Lectures in Plant Developmental Physiology, 2 cr. Department of Biological and Environmental Sciences Plant Biology Viikki Biocenter Spring 2006

Developmental Responses to Light Lecture 8 Cercis siliquastrum

Development of Arabidopsis seedling is strongly dependent on light

Light perception phytochromes, cryptochromes, phototropins. all photoreceptors consist of proteins bound to light absorbing pigments i.e. chromophores. the spectral sensitivity of each photoreceptor depends on its chromophore’s ability to absorb different wavelengths i.e. on the chromophore´s absorption spectrum. in response to light absorption, downstream signaling is mediated by the photoreceptor protein.

Phytochromes  120 kDa protein family  TKDs transmitter kinase domains  HKLD prokaryotic histidine kinase like domain (Quail 2002)

Phytochrome response modes- photoreversibility

Classical LFR reaction Phytochrome is synthesized as Pr i.e. seeds and seedlings grown in total darkness contain only this isomer. Responses promoted by a few minutes of dim light are prevented by subsequent brief exposure to dim far red light. LFR=Low fluence red/far red response

ARABIDOPSIS has 5 phytochrome genes: PHYA-E Phytochrome A is light labile and it is the predominant phytochrome present after prolonged dark periods in both imbibed seeds and seedlings. Phytochromes B-E are light stable and are the major type in light-grown plants.

Dark Light

Developmental responses to light Mosses, liverworts, ferns and some gymnosperms (e.g. conifers) show similar development in light and darkness. In angiosperms, gametophytic and embryonic development are largely insensitive to light. Light regulates development at all other stages of the life cycle of angiosperms. photomorphogenesis phototropism photoperiodism

Schäfer & Bowler EMBO reports 3: ” in all cases regulation of genes responsible for photomorphogenesis is predicted to require chromatin remodelling mediated by DET1/DDB1 nucleosome-binding complex” DTT=de-etiolated

Light induced germination is largely mediated by phytochromes In darkness: seedlings adopt an etiolated morhology: Rapid elongation of the hypocotyl or epicotyl, apical hook formation. In light: massive cell elongation in radicle and shoot coordinated with the mobilization of food reserves in the seed. Opening of the apical hook and growth of the cotyledons.

Phytochrome reponses HIR, high irradiance reaction, is not photoreversible. Also other UV-A and blue light photoreceptors are involved. –anthocyanin biosynthesis –inhibition of hypocotyl elongation growth LFR, low fluence response, classical example of reversible red / far-red reaction. PhyB. VLFR, very low fluence response, not reversible.

Schäfer & Bowler EMBO reports 3: VLFR=Very Low Fluence Response LFR=Low Fluence Response R-HIR=Red High Irradiance Response FR-HIR= Far red High Irradiance Response

Arabidopsis – seed germination some newly imbibed seeds germinate in darkness and this is promoted by low amounts of red light (low fluence). after several days in darkness they come dramatically more sensitive to light and will germinate in response to a broad spectrum of radiation (from UV-B to far-red) provided by VLFR. The extreme light sensitivity develops because of the accumulation of high quantities of phyA in the Pr form during prolonged darkness > germination is promoted even if a tiny fraction of the accumulated PrA is converted into PfrA. PrA displays some absorption over the whole spectrum > very low fluence of any wavelength may induce germination. more than one photoreceptor control germination which is supported by mutant studies.

germination in darkness requires phyB (in its Pfr B form). LFR promoted germination requires phyB. VLFR promoted germination requires phyA. Darkness Very low fluence illumination Low fluence red light

Arabidopsis – seed germinate in a wide range of environments Germination in darkness i.e. some seeds always germinate when temperature and soil moisture allow. Promotion of germination by red light, i.e. enhancement of germination on the soil surface in sunlight, which has a high ratio red / far-red light. PhyA mediated germination: –Buried seeds with light flashes –Buried seeds just below soil surface –Seeds on soil surface but beneath a heavy canopy

Gibberellins and seed germination Phytochromes mediate the effects of gibberellins on germination by influencing gibberellin biosynthesis and sensitivity. Red light induces and far red light represses transcription of the GA4 and GA4H genes of Arabidopsis.

