Plant Organ Primordia

Abstract Plant organ primordia are central to defining the shape and success of the plant through ever‐changing environmental conditions. The shoot (SAM) and root (RAM) apical meristems, which are composed of totipotent cells, generate the major plant organs including leaves, flowers and roots by initiating primordium formation. While the process of organ generation is unique to each meristem, major overarching themes have been observed, including important hormonal and gene interactions. The phytohormones auxin and cytokinin work to establish organ formation and define overall organ positioning, while homeotic genes work with the hormones to maintain stem cell fate and define meristem structure. These interactions, in conjunction with environmental pressures, govern the aerial and terrestrial form of the plant. In perennial plants, seasonal cues alter primordia development resulting in shifts between growth and growth suppression or senescence. The mechanisms and signals underlying these transitions remain unclear. Key Concepts Apical meristems are complex plant organs that give rise to above‐ and below‐ground tissues that define the overall plant structure. Interaction between members of the WUSCHEL and CLAVATA gene families regulate growth of the apical meristem. Hormone interaction between auxin and cytokinin initiates formation and determines position of aerial organs through hormonal gradients and inhibitory fields. Gravistimulation and auxin oscillation determine positioning of future lateral roots by priming cells, but environmental factors dictate whether initiation occurs. Shoot apical meristems experience vegetative and reproductive phases in which they produce either leaves or reproductive meristems. Perennial plants also experience a period of quiescence known as dormancy, which is induced by external cues such as day length and temperature. As shoot apical meristems prepare for dormancy, they switch from production of leaves and flowers to production of bud scales. Entrance into dormancy is regulated by an increase in abscisic acid, while dormancy release is accompanied first by an increase in cytokinin activity followed by an increase in gibberellic acid metabolism.