The shoot apical meristem (SAM) gives rise to above-ground organs and its establishement and homeostasis are regulated by well-studied genes. Briefly, stem cells express CLAVATA3 (CLV3), which encodes a secreted peptide that binds to membrane-bound kinase receptors, leading to the repression of the transcription factor WUSCHEL (WUS), which is itself expressed in the underlying organizing center. In turn, WUS directly activates the expression of CLV3 and this subtle balance between the two molecules restrains the stem cell pool. The loss of CLV3 activity leads to an increase in SAM size, and conversely, the loss of WUS activity abolishes the SAM.
By coupling atomic force microscopy (to probe cell wall rigidity) and confocal microscopy (to determine cell identity), we showed that stem cell identity correlates both spatially and temporally with increased stiffness at the cellular level. We have since shown that clv3 mutants have mechanical and cellular defects, in addition to tissular defects. Surprisingly, the clv3 SAMs are soft, instead of being stiff, as expected for stem cells. Indeed, many studies consider the clv3 phenotype as the result of stem cell over-proliferation, but our data rather suggest that clv3 SAMs mechanically differ from stem cells. Additionally, RNA in situ hybridization of genes expressed in different subdomains of the SAM showed that clv3 SAMs have mixed identities.
In this context, we re-examined the CLV3-WUS dogma of stem cell homeostasis. We studied how SAM morphometric parameters are influenced by genetic and mechanical properties, which might be a new parameter to study stem cells.