Determination of the unequal fate of cotyledons of a one-leaf plant, Monophyllaea

One-leaf plants, belonging to the family Gesneriaceae, were described for the first time more than 150 years ago. One such unusual plant, Monophyllaea, has only one leaf at maturity. Only one of the two cotyledons grows continuously, without the formation of true leaves, and this feature, known as anisocotyledonous development, has been repeatedly mentioned in textbooks of plant morphology. However, the mechanism for the determination of the one-leaf phenotype remains to be ascertained. In this study, meristematic regions were identified, by monitoring DNA synthesis, at the base of both cotyledons just after germination, while no such regions were found in the shoot apex. Surgical experiments with seedlings and analysis of the anisocotyledonous development revealed that the fate of the cotyledons is determined during their growth. Anisocotyledonous development seems to be the result of competition between the two cotyledons. The mechanism that governs the development of the shoot in the genus Monophyllaea is discussed in relation to apical dominance, which is the common mechanism that regulates shoot development in many plants.