Drying out defenses: drought reduces tree defense system

Drought stress is becoming increasingly common with climate change, with negative impacts on tree growth and survival. The mechanisms of drought mortality in trees are often attributed to carbon starvation or hydraulic failure, however disease and herbivory could also cause mortality under drought. For this to happen, tree defense systems against pathogens and herbivores would have to be weakened under drought – but are they?

Recently, in Tree Physiology, Sarah MacAllister and colleagues look at metabolic changes in Scots pine leaves during drought stress to determine whether plant defenses are weakened. They use an untargeted metabolomics approach – essentially measuring every metabolite in the leaves – to understand how drought stress affects leaves. They found divergence in leaf metabolism by 29 days of drought stress, with increases in aromatic amino acids (which serve as a carbon source for plant defense compounds), while glycosides and terpenoids (defensive compounds) decreased under drought stress. The authors surmise that the increase in aromatic amino acids could be due to increased protein degradation under severe drought stress, while the decrease in defensive compounds could be due to compromised photosynthetic function.

What are the implications of these findings? MacAllister and colleagues show that plant defenses become compromised during drought stress, making trees more susceptible to pathogens and herbivory. In turn, this would increase the likelihood that trees could die due to enemy attack during drought stress, rather than through carbon starvation or hydraulic failure. Furthermore, many unidentifiable compounds showed large changes in response to drought stress, underscoring a gap in our understanding of leaf metabolism during drought. It’s possible that some of these unknown compounds could relate to less expensive defense systems for the plant or simply metabolites required for surviving under water limitations.