

This tug-of-war between the pathogens adapting to the plants’ defenses and the plants adapting to their attacks has come about naturally throughout evolution.

Now, understanding these changes will make it possible to find new plant varieties that can carry out this molecular process against pathogens more effectively, as explained Youssef Belkhadir, researcher and group leader at the Gregor Mendel Institute in Vienna.

“One of the challenges is understanding the complexity inherent in root growth. Plus, roots have different ways of integrating and responding to cell damage, to perceiving molecular patterns associated with microbes and other types of stress,” noted Niko Geldner, professor at the University of Lausanne (Switzerland).

His research, using laser ablation of single cells, has shed light on how root damage, even when limited, has different implications that damage to leaves.

The studies also focus on prevention: there could be ‘dormant’ pathogens present in nature that reappear due to a change in precipitation. Another approach is to make plants more resistant to these pathogens.

Climate change is another worry and a great effort is being made to study the relationship between what is regulated by plant defenses and what is regulated by the environment. This is because plant growth (and that of pathogens, too) is determined by environmental temperatures: the warmer the environment, the more pathogens grow and plants have to deal with this additional stress, at higher temperatures and with less water.

One of the numerous areas of research in this field focuses on kinase receptors, a type of enzyme with hundreds of varieties that are encoded in the genome of plants. These enzymes are found on the cell surface and as the professor in the University of Zürich Department of Plant and Microbial Biology Cyril Zipfel stressed, “They control nearly every aspect of plant life, playing a central role: from reproduction to growth and response to the environment.”

Insects can also play a significant role, as they spread pathogens from one plant to another (known by the generic term phytoplasmas), causing significant changes to the architecture of the colonized plant, such as alterations to flower development or growing leaves that are like flowers, and modulating its defense mechanisms.

Pathogens can also have serious economic repercussions. One example is the witch-broom disease epidemic, spread by cicadas, which recently destroyed more than half of the lime trees in Oman and Iran, as well as infecting other citrus trees, like several types of lemons.

“One of the most spectacular biological phenomena is how these parasites can completely hijack a plant, so the host plant becomes a sort of ‘zombie’ focused on ensuring the survival of the parasites above its own survival,” described Saskia Hogenhout, project leader at the John Innes Center in Norwich, United Kingdom. Recently, a multi-layer bioclimatic model was published to help understand the epidemic patterns of these pathogens.¹

Another disease, called fire blight, affected plants in Switzerland in the past decade. However, in this case, the cause was climate change: higher precipitation levels and warmer springs, noted Sheng Yang He, professor at the MSU-DOE Plant Research Laboratory in East Lansing (United States).

Also, the four serious epidemics of fusarium wilt in wheat that China has suffered over the past five years were caused by the same circumstance, in this case greater precipitation and warmer summers. “The impact of higher temperatures affects both the defenses of the host plant and the virulence of the bacteria.² And the impact on growth can be seen both as a result of long-term and short-term temperature rises, for example with heat waves,” the expert explained.

From his point of view, the studies being done on interactions between plants and pathogens should start to look more at the multidimensional nature of the interactions of the disease, the environment and the microbiome in order to better reflect what is happening both to crops and in natural ecosystems.

Possibly the most well-known case, however, is the pathogen Phytophtfora infestans, which in the mid-19^th century destroyed all of the potato plantations in Ireland, causing a huge famine and enormous economic losses, in addition to the one million people who died of starvation and the other million forced to emigrate. Tolga Bozkurt, a researcher at Imperial College London, focuses his research on this pathogen and points to the key role of a process called selective autophagy combined with defenses to help build immunity.

Advances in high-performance molecular techniques, in addition to the analysis of computational data, have made it possible to better understand the impact plant hormones like cytotoxins and salicylic acid have on activating defenses and their consequences in terms of plant growth, as explained Cris Argueso, professor at Colorado State University (USA).

Another type of approach, known as meta-analysis of gene profiling for the whole genome, carried out by Teva Vernoux at the RDP Laboratory in Lyon (France), has made it possible to describe a network of genes that are kicked off by a small signaling molecule involved in flowering and the growth pattern of plants.