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LECTURE 4:PHOTOTROPISM

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LECTURE FLOW1. INTRODUCTION2. DEFINITION3. INITIAL STUDY4. PHOTROPISM MECHANISM5. PHOTORECEPTORS

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INTRODUCTION1. Plants can sense the direction, quality

(wavelength), intensity and periodicity of lightthat induces Phototropism Photomorphogenesis chloroplast differentiation and various other responses such as flowering and

germination.2. This behavior, called phototropism, is generally

assumed to improve light harvesting forphotosynthesis.

3. However, dark-grown etiolated seedlings, whichlack the developed chloroplast necessary forphotosynthesis, display very rapid phototropicresponses.

4. As such, the phototropic response of etiolatedseedlings probably does not initially enhancephotosynthesis.

5. Instead, the phototropic response of etiolatedseedlings is more likely to assist in seedlingemergence from the soil and around otherobstacles.

6. Once the seedling emerges from cover andreceives enough light to sustain photosynthesis,another process, called photomorphogenesis,inhibits shoot elongation and stimulateschloroplast biogenesis.

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Darwin Findings As early as 1880, Charles Darwin had noted:

"no one can look at the plants growing on a bank or onthe borders of a thick wood, and doubt that the youngstems and leaves place themselves so that the leaves maybe well illuminated”

Phototropism is especially important forgerminating seedlings, whereby the emergingshoot must grow towards the light to survive bymaximizing the capture of light forphotosynthesis

Darwin, together with the help of his botanist sonFrancis, left us an entire book, ‘The power ofmovements in plants’, describing his many,varied, and insightful observations on this topic.

Darwin's findings have provided an impetus foran entire field of study, the study of plant tropicresponses, or differential growth (curvature) ofplant organs in response to directional stimuli.

Darwin proposed thatplants could grow differentially (thus directionally) inresponse to external stimuli such as light or gravity.

The part of the plant that perceives the stimulusis separate and distinct from the part thatresponds to that stimulus were demonstrated.

In the case of phototropism, directional light is perceived in the apical portion of a

young seedling and ‘transduced’ to more basallylocalized portions of the shoot as a differential signalthat informs the plant which side is the closest to andwhich is the furthest from the light source such that abending response occurs.

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Finally, Darwin proposed thatan ‘influence’ (though he was unable to identify it)moves from the siteof stimulusperception to thearea of responsewhere bendingoccurs.

DEFINITION1. PHOTOTROPISM Phototropism is a growth movement induced

by a light stimulus‒ Growth towards a source of

light is called positivephototropism, that away fromthe source is termed negativephototropism.

‒ The tips of shoots are usuallypositively, that of rootsnegatively phototropic

Originally was phototropism calledheliotropism, because the plant growstowards the sun.

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The name was altered when it became clear thatplants react also towards artificial sources of light

Plants utilize light not only as an energy source,but also as a vital source of temporal andspatial information

Phototropism, or the directional growth of a plantorgan in response to directional blue light (BL),represents a predominant mechanism by whichplants reposition body parts as a strategy ofadaptation

Phototropic responses are distinguished fromother types of light-modulated directional growthresponses, such as nastic and circadian regulatedleaf movements, by two criteria.

First, the direction of phototropic curvatures isdetermined by the direction of the light stimulus, whilethe direction of nastic and circadian-regulatedmovements are not

Second, many leaf movement responses occur as aresult of reversible swelling/shrinking of specializedmotor, or pulvinar, cells whereas all stem and rootphototropic responses are driven by changes in cellelongation rates across the bending organ.

With respect to phototropic responses,unidirectional irradiation of seedlings with UV-A/blue light results in enhanced growth in thestem on the flank away from the light (“shadedside”) and generally represses growth on theflank facing the incident light (“lit side”) causingit to bend towards the light

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INITIAL STUDY1. Darwin in 1880 showed that the phototropic

response was lost when the tip of the emerging plant was cut off or covered with an

opaque cap.

2. Covering the base of the stem did notblock phototropic bending.

3. Boysen-Jensen showed in 1913 thatplacing a block of gelatin between the tipand the base did not block the response,showing that something was able todiffuse from the tip, through the gel, andinto the base.

4. When the tip was separated from thebase with a piece of mica, which blocksanything from flowing through, theresponse is blocked.

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1 2 3 4 5 6

5. Note that thereis more auxingrowth hormoneon the shadedside (green colorin the smallpicture insert).

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The Discovery of AuxinF. W. Went extracted the growth stimulant. He removed the tips of several coleoptiles of oat, Avena

sativa, seedlings. He placed these on a block of agar for several hours. At

the end of this time, the agar block itself was able toinitiate resumption of growth of the decapitatedcoleoptile.

The growth was verticalbecause the agar block wasplaced completely across thestump of the coleoptile andno light reached the plantfrom the side.

The unknown substance thathad diffused from the agarblock was named auxin

The Avena test soonrevealed thatsubstances with auxinactivity occur widely innature. One of themost potent was firstisolated from humanurine.

It was indole-3-acetic acid (IAA) and turnedout to be the auxin actually used by plants.

