Role of tonoplast aquaporins in Arabidopsis root development

Aquaporins (AQPs) are water channels allowing the fast and passive diffusion of water across cell membranes. It was hypothesized that AQPs contribute to cell elongation processes by allowing a fast water influx across the plasma membrane and the tonoplast in order to maintain the turgor pressure at a high level. Here, we show that, in Arabidopsis thaliana, the highly abundant tonoplast AQP isoforms TIP1;1, TIP1;2 and TIP2;1 contribute to the proper development of lateral roots by facilitating the emergence of the lateral root primordia (LRP). Whereas the single, double and triple tip mutants showed no or minor reduction in growth of the main root, the triple tip mutant exhibited a strong decrease in the number of lateral roots, which was due to a retardation of LRP emergence. This can be partially restored by the constitutive expression of TIP1:2. The expression pattern and location of TIP2;1 is strongly linked with LRP development. From these data, we concluded that the TIP1;1, TIP1;2 and TIP2;1 isoforms allow the spatial and temporal fine-tuning of the cellular water transport which is required during the highly regulated process of LRP development. In a side project, we studied the interaction of the Arabidopsis plasma membrane water channel PIP2;7 with the tryptophan-rich sensory protein (TSPO). TSPO is an endoplasmic reticulum and Golgi located protein involved in stress responsive heme scavenging and degradation. We confirmed their physical interaction by bimolecular fluorescence complementation and Forster resonance energy transfer. No direct impact of TSPO on the water channel activity and cellular trafficking of PIP2;7 was found when tested in Xenopus oocytes. The physiological consequence of the interaction still remains unknown.