The specification and maintenance of growth sites are tightly regulated during cell morphogenesis in all organisms. ROOT HAIR DEFECTIVE 2 reduced nicotinamide adenine dinucleotide phosphate (RHD2 NADPH) oxidase-derived reactive oxygen species (ROS) stimulate a Ca2+ influx into the cytoplasm that is required for root hair growth in Arabidopsis thaliana. We found that Ca2+, in turn, activated the RHD2 NADPH oxidase to produce ROS at the growing point in the root hair. Together, these components could establish a means of positive feedback regulation that maintains an active growth site in expanding root hair cells. Because the location and stability of growth sites predict the ultimate form of a plant cell, our findings demonstrate how a positive feedback mechanism involving RHD2, ROS, and Ca2+ can determine cell shape.

Reactive oxygen species (ROS) are emerging as important regulators of plant development. There is now abundant evidence that ROS play roles in cell growth and that spatial regulation of ROS production is an important factor controlling plant form. Here we will review evidence that supports a role for ROS in development, but first we will define what we mean by development. The body of the vascular plant sporophyte (diploid life cycle stage) is derived from meristems and much of the action of development occurs where organs are formed in and around meristems (Martienssen and Dolan, 1998). Organogenesis, the development of organs, involves an early patterning stage that roughs out boundaries where the organs will form. Within these boundaries, groups of founder cells divide and growth occurs, leading to the formation of structures containing arrays of differentiated cells. There are two major contributors to growth: First, there is cell division, which increases the number of cells in an organ; second, there is expansion of those cells. The huge increase in volume in plant cells as they transit from the founder population in and around meristems to the differentiated cells of the mature organ means that cell growth is an important component of plant development (Sugimoto-Shirasu and Roberts, 2003). Recent discoveries suggest that ROS may control development through their role in regulating cell growth. NADPH OXIDASES GENERATE ROS INVOLVED IN DEVELOPMENTROS that have been shown to play a role in development are produced by NADPH oxidases (NOXs) that generate the superoxide radical (O 2 c 2

The expression of 28 high light (HL)-responsive genes of Arabidopsis was analysed in response to environmental and physiological factors known to influence the expression of the HL-responsive gene, ASCOR-BATE PEROXIDASE2 (APX2). Most (81%) of the HL-responsive genes, including APX2, required photosynthetic electron transport for their expression, and were responsive to abscisic acid (ABA; 68%), strengthening the impression that these two signals are crucial in the expression of HL-responsive genes. Further, from the use of mutants altered in reactive oxygen species (ROS) metabolism, it was shown that 61% of these genes, including APX2, may be responsive to chloroplast-sourced ROS. In contrast, apoplastic/ plasma membrane-sourced H 2 O 2 , in part directed by the respiratory burst NADPH oxidases AtrbohD and AtrbohF, was shown to be important only for APX2 expression. APX2 expression in leaves is limited to bundle sheath parenchyma; however, for the other genes in this study, information on their tissue specificity of expression is sparse. An analysis of expression in petioles, enriched for bundle sheath tissue compared with distal leaf blade, in HL and control leaves showed that 25% of them had >10-fold higher expression in the petiole than in the leaf blade.However, this did not mean that these petiole expression genes followed a pattern of regulation observed for APX2.