Class III compensation, represented by KRP2 overexpression, depends on V-ATPase activity in proliferative cells.

Compensation refers to an increase in cell size when the cell number is significantly decreased due to the mutation or gain of function of a gene that negatively affects the cell cycle. Given the importance of coordinated growth during organogenesis in both animal and plant systems, compensation is important to understand the mechanism of size regulation. In leaves, cell division precedes cell differentiation (which involves cell expansion); therefore, a decrease in cell number triggers enhanced cell expansion (compensated cell expansion; hereafter, CCE). Functional analyses of genes for which a loss or gain of function triggers compensation have increased our understanding of the molecular mechanisms underlying the decrease in cell number. Nevertheless, the mechanisms that induce enhanced cell expansion (the link between cell cycling and expansion), as well as the cellular machinery mediating CCE, have not been characterized. We recently characterized an important pathway involved in cell enlargement in KRP2-overexpressing plants. Here, we discuss the potential axial role of plant KRPs in triggering enlargement in cells with meristematic features.