Regulation of Floral Development (Part III)

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PHYTOCHROME AND PHOTOPERIODISM

Phytochrome

•Phytochrome is a protein pigment that absorbs red and far-red light most strongly, but that also absorbs very low amount of blue light.

Phytochrome can interconvert between Pr and Pfr forms:

•Phytochrome is synthesized in a red light–absorbing form (Pr) which is blue to the human eye.

•Pr is converted by red light to a far-red light–absorbing form (Pfr) which is blue-green.

•Pfr, in turn, can be converted back to Pr by far-red light.

•This phenomenon is known as photoreversibility.

•Darkness also converts Pfr to Pr.

•Pfr is the physiologically active form of phytochrome.

Photostationary state

•The proportion of phytochrome in the Pfr form after saturating irradiation by red light is only about 85%.

    Reason: Most of Pr absorb red light and are converted to Pfr, but some Pfr also absorbs red light and are converted back to Pr

•Similarly, under far-red light irradiation, an equilibrium of 97% Pr and 3% Pfr is achieved.

    Reason: Very small amount of far-red light absorbed by Pr makes it impossible to convert Pfr entirely to Pr by far-red light.

•This equilibrium is called a photostationary state.

Phytochrome is a dimer composed of two polypeptides

•Native phytochrome occurs as a homo-dimer.

•Each subunit consists of:

        •a light-absorbing pigment molecule called the chromophore

        •a polypeptide chain called the apoprotein.

•The chromophore is attached to the apoprotein through a thioether linkage to a cysteine residue

•Both the chromophore and protein undergo conformational changes during Pr-Pfr interconversion.

PHY genes encode two types of phytochrome:

•Phytochrome is encoded by a multigene family consisting of five members: PHYA, PHYB, PHYC, PHYD, and PHYE.

Type I phytochrome:

•Encoded by PHYA gene

•Transcriptionally active in dark-grown seedlings

•Expression is strongly inhibited in light in monocots and less dramatically in dicots.

•PfrA is also unstable.

Type II phytochrome:

•Encoded by PHYB, PHYC, PHYD, PHYE genes

•Expression of their mRNAs is not significantly changed by light.

•Proteins are more stable in the Pfr form.

Phytochrome is the primary photoreceptor in photoperiodism

In SDPs, a night break becomes effective only when the supplied dose of light (Red light) is sufficient to saturate the photoconversion of Pr to Pfr.

•A subsequent exposure to far-red light, which photoconverts the pigment back to the physiologically inactive Pr form, restores the flowering response.

Reason:

      •During night, Pfr gets converted to Pr.

    •A long night length is required to convert sufficient amount of Pfr to Pr.

   •If a flash of red light is given, Pr absorbs red light and gets converted back to Pfr.

    •So, the effective night length gets reduced and sufficient Pfr is not generated.

    •If the flash of red light is followed by a flash of far-red light, the Pfr again gets converted to Pr and this nullifies the effect of red light flash.

                      Pfr/Pr > 1   …….Flowering in LDPs

                      Pfr/Pr < 1   …….Flowering in SDPs

•In LDPs, a night break of red light promoted flowering, and a subsequent exposure to far-red light prevented this response.

A Blue-Light Photoreceptor Also Regulates Flowering

•In some LDPs, such as Arabidopsis, blue light can promote flowering.

•Cryptochrome is the major blue light photoreceptor that regulates flowering.

GENETIC AND MOLECULAR REGULATION OF FLOWERING

Transition to flowering involves multiple factors and pathways

Five genetically distinct developmental pathways control flowering. These are:

        ØPhotoperiodic pathway

        ØVernalization pathway

        ØAutonomous pathway

        ØSucrose pathway

        ØGibberellin pathway

Photoperiodic pathway

It involves phytochromes (PHYA and PHYB acting antagonistically) and cryptochromes.

In Arabidopsis (LDP)

•In the morning, PHYB represses CONSTANS (CO) gene expression.

•In the evening, PHYA and cryptochrome interacts with circadian clock genes to induce CO expression.

•CO is a transcription factor which induces FLOWERING LOCUS T (FT).

•FT protein (‘Florigen’) travels from the induced leaf to the shoot apex via the phloem.

•FT forms a complex with transcription factor FD.

•FT-FD complex activates downstream genes like SOC1, LFY, etc which in turn activates the floral homeotic genes for floral development.

In rice (SDP)

•In the morning, PHYB represses Heading date 1 (Hd1) which is the homologue of CO.

•In the evening, PHYA interacts with circadian clock genes to induce Hd1 expression.

•Here, Hd1 acts as an inhibitor of flowering.

•Absence of Hd1 stimulates expression of Hd3a.

