How do plants perceive damage to themselves?

Plants use receptor proteins on the surface of their cells to recognise danger signals from pathogens. A paper recently published by Li, Wang and Mou in Plant Physiology discusses recent work that finds that plants sense signals from damaged cells in a similar manner. As pathogens are recognised from pathogen-associated molecular patterns (PAMPs) so too damage is detected through similar chemicals, damage-associated molecular patterns (DAMPs).

“A large and compelling body of evidence has accumulated in recent years, which supports an important role for DAMPs in plant immune responses,” write Li and colleagues in their article. “Nevertheless, the identity of DAMPs in plants remains to be unambiguously defined.”

The botanists argue there are broadly two kinds of DAMP, named primary and secondary DAMPs. The primary DAMPs are the kinds of molecules that get produced when cell structures break down. Plants have evolved the ability to recognise the debris of broken cells. The secondary DAMPs are actively created by damaged cells as a warning to other cells in the plant.

One problem the authors discuss is that some DAMPs seem to be released by cells that are not dying. This release would run counter to the danger model that PAMPs and DAMPs are interpreted in, but they say that the situation may be complicated for a couple of reasons.

“First, some DAMPs may play dual functions in plants. For instance, as in animals, eATP in plants not only acts as a DAMP in wound response, but also plays a major role in growth control,” write the authors. “The constitutive eATP and actively released ATP may be crucial for cell viability and growth changes. Second, the amount of DAMPs actively released may not be sufficient for immune activation. For example, in response to cold stress (4°C for 7 days), the concentration of eATP in the extracellular root medium of seven-day-old Arabidopsis seedlings is ~8 nM, whereas that in the fluid released at the sites of physical wounding is ~40 μM. The eATP concentration under cold stress is likely too low to activate the eATP receptor DORN1 (Kd, ~46 nM) for wound response. These results suggest that DAMPs may induce immune responses in a concentration-dependent manner, or there may be a threshold below which DAMPs do not activate immune response. And third, since plants lack specialized immune cells and adaptive immunity, cell-autonomous immunity may play a more important role in plants than in animals. Plants might have thus evolved mechanisms to actively release high amounts of DAMPs for activation of cell-autonomous immunity. Clearly, further investigations are required to determine whether sufficient DAMPs can be released in the absence of cell death for immune activation in plants.”

The payoff for understanding how plants perceive damage could be healthier ecosystems. “It is expected that a deeper understanding of plant DAMPs and the plant immune system could significantly help design new strategies to breed crop varieties with increased resistance against pathogens and/or herbivores,” conclude Li and colleagues. So, while the biology is molecular, the importance is planetary.