Rice leaves undergo a rapid metabolic reconfiguration during a specific stage of primordium development

Leaf development is crucial to establish the photosynthetic competency of plants. It is a process that requires coordinated changes in cell number and differentiation, transcriptomes, metabolomes and physiology. However, despite the importance of leaf formation for our major crops, early developmental processes for rice have not been comprehensively described. Here we detail the temporal developmental trajectory of early rice leaf development and connect morphological changes to metabolism. In particular, a developmental index based on the patterning of epidermal differentiation visualised by electron microscopy enabled high resolution staging of early growth for single primordium metabolite profiling. These data demonstrate that a switch in the constellation of tricarboxylic acid (TCA) cycle metabolites defines a narrow window towards the end of the P3 stage of leaf development. Taken in the context of other data in the literature, our results substantiate that this phase of rice leaf growth, equivalent to a change of primordium length from around 5 to 7.5 mm, defines a major shift in rice leaf determination towards a photosynthetically defined structure. We speculate that efforts to engineer rice leaf structure should focus on the developmental window prior to these determining events.