Root phenotyping: from component trait in the lab to breeding.
Plant organ phenotyping by noninvasive video imaging techniques provides a powerful tool to assess physiological traits, circadian and diurnal rhythms, and biomass production. In particular, growth of individual plant organs is known to exhibit a high plasticity and occurs as a result of the interaction between various endogenous and environmental processes. Thus, any investigation aiming to unravel mechanisms that determine plant or organ growth has to accurately control and document the environmental growth conditions. Here we describe challenges in establishing a recently developed plant root monitoring platform (PlaRoM) specially suited for noninvasive high-throughput plant growth analysis with highest emphasis on the detailed documentation of capture time, as well as light and temperature conditions. Furthermore, we discuss the experimental procedure for measuring root elongation kinetics and key points that must be considered in such measurements. PlaRoM consists of a robotized imaging platform enclosed in a custom designed phytochamber and a root extension profiling software application. This platform has been developed for multi-parallel recordings of root growth phenotypes of up to 50 individual seedlings over several days, with high spatial and temporal resolution. Two Petri dishes are mounted on a vertical sample stage in a custom designed phytochamber that provides exact temperature control. A computer-controlled positioning unit moves these Petri dishes in small increments and enables continuous screening of the surface under a binocular microscope. Detection of the root tip is achieved by applying thresholds on image pixel data and verifying the neighbourhood for each dark pixel. The growth parameters are visualized as position over time or growth rate over time graphs and averaged over consecutive days, light-dark periods and 24 h day periods. This setup enables the investigation of root extension profiles of different genotypes in various growth conditions (e.g., light protocol, temperature, growth media) and is especially suited for the detection of diurnal or circadian growth rhythms.