Vacuolar proton-conducting ATPases (V-ATPases) are key components in the control of pH and ionic homeostasis in eukaryotes. They feature centrally in plant cell biology because, in plants, proton gradients support the bulk of secondary active transport of solutes and ions. However, genetic analysis of V-ATPase function in plants has been lacking. The recent paper by Schumacher et al.1xThe Arabidopsis det3 mutant reveals a central role for the vacuolar H(+)-ATPase in plant growth and development. Schumacher, K. et al. Genes and Development. 1999; 13: 3259–3270Crossref | PubMed | Scopus (200)See all References1, opens new horizons in the field, demonstrating not only the central role of V-ATPases in plant growth, but also unravelling strong, if perhaps unexpected, connections between the V-ATPase and light regulated developmental pathways.deetiolated3 (det3) is one of twenty monogenic mutants in Arabidopsis that show aberrant activation of light morphogenetic pathways in the dark. This paper shows that det3 plants also show conditional defects in cell expansion. Positional cloning of the det3 gene demonstrates that it encodes subunit C of the V-ATPase, which is believed to be required for holoenzyme assembly. det3 mutants show a twofold reduction in subunit C mRNA levels as a result of a splicing defect. The authors show that the reduction in holeonzyme activity can be either 60% or 30%, depending on growth conditions. This indicates that the assembly efficiency of subunit C is enhanced under specific circumstances and that it could be a control point for rapid changes in holoenzyme activity. How then does the elaborately controled V-ATPase fit into the pathways of light-regulated development? det3 mutants show reduced sensitivity to Brassinosteroids (BRs), a class of plant growth regulators that have a key role in light-dependent development. An exciting possibility is that BRs act through the V-ATPase to rapidly regulate cell expansion by exploiting the different modes of holoenzyme assembly. This paper will allow researchers to pose novel questions regarding the organization of BR-signalling pathways in plants and the intricacies of V-ATPase regulation and function.