Beyond the norm: Gigantism, monstrosities, and growth control in cacti meristems

Cacti are a diverse group of succulent plants that have captivated botanists and enthusiasts alike for centuries. Their unique characteristics and adaptations to arid environments make them fascinating study subjects. One important reason to study cacti is the particular changes in the development of the shoot apical meristem (SAM), such as the growth of giant SAMs, determinacy, fasciation, and dichotomous branching. Studying these processes in cacti might provide valuable insights into the underlying genetic and developmental processes governing normal and abnormal growth and a deeper appreciation for their adaptability in challenging environments. This review will attempt to shed light on the distinctive cacti SAM patterns and discuss how “abnormal” growth patterns might have originated and given rise to evolutionary novelties. Additionally, we will explore tentative molecular pathways and genetic factors that regulate the division and differentiation of cacti SAM cells using data primarily from the model plant Arabidopsis thaliana. Most relevant findings show that the zonation in cacti seedlings after germination is incomplete, contrasting with what is observed in A. thaliana. Additionally, larger SAMs are believed to be linked to larger zones. WUSCHEL (WUS), CLAVATA (CLV), and SHOOT MERISTEMLESS (STM) are promising candidates that may help explain various phenomena in the cacti SAM. To understand the morphogenetic mechanisms that establish zonation and size in cacti, it is necessary to search for orthologues and employ molecular biology techniques, such as complementation analysis.