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17 Photomorphogenesis: responding to light

Light perception in plants. Because plants do not enjoy the luxury of being able to change their environment or seek shelter from adverse conditions by changing their location, they must be more sensitive to changes in their surrounding so they can adapt accordingly.Plants can sense light gradients

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17 Photomorphogenesis: responding to light

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    2. Light perception in plants Because plants do not enjoy the luxury of being able to change their environment or seek shelter from adverse conditions by changing their location, they must be more sensitive to changes in their surrounding so they can adapt accordingly. Plants can sense light gradients and detect subtle differences in spectral composition.

    3. Photomorphogenesis Photomorphogenesis is referring to the response of plant to light, which is the central theme in plant development.

    4. Photoreceptors Most photomorphogenic responses in higher plants appear to be under control of one (or more) of four classes of photoreceptors: 1. Phytochromes (red and far-red) 2. Cryptochrome (blue and UV-A): seedling development and flowering 3. Phototropin (blue and UV-A): differential growth in a light gradient 4. UV-B receptors: unknown

    5. Chapter outline Red and far-red responses Blue and UV-A responses Interactions between photoreceptorsUV-B responses

    6. Phytochromes Phytochromes are plants photoreceptors. Phytochromes are photochromic. They can absorb red (665nm) and far-red (730nm) light and they have two forms, red-absorbing form (Pr) and far red-absorbing form (Pfr).

    7. Figure 17.1

    8. Phytochrome is photoreversible Pr and Pfr forms of phytochrome can change to the other form when expose to red or far-red light, respectively.

    9. Phytochrome is photoreversible

    10. The photoreversibility of phytochrome comes from its chromophore, phytochromobilin (PFB)

    11. PFB is covalently linked with the N-terminal part of phytochrome

    12. Conformational change of PFB results in Pr ?? Pfr change

    13. Pfr form is the active from of phytochrome

    14. Phytochrome is down regulated after activation The down regulation of phytochrome involved mRNA and protein degradation. Also, the expression of phytochrome will be down regulated at transcriptional level after activation.

    15. Figure 17.5

    16. Figure 17.7

    17. Pr and Pfr forms of phytochrome is always in a dynamic equilibrium

    18. Phytochrome responses can be grouped into three groups

    24. Table 17.3

    36. Phytochrome under natural conditions Under natural conditions, phyA may just detect the presence/absence of light since it only accumulate under dark-grown conditions. Other phytochrome response observed under natural conditions is shade avoidance syndrome.

    37. Light under canopy is far-red enriched

    38. Shade avoidance is triggered by far-red light, which can be shown in end-of-day treatment

    39. Shade-avoidance syndrome

    40. Figure 17.14

    42. Phytochrome signal transduction Phytochrome is a protein kinase. When activated, it will phosphorylate other proteins and begin signal pathways.

    43. Proteins that are phosphorylated by phytochrome

    44. Phytochrome regulates gene expression A lot of nuclear-encoded genes are regulated by phytochromes, including the small subunit of rubsico (RBCS) and the light-harvesting chlorophyll a/b binding proteins (CAB). Some proteins are positively regulated, like RBCS and CAB; others are negatively regulated, like phyA and NADPH-protochlorophyllide oxidoreductase.

    45. Figure 17.17

    46. Phytochrome also regulates other transcription factor’s activities PIF3 (phytochrome interacting factor 3) is a transcription activator. When phytochrome activates (Pr ? Pfr), the Pfr form binds to PIF3 and activates it. Then activated PIF3 will activate transcription of a large variety of proteins containing G-box motifs.

    47. Figure 17.18

    48. Blue and UV-A light responses Cryptochrome Phototropin

    49. Cryptochrome is a flavoprotein

    50. Cryptochromes Cryptochromes are blue/UV-A photoreceptors mediating seedling development/flowering responses in plants. In Arabidopsis, there are two cryptochromes, cry1 and cry2. The structure of cry2 is also similar to cry1 with two chromophores. Cry2 has a role in determining flowering time.

    51. Phototropin Phototropin was orginally isolated as nph1 (nonphototropic hypocotyl 1).

    52. Phototropin Phototropin is also a flavoprotein with two flavin mononucleotide (FMN) chromophores. FMN chromophores binds to domain called LOV (light, oxygen and voltage) domain.

    53. Phototropin could be a blue-light dependent protein kinase

    54. Interactions between Photoreceptors

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