Thymol mediates three-way interactions between Thymus (Lamiaceae), legumes, and rhizobia

Background/Question/Methods

Researchers have long known that species in the mint family (Lamiaceae) exude essential oils containing biologically active compounds. We are interested in exploring the potential ecological effects of these secondary compounds in natural plant communities and agroecosystems. Thymol (a principal component of the essential oil of Thymus spp.) is an anti-microbial compound used commercially for its broad-spectrum activity. This study examined the effect of thymol (in biologically realistic concentrations) on nodule formation in Medicago sativa (alfalfa) and Vigna unguiculata (cowpea). Experiments were initiated in a greenhouse at the UVA Blandy Experimental Farm in Boyce, VA in summer 2012 and 2013. For alfalfa, we used three treatments: (1) control, (2) addition of 0.3 ng pure thymol (3) addition of thyme essential oil containing 0.3 ng thymol. For cowpea, we examined three extra-early maturing varieties of cowpea developed by the IITA in Nigeria with treatments (1) and (2) above as well as (3) growth with thyme seedlings. Inoculated seeds were sown in a high-drainage soil mixture in 50-cell deep plug trays. Trays were placed in randomized blocks with supplemental lighting (16:8 light L:D cycle), with watering as needed, and with weekly fertilization using 1/5 Hoagland solution without nitrogen.

Results/Conclusions

After three weeks, the thymol treatment significantly increased the number of nodules and seedling root weight in alfalfa. In older alfalfa seedlings (five weeks), the thymol treatment also significantly increased shoot weight. In all three varieties of cowpea, significantly more nodules per seedling were observed in treatments with thyme or thymol after five weeks. Two of the three varieties of cowpea also showed significantly larger roots in treatments with thyme and thymol. This study reinforces the importance of a wider community-level view in evaluating the interactions between legumes and their symbionts, particularly since mints (Lamiaceae) co-occur worldwide with legumes (Fabaceae) in natural communities. These results also have implications for improving plant nutrition and soil fertility in agroecosystems. Drought-tolerant legumes like cowpea are critical to food security in rainfed systems such as the Sahel that are threatened by increasing aridity due to global climate change. Further research is underway with carvacrol and other exudates from Lamiaceae to examine impacts on plant-microbial mutualisms. Much is still to learn about the role of bioactive root exudates in mediating multi-species interactions contributing to defense or mutualisms in the rhizosphere. A community-level perspective is essential since these interactions are likely to be diffuse and context-dependent.