Blue light-dependent phosphorylation of PILS proteins

Plants are constantly exposed to different environmental conditions and light is among the most relevant environmental signals. Plants grow towards a light source, exhibiting phototropism. Phototropism, originally studied by Charles and Francis Darwin, describes the organ growth in accordance with the direction of the light source. Already the work of Darwin suggested the presence of a light-dependent activation of a matter (later identified as the plant hormone auxin) in the upper part of the plant, which transmits its growth effects to the lower part. However until today, the source of auxin in the shoot tip remains unclear. We provide evidences that intracellular auxin transport facilitators of the endoplasmic reticulum localized PILS protein family function in cellular availability of auxin for non- cell autonomous auxin responses. Protein function is often modulated by small modifications, such as the addition of a phosphate. This phosphorylation is also very common to regulate transporters. We have already identified several such modifications in PILS proteins. Here, we investigate how the blue-light receptor PHOT1-dependent phosphorylation of PILS proteins influence cellular auxin efflux, which has developmental importance during phototropic responses. Moreover, we will approach how phosphorylation determines the auxin carrier function of PILS proteins. Finally, we will map the site of PHOT1/PILS-dependent cellular re-mobilization of auxin and study its mechanistic contribution to polar auxin transport rates. Our work will provide fundamental insight into the regulation of auxin homeostasis with a specific emphasis on light-dependency.

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