Neighbour detection and pathogen defence during competition for light

In dense vegetation plants have to compete for light. Plants at risk of becoming overgrown by neighbours therefore respond with an escape response called the shade avoidance syndrome, a suite of responses that serve to ensure light capture by the photosynthesizing organs. These include upward movement of the leaves (hyponasty) and elongation of stems or petioles. In order to respond adequately to neighbours, plants use changes in the light quality to perceive the presence of proximate vegetation. The earliest known neighbour detection signal is a decrease in red:far-red (R:FR) light. In this thesiswe studied competition for light in dense stands of the model plant Arabidopsis thaliana. We show that in these Arabidopsis canopies hyponasty in response to neighbours occurs prior to a considerable decrease in R:FR. The early hyponastic response was induced by touching of leaves from neighbouring plants and we introduce touch as a new neighbour-derived signal to induce shade avoidance responses. Depletion of blue light is another light signal that has been implicated in shade avoidance and occurred at a later stage of competition in dense stands of Arabidopsis. We show that perception of reduced blue fluence rates has an additive effect on shade avoidance responses to low R:FR. The co-action between low R:FR and low blue perception was found to be dependent on the plant hormones auxin and brassinosteroid, but also requires other yet unidentified players. Apart from competition for light, plants growing at high density may simultaneously have to cope with other environmental stresses such as pathogen attack. We studied the interaction between shade avoidance and plant immune responses in Arabidopsis. It was shown that plants become more susceptible to pathogens when they express shade avoidance responses. Induction of a shade avoidance response, however, was not affected in plants with a simultaneously activated immune response. This was found to hold true for two different immune responses (salicylic acid- and jasmonic acid-dependent) and indicates that competition for light is prioritised over defence against pathogens. The interaction between shade avoidance and salicylic acid-dependent pathogen defence was found to involve enhanced nuclear localization of the transcriptional co-activator NON-EXPRESSOR OF PR GENES (NPR)1, which normally leads to enhanced expression of SA-induced genes. The repression of SA-associated transcription during shade avoidance is possibly regulated through reduced phosphorylation of NPR1 that is necessary for full induction of the transcription cycle. Plant stress responses are mostly studied in isolation. The repression of plant immune responses in plants that simultaneously have to compete for light and defend against pathogens illustrates the necessity to study plant stress responses in combination