Chemical communication: An evidence for co-evolution between plants and soil organisms

What we call ``humus'' is a huge collection of organic molecules, among which physiologically active compounds with important ecological roles can be found. These molecules are produced by soil organisms and plants. Soil organisms physically associated with plants such as some bacteria are well known for their effects on plants through signal molecules inducing changes in plant hormone signaling pathways and growth. It has been recently shown that soil animals such as earthworms, which live in the bulk soil without physical contact with plant roots, are also able to modify plant development, immunity and growth through signal molecules. Modification of plant growth and fitness through the emission of signal molecules is in fact a general mechanism in the aboveground-belowground interactions. Plants also produce root exsudates among which many molecules are interpreted as signals by soil organisms. Because of this reciprocal signal emission and subsequent adaptations with important consequences on plants and soil organisms' fitness, I propose that coevolution exist between plants and soil organisms.