Interaction of phytochromes and gibberellin

Seedling etiolation and photomorphogenesis: Etiolation is an adaptation to germination below the soil surface Etiolation i.e. skotomorphogenesis Photomorphogenesis i.e. de-etiolation

Light perception by the seedling Photomorphogenesis can be induced by a broad spectrum of illumination and full de-etiolation requires continuous illumination. Some aspects such as changes in gene expression or the inhibition of hypocotyl elongation may be induced by brief pulses of light. Signal transduction downstream of the photoreceptors induces photomorphogenesis in two ways. –Photosynthetic genes may be activated by direct positive regulation downstream of light perception –Chromatin remodelling –Light inducing signalling inactivates negative regulators of photomorphogenesis.

Photomorphogenesis is promoted by continuous red light, far red light and UV-A/ blue light in wild type seedlings. For the response to red light phyB is needed, for far red light phyA is needed, for UV-A / blue light cry1 is needed

Cop/det/fus mutants show some degree of photomorphogenesis in the dark det = de-etiolated cop = constitutively photomorphogenic Fus = fusca At least 11 mutants display a whole suite of photomorphogenic characteristics i.e. the mutants are pleiotropic Most of the Cop/det/fus mutants were identified by more than one mutant screen which means that the genes have more than one name.

Negative regulators of photomorphogenesis Blue light Red light DET = de-etiolated COP = constitutively photomorphogenic Wild type det/cop mutant

COP9 complex or signalosome (CSN) Some of the COP/DET/FUS genes, including at least COP8, COP9, FUS5 and FUS6 encode components of a multisubunit protein complex. COP9 was the first member of the complex to be identified. Biochemically COP9 is a multifunctional regulator of protein turnover.

In darkness COP1 is in the nucleus, where it accelerates the proteolysis of the HY5 transcription factor. In the light COP1 is in the cytoplasm. COP1 is constitutively cytoplasmic in det1 mutants and in mutants that lack COP9 complex.

Schäfer & Bowler EMBO reports 3: ” in all cases regulation of genes responsible for photomorphogenesis is predicted to require chromatin remodelling mediated by DET1/DDB1 nucleosome-binding complex”

COP1 the movement of COP1 may be a mechanism for the maintenance rather than initiation of photomorphogenesis COP1 is a ubiquitin/protein ligase that acts by attaching ubiquitin. Important target is HY5 =transcription factor ELONGATED HYPOCOTYLS The activity of HY5 and the extent of photomorphogenesis is regulated by: –Gene is transcribed at a greater rate in the light than in the dark –HY5 protein is phosphorylated in darkness causing a reduction in activity –HY5 protein has a shorter half-life in darkness due to more rapid ubiquitination of HY5 by COP1.

Phototropism due to directional illumination Shoots are positively and roots negatively phototropic. Leaves have more complex phototropic responses that affect both the position and orientation of lamina. This allows the formation of leaf mosaics in which mutual shading between leaves in canopy is minimized. Phototropism normally occurs through differential cell expansion. Cell expansion on illuminated side of the stem decreases and on the shaded side often increases.

Chloroplasts move to maximize or minimize the absorption of light Cross-section through the leaf of Arabidopsis thaliana dim light bright light

Chloroplast movement in green algal genus Mougeotia

Perception of light and signal transduction in phototropism Primarily mediated by blue/UV-A photoreceptors. Requirement for auxin signalling: –Auxin redistribution (Cholodny-Went hypothesis) and or changes in auxin sensitivity.

Cholodny-Went theory

Photoperiodic control of flowering control of flowering by daylength. Long-day plants: flower when days are long & nights short and photoperiods above a critical length (facultative LDPArabidopsis). Short-day plants: flower when days are short & nights long and photoperiods below a critical length (obligate SDP soybean). Day-neutral plants: photoperiod not a factor in flowering (tobacco). The photoperiodic control of flowering ensures that flowers are produced in a favourable season and allows floral synchronization in local populations leading to more efficient cross- pollination.

Measuring the photoperiod Measuring the photoperiod, importance of night length

Exposing only particular sections of the shoot to inductive photoperiods suggests that perception of night length occurs in mature leaves and inductive photoperiods stimulate mature leaves to produce positive or negative flower promoting signal (florigen).

A function of Circadian clock in the photoperiodic response

Photoreceptors and circadian clock in the regulation of CO expression

Light sets an inducible endogenous rhythm in relation to a treshold for response. When this inducible phase is above the treshold and coincides with light detection a photoperiodic response is induced or repressed. The external coincidence model of photoperiodism