Occam's Razor Applied to Hormonology (Are Cytokinins Produced by Plants?)

"One wonders if cytokinins may be important generally in associations between higher plants and microorganisms" (Miller, 1968). Cytokinins are a class of compounds that are defined by their ability to stimulate cell division in plants. They are found throughout the plant, but are most abundant in actively growing tissues. Their isolation from corn (Letham, 1963; Miller, 1965), following the discovery of their effect on plant cells in culture (Miller et al., 1955), led to the assumption that cytokinins are endogenously produced plant-growth regulators. This assumption gained the status of fact, although it was never proven. Conversely, cytokinin production by plant-associated microorganisms is well documented. This Scienitific Correspondence examines the assumption that plants produce cytokinins and presents an alternative hypothesis: cytokinins are produced exclusively by the microbial symbionts of plants.