Juvenile-to-adult phase transition in a common wheat cultivar Norin 61, and accompanying changes in leaf transcriptome

Higher plants experience morphological and physiological changes during the vegetative stage called juvenile-to-adult (JA) phase transition. Despite the advanced studies in Arabidopsis, maize, and rice, the JA phase transition remains unexplored in wheat. This study aimed to elucidate when and how the transition occurs in wheat by investigating the temporal changes in leaf morphology, expression of its regulators, transcriptome, and photosynthetic activity in the common wheat cultivar Norin 61. As a result, leaf blade size, leaf tip shape, and trichome density on leaf blades exhibited major changes from the first to second leaf stages. The expression level of microRNA 156, a regulator of JA phase transition in plants, was the highest in the first leaf stage and decreased following the plant growth, whereas that of its targets, SQUAMOSA PROMOTER BINDING PROTEIN-like (SPL) genes, increased. Additionally, transcriptome profiles dramatically changed from the second to third leaf stages and from the fourth to fifth leaf stages, which could be characterized by the change in activity of photoreactions, material transport, and phytohormone signaling. Unlike rice, wheat showed high photosynthetic rates per unit area even in the first leaf, which may be a unique and noteworthy characteristic in wheat. Taken together, we conclude that wheat initiates the JA phase transition after the first leaf stage and reaches the adult phase before the fourth leaf stage; it subsequently enters the reproductive stage. The present study will provide a foundation for advanced studies on wheat JA phase transition.