April Showers Bring May Flowers - But Microbes Keep them Growing
Spring is in the air and in honor of this, last week we read about geosmin, a molecule produced by soil-dwelling bacteria that gives dirt its distinctive, earthy smell. This smell, combined with the emergence of spring flowers, reminds me of the fascinating relationships that exist between plants and their associated microbes (aka microbiomes). Just as humans have a complex relationship with microorganisms, some make us sick while others aid our health, plants too coexist with a mixture of mostly helpful but sometimes harmful microbes. While geosmin may overwhelm our noses, plants are able to detect a number of compounds produced by their neighboring microorganisms.
Many of these airborne molecules belong to a group of diverse, small compounds produced by bacteria, called Bacterial Volatile Compounds or BVCs for short. BVCs can influence how plants grow and respond to disease-causing microbes by triggering different responses. For example, the compound indole promotes overall plant development as well as the sideways growth of roots in Arabidopsis thaliana (a type of mustard green that is used frequently in plant studies). Tridecane, another BVC, stimulates signaling pathways in Arabidopsis that lead to the defense against plant pathogens.
One particularly interesting BVC, 2,3-butanediol, can lead to either disease or resistance depending entirely on who produces it and when. 2,3-butanediol is a small organic molecule produced by a large number of plant-associated bacteria, such as the pathogenic Pectobacterium carotovorum and the beneficial Bacillus subtilis. When produced by P. carotovorum, 2,3-butanediol assists in the progression of disease by creating the alkaline (pH >7) environment plant cell wall-degrading enzymes need in order to eat away at their target. However, if B. subtilis is the first to produce 2,3-butanediol, the subsequent disease-causing actions of P. carotovorum and similar pathogens are thwarted by 2,3-butanediol triggering the host plant’s immune system. While the nature of this protective action is still being uncovered, researchers are exploring the use of BVCs, like 2,3-butanediol, and BVC-producing bacteria to enhance crop yields.
The use of BVCs to stimulate plant growth and promote resistance to disease has growing appeal in agriculture. However, because BVCs quickly evaporate and spread throughout the air, direct application of these compounds to plants or soil is tricky in practice. The applied BVCs could be lost to the atmosphere before they’ve done their task. But despite this complication, some field studies have yielded promising results. Meanwhile, other researchers sidestep this potential problem entirely by focusing on the source of helpful BVCs, the plant-associated microbes themselves.
Perhaps the best studied plant-associated microbes are the Plant Growth-Promoting Rhizobacteria (PGPRs). PGPRs are so named because of their beneficial effects on plant growth, through BVCs and other means. Additionally, they reside in the plant’s rhizosphere, or the area immediately surrounding the root. Individually, PGPRs enhance plant growth and/or resistance to disease, but as researchers test the application of PGPR mixtures, we’re learning just how complex the microbial community and its influences are. While some combinations lead to enhanced effects, others remain no different than single PGPR additions. These collective findings indicate that we still have a lot to learn about how microbes assist plants and how they interact with each other in doing so. However, it remains clear that these roles are critical to the growth and survival of our photosynthetic friends.
Understanding that microbes are important members of our gardens adds a new level of appreciation to enjoying the splendor of spring greenery. Just as human microbiome studies tell us that we wouldn’t be who we are without our microbial entourages, neither could our gardens, forests, and fields, without their tiny cohabitants. So as you take time out of your busy day to stop and smell the roses, consider what they’re smelling in return, and how that allows them to flourish and bloom.
Chung, J. H., Song, G. C., & Ryu, C. M. (2016). Sweet scents from good bacteria: Case studies on bacterial volatile compounds for plant growth and immunity.Plant Molecular Biology, 90(6), 677-687. doi: 10.1007/s11103-015-0344-8.
Hagan, A. (2016). Peanuts and Probiotics. https://misciwriters.com/2016/01/05/peanuts-and-probiotics/
Rosier, A., Bishnoi, U., Lakshmanan, V., Sherrier, D. J., & Bais, H. P. (2016). A perspective on inter-kingdom signaling in plant-beneficial microbe interactions.Plant Molecular Biology, 90(6), 537-548. doi: 10.1007/s11103-016-0433-3.
Ryu, C.-M., Farag, M. A., Hu, C.-H., Reddy, M. S., Wei, H.-X., Pare, P. W., & Kloepper, J. W. (2003). Bacterial volatiles promote growth in Arabidopsis.Proceedings of the National Academy of Sciences, 100(8), 4927-4932. doi: 10.1073/pnas.0730845100.
Schlaeppi, K. & Bulgarelli, D. (2015). The Plant Microbiome at Work.Molecular Plant-Microbe Interactions, 28(3), 212-217.