Leaf movement: auxin-mediated light signalling over spatial scales

Light is vital for plants as it powers the photosynthesis that provides them with the energy to grow. In dense vegetation, light absorption by neighbours limits the available light to individual plants. Therefore, plants adapt their growth to optimize light capture is dense vegetation. This adaptive growth includes elongation and upward movement of stems and leaves and serves to elevate the leaves towards the light. These so-called shade avoidance responses are regulated through signalling of wavelength-specific photoreceptors. As red (R) and blue light are absorbed by leaves and used in photosynthesis, while other wavelengths such as green and far-red (FR) are not, the spectral composition changes in shade. In addition, even before actual shading occurs, the light spectrum changes through specific horizontal reflection of FR from the leaves. The specific absorption of R and reflection of FR leads to a reduction in the ratio of R/FR light in a vegetation. These changes are mimicked by changes in the activity of the phytochrome B (phyB) photoreceptor. In sunlight, where the R/FR is high, phytochrome mainly exists in it’s active, growth-repressing form. Reductions in R/FR lead to reduced phytochrome activity which alleviates the repression of growth. When rosette plants such as Arabidopsis thaliana (Arabidopsis) are grown in close proximity to their neighbours, they will first detect those neighbours at their outermost leaf tips through horizontal FR reflection. Here we describe that FR enrichment at the leaf tip of Arabidopsis leads to upward bending, or hyponasty, at the base of the leaf stalk, the petiole. The distance between the sensing and bending part of the leaf suggests long distance light signalling. We discovered that directed transport of the plant hormone auxin relays the light signal from tip to petiole base, where differential elongation between the two sides of the petiole causes the leaf to bend upwards. We reveal that auxin is synthesised in the leaf tip in response to FR enrichment and transported towards the abaxial, lower, side of the petiole via PIN-FORMED auxin transport proteins. In the abaxial petiole, auxin stimulates epidermal cell growth in a process that requires a second growth-promoting plant hormone, gibberellin. We show that this dual hormonal regulation is necessary for hyponastic leaf movement in response to light. Our results reveal how plants can spatially relay information about neighbour proximity from their sensory leaf tips to the petiole base, thus driving adaptive growth