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 Ca ²⁺ influx into the cytoplasm that is required for root hair growth in Arabidopsis thaliana . We found that Ca ²⁺ , 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 Ca ²⁺ can determine cell shape.

In flowering plants, pollen germinates on the stigma and pollen tubes grow through the style to fertilize the ovules. Enzymatic production of reactive oxygen species (ROS) has been suggested to be involved in pollen tube tip growth. Here, we characterized the function and regulation of the NADPH oxidases RbohH and RbohJ (Respiratory burst oxidase homolog H and J) in pollen tubes in Arabidopsis thaliana. In the rbohH and rbohJ single mutants, pollen tube tip growth was comparable to that of the wild type; however, tip growth was severely impaired in the double mutant. In vivo imaging showed that ROS accumulation in the pollen tube was impaired in the double mutant. Both RbohH and RbohJ, which contain Ca²⁺ binding EF-hand motifs, possessed Ca²⁺-induced ROS-producing activity and localized at the plasma membrane of the pollen tube tip. Point mutations in the EF-hand motifs impaired Ca²⁺-induced ROS production and complementation of the double mutant phenotype. We also showed that a protein phosphatase inhibitor enhanced the Ca²⁺-induced ROS-producing activity of RbohH and RbohJ, suggesting their synergistic activation by protein phosphorylation and Ca²⁺. Our results suggest that ROS production by RbohH and RbohJ is essential for proper pollen tube tip growth, and furthermore, that Ca²⁺-induced ROS positive feedback regulation is conserved in the polarized cell growth to shape the long tubular cell.

Summary: The establishment of organ boundaries is a fundamental process for proper morphogenesis in multicellular organisms. In plants, the shoot meristem repetitively forms organ primordia from its periphery, and boundary cells are generated between them to separate their cellular fates. The genes CUP‐SHAPED COTYLEDON1 (CUC1) and CUC2, which encode plant‐specific NAC transcription factors, play central roles in establishment of the shoot organ boundaries in Arabidopsis thaliana. Here we show that CUC1 protein activates expression of LIGHT‐DEPENDENT SHORT HYPOCOTYLS 4 (LSH4) and its homolog LSH3 in shoot organ boundary cells. Both genes encode nuclear proteins of the Arabidopsis LSH1 and Oryza G1 (ALOG) family, the members of which are widely conserved in land plants. Expression of LSH4 and LSH3 is detected in the boundary cells of various shoot organs, such as cotyledons, leaves and floral organs, and requires the activity of CUC1 and CUC2. Experiments using the glucocorticoid receptor system indicate that transcription of LSH4 and LSH3 is directly up‐regulated by CUC1. Constitutive expression of LSH4 in the shoot apex causes inhibition of leaf growth in the vegetative phase, and formation of extra shoots or shoot organs within a flower in the reproductive phase. Together, our results indicate that CUC1 directly activates transcription of the nuclear factor genes LSH4 and LSH3, which may suppress organ differentiation in the boundary region.