•Hd3a (‘Florigen’) travels from the induced leaf to the shoot apex via phloem for floral development.

Coincidence model

•Transition to flowering occurs when the expression peak of CO coincides with the light phase.

•PHYA/PHYB perceive light signals and entrain the clock

•The clock genes control the diurnal expression of CO and its expression peaks between 12 hours of light and dawn (SuaÂrez-LoÂpez et al., 2001; Nature).

•In long days, the CO expression peak coincides with the light phase (coincidence phase). 

•In light, the CO protein is stable and can activate FT expression, promoting flowering. 

•In short days, CO expression peaks in the dark (no coincidence of light phase and CO expression). 

•In the dark, CO protein is degraded and thus cannot induce FT expression and further processes.

(Singh et al., 2016; New Phytologists)

Florigen concept

•During flowering, a biochemical signal is produced in photoperiodically induced leaves and is then transmitted to the shoot apex where it stimulates flowering.

•This mode of action resembles a hormonal effect.

•Hence, Mikhail Chailakhyan postulated the existence of a universal flowering hormone, florigen (1930s).

•Evidence: non-induced receptor plants were stimulated to flower by being grafted onto a leaf or shoot from photoperiodically induced donor plants.

•Movement of the floral stimulus from a donor leaf to the stock across the graft union correlated closely with the translocation of 14C-labeled assimilates via vascular continuity across the graft union.

Identification of florigen

•Since 1930s, many attempts to isolate florigen have remained unsuccessful.

•A major breakthrough is the identification of FLOWERING LOCUS T (FT) in Arabidopsis.

•FT is a downstream target of CO and is expressed in companion cells of leaves (like CO).

•FT is a small globular protein induced by CO in response to long days.

•FT then travels from leaf to shoot apex via phloem to induce flowering.

•It can travel through grafting unions and induce flowering in competent recipient plants.

•Therefore, FT (or its homologue, Hd3a in rice) is the florigen.

Regulation of floral induction by FT

•FT mRNA is synthesized in leaf companion cells.

•Endoplasmic reticulum protein, FT INTERACTING PROTEIN 1 (FTIP1), helps the transport of FT  from companion cells to sieve tubes.

•FT protein then moves via phloem translocation stream to shoot apex.

•In the shoot apex (floral meristem), FT enters the nucleus to bind FLOWERING LOCUS D (FD) which is a transcription factor.

•FT-FD complex activates floral identity genes like SOC1 and AP1.

•SOC1 then activates LFY which further activates the floral homeotic genes for floral development.

VERNALIZATION: PROMOTING FLOWERING WITH COLD

•Vernalization is the process whereby flowering is promoted by a cold treatment given to a fully hydrated seed (i.e., a seed that has imbibed water) or to a growing plant.

•Commonly, a combination of cold treatment followed by long-day exposure is required.

Vernalization results in the competence to flower at the shoot apical meristem

•The effective temperature range for vernalization is from just below freezing to about 10°C.

•The effect of cold increases with the duration of the cold treatment until the response is saturated.

•Vernalization can be lost as a result of exposure to devernalizing conditions, such as high temperature, but the longer the exposure to low temperature, the more permanent the vernalization effect.

•Vernalization takes place primarily in the shoot apical meristem.

•Localized cooling causes flowering when only the stem apex is chilled and is largely independent of the temperature experienced by the rest of the plant.

•In case of seed vernalization, fragments of embryos, consisting essentially of the shoot tip, are sensitive to low temperature.

•Vernalization results in the acquisition of competence of the meristem to undergo the floral transition.

Vernalization involves epigenetic changes in gene expression

Changes in gene expression that is stable even after the signal that induced the change (in this case cold) is removed are known as epigenetic regulation.

•The involvement of epigenetic regulation in the vernalization process has been confirmed in the LDP Arabidopsis.

•In Arabidopsis, a gene, FLOWERING LOCUS C (FLC), acts as a repressor of flowering.

•FLC is highly expressed in non-vernalized shoot apical meristems and negatively regulates SOC1.

•After vernalization, FLC is epigenetically (conversion from euchromatin to heterochromatin by histone methylation) switched off permitting flowering in response to long days to occur.

•In the next generation, however, the gene is switched on again, restoring the requirement for cold.

Autonomous pathway

•Here, flowering occurs in response to internal signals—the production of a fixed number of leaves, age, etc.

•The autonomous pathway acts by reducing the expression of the flowering repressor gene FLOWERING LOCUS C (FLC), an inhibitor of LFY.

Sucrose pathway

•Sucrose stimulates flowering in Arabidopsis by increasing LFY expression.

Gibberellin pathway

•Required for early flowering and for flowering under non-inductive short days.

•GA induces GA-MYB (transcription factor) mediated activation of LFY.

 

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