Environmental modulation of Arabidopsis light foraging strategies

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As ectothermic, sessile organisms that cannot choose their surroundings, plants must adapt their growth and development to the prevailing environmental conditions. Plants monitor their surroundings by integrating multiple stimuli, of which light and temperature signals are the most important. Light signals are perceived using a family of photoreceptors named phytochromes. Upon absorption of red wavelengths, phytochromes are converted to their active form which inhibits stem elongation and promotes leaf development. Absorption of far-red wavelengths, however, converts phytochromes back to their inactive form. When growing in crowded communities, plants compete with neighbouring vegetation for light to fuel photosynthesis. The presence of neighbouring vegetation is detected as a reduction in the ratio of red to far red wavelengths (low R:FR ratio) in the light reflected from or transmitted through green tissues. This change in light quality results in a reduction in active phytochrome in shaded plants and initiates a suite of elongation responses termed the shade avoidance syndrome, often at the expense of leaf development. These responses serve to elevate leaves towards gaps in the canopy and can be studied in the laboratory by growing plants in white light supplemented with far-red wavelengths. The majority of published experiments use growth temperatures in excess of 20oC to study shade avoidance responses. When grown at cooler temperatures, however, the leaf expansion response of Arabidopsis to low R:FR ratio is strikingly reversed. Plants grown in low R:FR ratio at 16oC display significantly increased leaf area, thickness and plant biomass when compared to high R:FR ratio-grown controls. Our observations suggest that Arabidopsis displays plasticity in light foraging strategy with the precedence of each strategy being determined by ambient growth temperature. At temperatures >20oC, Arabidopsis plants forage for light by raising and elongating leaves/petioles towards gaps in the canopy (shade avoidance syndrome 1, SAS1). At cooler temperatures, however, an alternative strategy is adopted, whereby plants dramatically expand leaf area and thickness, which is likely to increase photon capture (SAS2). In this proposal we aim to perform detailed analyses of leaf anatomy and photosynthetic performance in plants grown in high and low R:FR ratio across a broad spectrum of growth temperatures. We propose to investigate the possibility that different shade avoidance strategies are seasonal through experiments in unheated glasshouses in multiple seasons. The adaptive value of plasticity in shade avoidance strategy will be investigated through competition and fitness experiments using mutants and accessions displaying only SAS1. Natural genetic variation in shade avoidance strategy will be exploited using QTL mapping to identify regions of the genome responsible for such variation. This will ultimately enable the identification of key genes involved in this process. In addition, we have observed co-incidence of SAS2 and expression of the CBF regulon, a suite of genes involved in cold acclimation and acquisition of freezing tolerance. We therefore propose to map QTLs associated with low R:FR ratio-mediated induction of the CBF regulon and compare with QTLs identified for SAS2.