Regulation of shoot apical meristem function by abscisic acid and hormone crosstalk

The shoot apical meristem (SAM) is the source of stem cells that allows to generate all the aerial parts of the plant, such as leaves, stems, or flowers. For proper SAM activity during plant development, it is necessary to maintain an equilibrium between stem cell proliferation and differentiation. This balance is controlled by the phytohormones cytokinin (CK) and auxin, which promote proliferation and differentiation, respectively. However, less is known about the contribution of the hormone abscisic acid (ABA) in meristem function. Confocal live imaging analyses of ABA response reporters in the SAM suggest that ABA may participate in the regulation of SAM homeostasis. Moreover, ABA response is spatially patterned in a complementary fashion to auxin and CK signalling, and the expression of auxin and CK response markers decreases in the SAM after ABA treatment, indicating that ABA may act antagonistically to these hormones. These experiments together with the characterization of ABA-related mutants suggest that ABA may promote the undifferentiated status of the stem cells in the SAM and repress organ initiation and development. This study provides the first clues about the potential role of ABA on the control of SAM maintenance and activity.