BACKGROUND AND AIMS: The Shoot Apical Meristem (SAM) plays a key role in developmental biology by generating all aerial organs. Most research has focused on model plants like Arabidopsis thaliana, but studying plants in arid environments, such as cacti, can reveal evolutionary insights. Cacti have large SAMs that correlate with stem size. This study examines the development of the SAM in Mammillaria san-angelensis and explores the relationship between SAM size and plant allometry during development. METHODS: We analyzed plant morphology, specifically hypocotyl and stem dimensions, along with five SAM anatomical traits across eight developmental stages, from one day post-germination to a 4-year-old plant, using conventional microscopy and anatomical techniques. We compared the vegetative SAM, floral meristem, and areole meristem, and performed correlation analysis to link SAM with plant traits. KEY RESULTS: We identified key developmental processes during the early stages of plantlet growth, including the morphogenesis of SAM and the initiation of dimorphic areole formation. We noted the increase in size and the change from two to three layers in the SAM, along with alterations in its dome formation. We found that the size of the SAM can explain the allometric growth stem width and length. CONCLUSIONS: The correlation of larger SAM with larger stem seen in adult cacti across the Cactaceae family was recapitulated during the development of a single species (M. san-angelensis). Altogether, we set the developmental framework for future genetic research to characterize the evolutionary bases of SAM size increase, the meristem variants of M. san-angelensis, and the power of their understanding to explain the origins of gigantism, succulence and the diversity of shapes and sizes in the cacti family.
López-Ruíz et al. (Thu,) studied this question.