How far-red light enrichment modulates tomato resistance towards Botrytis cinerea

Abstract

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.