Went also discovered that it is the unequaldistribution of auxin that causes the bending inphototropism.

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When a coleoptile tip that has previously beenilluminated from one side is placed on twoseparated agar blocks, the block on the side thathad been shaded accumulates almost twice asmuch auxin as the block on the previously lightedside. Hence the more rapid cell elongation on theshady side of the plant

PHOTROPISM MECHANISM It is well accepted that lateral

redistribution of the phytohormone auxinunderlies the bending of plant organstowards light

The questions arising are How the light is perceived or sensed by plants? How the information of light presence is

transduced to Auxin

Despite more than a century of research,it is still unresolved how light regulates auxin distribution and where this occurs in dicots

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Comparison of the action spectrum of an Avena coleoptile’s phototropicbending (green curve) and the absorption spectra of carotenoids (orangecurve) and flavins (yellow curve; according to H. MOHR, P. SCHOPFER,1978, according to W. SHROPSHIRE and R. B. WITHROW, 1959)

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PHOTORECEPTORS Phototropins are the primary photoreceptors

controlling phototropism in aerial organs ofhigher plants that perceive directional blue-light(BL) cues and then stimulate signaling, leading torelocalization of the plant hormone auxin

Phototropins regulate not just phototropism, buta number of additional blue light responses,including stomatal opening, chloroplastmovements, leaf movements and expansion, andrapid inhibition of stem growth.

The first of these photoreceptors identified at themolecular level is phototropin 1 (phot1),originally designated NPH1 (for its non-phototropic hypocotyl mutant phenotype).

The PHOT2 genewas subsequentlyidentified based onits high degree ofsequencehomology toPHOT1

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The structure of plant phototropins can beseparated into two parts:

‒ LOV1: a N-terminal photosensory input region coupled to‒ LOV2: a C-terminal effector or output region that

contains a classic serine/threonine kinase motif

Domain structures of the phot1 and phot2 receptors. Theamino terminal photosensory LOV domains are shown inblue with their associated FMN co-factor (above each LOVdomain). The carboxyl-terminal protein kinase domain ofeach phot is shown in red.

Phototropic response of wild-type Arabidopsisseedlings (WT) and a phototropin-deficientmutant (p1p2) irradiated with blue light fromthe right.

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(A) Cartoon illustratingthe domainstructure ofphototropin bluelight receptors.

(B) Structural model ofthe phototropinLOV2 domain inthe dark state.

Phototropinstructure

The position of the FMN chromophore is indicated.

B

A

LOV-domain photochemical reactivity. (A) Purifiedpreparation of bacterially expressed LOV2. (B) Schematicrepresentation of LOV-domain photochemistry. Details ofthe reaction are described in the text.

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1. Phototropin activation byblue light. In the dark,phototropin isunphosphorylated andinactive.

2. The absorption of lightinvokes a photochemicalreaction within the LOVdomains

3. Photoexcitation of themain light sensor, LOV2,causes a disordering ofthe Jα-helix, and theactivation of the C-terminal kinase domain,which consequently leadsto autophosphorylation ofthe photoreceptor.

The relative positions ofphosphorylation sites areindicated

A number of phototropin-interacting proteinshave been isolated NPH4/ARF7 is the auxin response factor (transcriptional

activator, a key regulator of hypocotyl phototropism andgravitropism in Arabidopsis) that responds to changes inlocal auxin concentrations to directly mediate expressionof genes likely encoding proteins necessary fordevelopment of phototropic curvatures

NPH3 (Non-Phototropic Hypocotyl 3, phototropin-interacting protein), a novel protein that directlyinteracts with phot1 or serves as a protein scaffold toassemble components of a phototropin receptorcomplex. NPH3 is required for optimal leaf positioningand leaf flattening in Arabidopsis.

RPT2 (Root Phototropism 2) = a protein closely relatedto NPH3 that also interacts with phot1, and is requiredfor both phototropism and stomatal opening by blue light

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cryptochromes (crys) = BL absorber can act asmodulators of phot-dependent signal-response.phytochromes (phys) = RL absorber can act asmodulators of phot-dependent signal-response

AUX1—a gene encoding a high-affinity auxin influxcarrier previously associated with a number of rootresponses

ABCB19 (ATPBINDING CASSETTE B19) = the auxinefflux transporter at and above the hypocotyl apex as asubstrate target for the photoreceptor kinasePHOTOTROPIN 1 (phot1)

Members of the PIN-FORMED family are the primarymediators of directional auxin fluxes that regulate plantdevelopment [8]. PIN3 = an auxin transporter that isproposed to mediate lateral auxin fluxes by differentiallyrestricting auxin to the vascular cylinder [6].

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1. In darkness, auxin primarily moves through thevascular tissues in the petioles and hypocotyl,and also through epidermal tissues

2. Upon exposure to directional blue light, auxinmovement is temporarily halted at thecotyledonary node and the seedling stopselongating vertically

3. This is followed by a redistribution of auxin atthe cotyledonary node to the shaded side of theseedling and a channeling to the elongation zonebelow

4. Subsequent elongation of the cells on theshaded side of the hypocotyl results indifferential growth and bending towards the lightsource

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