Water deficit and growth. Co-ordinating processes without an orchestrator?

► Water deficit reduces carbon accumulation, cell number and tissue expansion. ► Tissue expansion is loosely co-ordinated with cell division and carbon accumulation. ► The co-ordination between carbon accumulation, cell number and tissue expansion results from feedbacks between parallel processes and common mechanisms for several organs. ► Absence of a central co-ordination would have profound implications for plant modelling and plant breeding in dry environment. Water deficit affects plant growth via reduced carbon accumulation, cell number and tissue expansion. We review the ways in which these processes are co-ordinated. Tissue expansion and its sensitivity to water deficit may be the most crucial process, involving tight co-ordination between the mechanisms which govern cell wall mechanical properties and plant hydraulics. The analyses of sensitivities, time constants and genetic correlations suggest that tissue expansion is loosely co-ordinated with cell division and carbon accumulation which may have limited direct effects on growth under water deficit. We therefore argue for essentially uncoupled mechanisms with feedbacks between them, rather than for a co-ordinated re-programming of all processes. Consequences on plant modelling and plant breeding in dry environment are discussed.