Potato-associated bacteria can act as bodyguards

Bacteria found with potatoes can help protect their hosts against late blight. The aid comes from their volatile emissions. The results of a study into protection against Phytophthora infestans was published in Scientific Reports. “This work brings new perspectives for plant protection against the devastating Irish Famine pathogen, while opening new research avenues on the role of sVOCs [sulfur-containing volatile organic compounds] in the interaction between plants and their microbiome.”, write Delphine Chinchilla and colleagues.

Oomycetes, microscopic fungus-like organisms, are a major problem for plants. Phytophthora infestans, the infection that turn potatoes in Ireland into mush, is a particularly successful pest. It manages to combine asexual reproduction for speed of attack, with sexual reproduction to ensure genetic diversity. “The fast disease spread is facilitated by the massive production of two types of asexual spores: sporangia that can be dispersed by wind and rainfall, and zoospores, which are motile, bi-flagellated spores able to swim towards stomata or daughter tubers,” write Chinchilla and colleagues and colleagues in their paper.

Sexual reproduction allows P. infestans to overcome fungicides. Breeding resistance into the plants themselves would help stay infection for a while, but sooner or later P. infestans would be able to overcome it. So to aid in plant protection, Chinchilla and colleagues looked at the microbiome.

The microbiome is the sum of all the microscopic organisms in a location. The part that Chinchilla and colleagues was interested in contained bacteria. In earlier work the team had already found that volatile molecules emitted by bacteria could aid plant health. Some volatiles could signal to a plant to prepare defences against an attacker, while others could interfere with pathogens directly. This work found that volatile emissions that contained sulphur could help combat P. infestans.

“In contrast to elemental sulfur, which has long been used in crop protection against fungi, the discovery that volatile organic sulfur compounds also have strong crop protection potential is more recent,” write Chinchilla and colleagues. “Dimethyl disulfide (DMDS), which is produced by many bacteria and by some plant species such as Alliaceae and Brassicaceae, has received most attention and is being used in practice for soil fumigation against weeds, nematodes and pathogenic fungi. However, in our in vitro characterization of the biological effect of bacterial sVOCs on different life stages of P. infestans, the protective activity of DMDS was largely surpassed by that of two others sVOC, dimethyl trisulfide (DMTS) and S-methyl methane thiosulfonate (MMTS).”

If you’re thinking S-methyl methane thiosulfonate sounds familiar, then that might be because they’re common compounds found in brassicas, say the authors. There’s a good chance you’d recognise the smell if it were under your nose, as it’s a useful danger signal.

It turns out that sulphur is not just bad news for you, it’s bad news for P. infestans too. Rather than inducing changes in the plant, the sVOCs emitted by bacteria interfered with the proteome of P. infestans itself. This disrupted the organism’s ability to attack a plant. “We observed strong specificity in the proteome changes caused by exposure to the individual sVOCs, with only few proteins commonly regulated by DMDS, DMTS and MMTS or by the 5 sVOCs. This specificity is consistent with their differential activities on hyphae and spores,” write the scientists.

One of the important findings of the study was that S-methyl methane thiosulfonate (MMTS) appears not to be phytotoxic. This means that the compound that is damaging the oomycetes is not also damaging the plants. This leaves open the possibility safer crop protection, though work remains, write the team. “Although MMTS was not phytotoxic in our experimental setup, future studies shall investigate the putative toxicity of this sVOC towards non-target organisms to evaluate its suitability for crop protection.”