Biophysical mechanisms for morphogenetic progressions at the shoot apex

Leaf primordia, first visible as small bumps, are produced in a cyclical pattern at the edges of the shoot apex, a smooth region at the top of the stem. Their formation is a biomechanical process. This review first considers hypothetical construction mechanisms and then summarizes research that provides information about how and where the primordia are made. Studies of growth at the primordium site indicate the importance of growth parallel to the surface in generating the forces for primordium emergence. The symmetry of the pattern of reinforcement by cellulose microfibrils correlates with the subsequent pattern of primordium production. Finite element models of the apex reveal that lateral bulging of the apex results in a gradient of shear stress, with high shear at the future primordium site. In contrast, tension parallel to the surface is lowest at the primordium site. Response of apical surface tissue to punctures indicates that an existing primordium can exert a pulling force tangential to its base and a compressive force perpendicular to its base. These observations lead to identification of a continuous biophysical cycle for apex morphogenesis, in which most of the steps are direct physical consequences of the previous step. Biophysical processes, subject to input from genetic, hormonal, and environmental sources, are thus involved in the construction and patterning of leaf primordia.