Shade avoidance : how auxin controls photoreceptor-mediated shoot elongation

Abstract

Plants perceive the threat of competing neighbours through various signals. They carry sophisticated photoreceptor systems to signal this and subsequently activate an interacting network of various hormones and transcriptional regulators. The complete signal detection and signal transduction network together defines the SAS to be expressed. The aim of the studies in this PhD thesis is to elucidate the hormonal regulation of auxin in the elongation response as part of the SAS. Understanding this mechanism is of interest, not only from a fundamental point of view, but also from an applied point of view. Understanding the hormonal regulation of the SAS could be a target to improve harvest yield of commercial crops. In chapter 2 the blue light-mediated shade avoidance of Arabidopsis seedlings is described. This elongation response is only partly under the control of auxin and requires combined auxin and brassinosteroid action. In this Chapter we investigated cell wall modifying proteins, which are known to regulate cell elongation, as possible regulator of shade avoidance by both auxin and brassiniosteroids. We show that different XTHs were regulated during blue light-mediated shade avoidance and that the requirement of both auxin and brassinosteroid action could be explained by the different XTHs they regulated in the elongation response. Blue light is not the only light signal that can induce shade avoidance in Arabidopsis seedlings. Also reduction of the R:FR induced hypocotyls to elongate, as we show in chapter 3. The data show that, unlike blue light-mediated shade avoidance, auxin action explains the full response. We demonstrate that PIN3, a regulator of polar auxin transport, changes its cellular location in the hypocotyl towards the lateral side of the endodermal hypocotyl cells upon the FR treatment. As a consequence the direction of auxin transport is changed and results in auxin accumulation throughout over the hypocotyl thus inducing elongation. In Chapter 4 the role of auxin (transport) in shade avoidance of full grown plants is studied, not only as a result of changes in light quality but also in dense stand experiments. To examine this, the pin3-3 mutant which lacks the elongation response upon the FR treatment in seedlings, was used. Petioles of pin3-3 are shown to have no response upon FR-treatment but also a delayed response to neigbouring plants. This enabled us to further investigate the role of auxin transport in the response to neighbouring plants but also the functional significance of the SAS for rosette species such as Arabidopsis. Chapter 5 shows how the cellular localization of PIN3 is dependent on intact microtubule (orientation). Disrupting the microtubules inhibits the expression levels of the cell wall modifying protein family of XTHs which are needed for the shade avoidance response of Arabidopsis petioles, through the inhibition of polar auxin transport.