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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.

Light is essential for plant growth. Red and blue light are the major wavelength which are absorbed by leaf tissue and used for photosynthesis while others, i.e yellow and green light are reflected giving the plants their natural color. Far-red light (FR), which is invisible to the human eye, is transmitted by green plant tissues and constitute an important signal for plants to sense neighboring vegetation. At high planting density, the absorption of red light and the reflection of far-red light lead to a decrease in the ratio between red and far-red light (R:FR). Upon detection of a low R:FR, plants elongate and move their leaves upwards (hyponasty) in order to outcompete their neighbors and capture more light above the canopy. These changes in plant architecture called “shade avoidance” have been shown to enhance plant susceptibility to herbivores and pathogens including B. cinerea. In this thesis, we investigated how low R:FR can manipulate defense responses of tomato plants towards B. cinerea. The addition of FR LEDs to a white light background or a red and blue light background could change tomato morphology leading to stem and petiole elongation; typical traits of shade avoidance. The addition of FR also led to an increase in susceptibility to B. cinerea when perceived before the inoculation occurred. Through an RNA sequencing analysis, we investigated how FR exposure interferes with the plant’s capacity to defend itself against B. cinerea following a 30-hour time course. We found that the increased susceptibility caused by FR-enrichment relied on a delay in pathogen detection and in plant defense activation compared to control conditions. Plant defense against B. cinerea are known to be mainly based on the production of plant defense hormones, namely jasmonic acid and ethylene. Our data revealed that FR delayed the induction of a set of six genes known to be regulated by those two hormones. These genes, known as PROTEINASE INHIBITOR (PI) genes. We validated that jasmonic acid and ethylene are both needed for the full induction of PI which did not occur after FR exposure. The lack of PI induction in FR-treated plants could point at a decrease in hormone sensitivity, which could result in a delay in plant defense activation against B. cinerea. The RNA sequencing results also hinted at altered carbohydrate metabolism and we hypothesized that FR might cause changes in the sugar status of the plant indirectly promoting the growth of the fungus in plant tissue. Plants experiencing FR light accumulate more monosaccharides (glucose and fructose) than plants in control conditions which was correlated with a faster development of the fungus. This sugar-mediated susceptibility was also observed when only a single leaflet was exposed to FR. Soluble sugar levels were increased at the site of illumination but also in older leaves (located below) which did not receive FR themselves. Interestingly, wherever an increase in soluble sugars was observed, also an increase in plant susceptibility was found, suggesting that soluble sugars play a role in the FR-induced susceptibility in tomato.

Shade avoidance responses are suit of adaptive reactions for gaining sufficient light by plants grown in shading conditions. This phenomenon consists of enhanced elongation of shoot-like organs, upward movements of leaves and accelerated flowering and fruit-set. Adaptive reactions result from the activity of many plant growth regulators. Our study was focused on DELLA proteins which are components of gibberellin (GA) plant hormone signaling in regulating shade avoidance of Arabidopsis petioles and hypocotyls. We confirmed the involvement of these proteins in GA-regulated shade avoidance and showed interaction of these proteins with auxin and ethylene which partly regulates shade avoidance responses.