Role of Reactive Oxygen Species during Cell Expansion in Leaves1[OPEN]

The conditions under which plants grow greatly fluctuate and require that plants continuously monitor their environment and adjust their developmental program accordingly. Recent advances have indicated a clear and distinct role for reactive oxygen species (ROS) in both environmental stress sensing and guiding plant development. Leaf growth is a flexible process in which the final shape and size of the organ is tailored to the environment. Both during development under controlled conditions as well as during abiotic stress, cell expansion in leaves is in part controlled through the regulation of apoplastic ROS homeostasis. The effect of different ROS types on cell wall properties is well documented, but how plants control apoplastic ROS homeostasis is poorly understood. Furthermore, ROS appear to influence other cellular processes that guide cell expansion, includingwater uptake and cytoskeleton dynamics. Here, an overview of our current understanding on the role of ROS in leaf cell expansion is given and avenues for future research directions are highlighted. ROS have long been documented as damaging molecules that especially accumulate during stress in plants. Abiotic stresses, such as drought and salinity, are characterized by an initial growth reduction of leaves and the induction of programmed cell death under prolonged stress conditions (Loggini et al., 1999; Hernández et al., 2001). The notion that alterations in the growth rate of leaves correlate with ROS homeostasis have paved the way for a more fundamental role of ROS in the regulation of plant development. Although during abiotic stress ROS levels rise, this does not necessarily apply to the growth zone of the leaf. For instance, studies on leaf expansion in maize (Zea mays) under saline conditions revealed that not an increase in ROS but a decrease caused retarded leaf growth (Rodríguez et al., 2004).Moreover, exposure ofmaize to salinity or drought causes an increase in the antioxidant capacity of the leaf and thereby restricts cell expansion (Bernstein et al., 2010; Kravchik and Bernstein, 2013; Avramova et al., 2015). Then again, an increase in ROS levels also can result in a restriction of cell growth under abiotic stress conditions (MacAdam and Grabber, 2002; Simonovicova et al., 2004), indicating that ROS have a dual role in the regulation of cell expansion. Still, ROS homeostasis does not act alone, as abiotic stresses like salinity, drought, or osmotic stress all interferewith thewater balance and cause a reduction in cell turgor, which affects the mechanical power of the cell to expand (Schopfer, 2006). The initial observations under abiotic stress have attracted a broad research interest in the relationship between cell growth and ROS. Nowadays, it is clear that ROS homeostasis does not only control growth under stress conditions but also during plant development. ROS, like hydrogen peroxide (H2O2), hydroxyl radicals (_OH), and superoxide radicals (O2 ), have been implicated in developmental processes and emerged as key signaling molecules in plants (Schmidt and Schippers, 2015). For instance, during root hair and pollen elongation, ROS play fundamental roles in the spatial regulation of polar cell growth (Takeda et al., 2008; Kaya et al., 2014). Still, the role of ROS in the regulation of leaf development remains unclear. The leaf emerges at the flank of the shoot apical meristem and at first grows through active cell division (Beemster et al., 2005; Polyn et al., 2015; Schippers et al., 2016). The final size of the leaf is determined by the subsequent