Unexpected findings: when submerged plants are wounded
Although our first hypothesis was incorrect, we continuously examined the phenomenon and found that plants deactivate their ability to resist against herbivores after they have experienced flooding
First hypothesis on plant wound sensing
In 2017, I started a new career in Korea Research Institute of Bioscience and Biotechnology (KRIBB) located in Daejeon, Korea after the post-doctoral fellowship in Seoul National University (SNU). I rearranged all my research materials in SNU and moved to Daejeon. In Seoul, my main research topic was plant responses to high temperatures. Last study in SNU was examination of molecular mechanisms on hyponastic movement of leaves in response to warm temperatures. During the research, several plants were exposed to strong airflow in our growth room, and I accidentally found that plants suppress daily hyponastic movement when they are exposed to wind or mechanical stimulation.
After that, I decided to study molecular mechanisms on plant responses to mechanical stimulation in KRIBB. Because it was unknown how plants recognize touch or wound, I discussed on that issue with my colleagues in our lab to build hypothesis on plant sensing mechanisms of mechanical stimulations. In 2018, one day I have read a post on public web portal that oxygen plays important roles during wound healing of animal tissues. Plant leaf tissues are surrounded by epidermal cells with cell wall and cuticle, thus internal aeration is tightly controlled by the stomatal apertures. Based on the role of oxygen in animal wound responses and characteristic of plant cells, my colleagues and I hypothesized that disruption of plant cells by wounding would cause massive aeration, which possibly be an initial signal for wounding.
To examine our hypothesis, we submerged the Arabidopsis plants and wounded in the water. Surprisingly, plant wound responses were largely suppressed by the treatment! We were happy for the results at that time. To verify our hypothesis, we submerged and reaerated plants and then wounded the reaerated leaves. If abrupt aeration is a sign for wounding, then reaerated plants should induce wound responses. However, still the reaerated plants did not show wound responses. These results indicated that our hypothesis is incorrect, but alternatively we had another question: why plants inactivate wound responses after they have experienced submergence?
Hypoxia vs Ethylene
Because submergence triggers hypoxia, we firstly thought that hypoxia disrupts wound responses. To test the hypothesis, we built a portable and transparent glovebox for treatment of hypoxia using Argon (Ar) gas instead of water. Treatment of Ar gas for 3 h triggered hypoxia and inhibition of wound responses, but the effects were not as strong as submergence. In addition, while Ar gas needs 3 h to affect wound responses, only 10 min of submergence effectively suppressed wound responses. Critically, short-term submergence (10 min ~ 1 h) did not induce hypoxia in plants because there are plenty of oxygen in plant cells as a result of photosynthesis. Before hypoxia, submergence induces ethylene responses within minutes in plants due to restricted gas diffusion in water. We thus changed our hypothesis: ethylene suppresses wound responses. We analyzed role of ethylene, and finally found that ethylene signals activated during the submergence partially block plant wound responses.
Submergence inactivates herbivore resistance of plants
Plant wound responses induce jasmonic acid (JA) biosynthesis, which is important for herbivore resistance. We tried to analyze plant-herbivore interactions after submergence treatment, but it was our first time to perform herbivore resistance assays. We searched commercially available Pieris rapae caterpillars (Brassicaceae specialist) in our country and found a farmer selling the caterpillars. Because he stops breeding caterpillars during winter, we tried our best to successfully perform herbivore assays before November. After few trials, we finally gained results that plants that had been submerged have reduced resistance to herbivore after they are reaerated.
There is an unidentified question: why plants inactivate herbivore resistance after submergence? What is the benefit for these events? It depends on plant species, but our hypothesis is that plant suppresses JA responses to elevate salicylic acid (SA)-dependent pathogen resistance after submergence. For Arabidopsis, outbreak of bacterial pathogens after submergence possibly be more threatening event than herbivore attack. Because JA antagonizes SA signaling, plants might inactivate JA responses to elevate SA responses. The trade-off between JA and SA responses would be the results of evolution, which is beneficial for survival. We gained unexpected results during the study, but we could find another important strategy of plants in response to submergence.