Chapter 17. Phytochrome and Light Control of Plant Development

Corn and bean seedlings grown in the light or the dark De-etiolated etiolated De-etiolated etiolated Light-grown corn Dark-grown corn Light-grown bean Dark-grown bean Developmental changes by light (photomorphogenesis) (transformation from the etiolated to the green state) A decrease in the rate of stem elongation The beginning of apical-hook straightening The initiation of the synthesis of pigments

Phytochrome: red light development photosynthesis Function of light stomata opening flowering Phototropin: blue light photoreceptor Cryptochrome: blue light Phytochrome: red light

The photochemical and biochemical properties of phytochrome

Typical photoreversible responses induced by phytochrome in variety of higher and lower plants

Lettuce seed germination is a typical photoreversible response controlled by phytochrome Red, 650-680 nm Far-red, 710-740 nm dark

Phytochrome can interconvert between Pr and Pfr forms Pr : red – light absorbing form Pfr: far- red - light absorbing form

Absortion spectra of purified oat phytochrome in the Pr and Pfr forms overlap Red light Pr :15% Pfr :85% Far-red light Pfr : 3% Pr : 97%

Pfr is the physiologically active forms of Phytochrome Wild type: short hypocotyl by white light hy mutant (phytochrome mutant): long hypocotyl by white light If the phytochrome-induced response to white light is caused by the absence of Pr, such phytochrome-mutants should have short hypocotyl in both darkness and white light. Instead, the opposite occurs.

De-etiolated etiolated De-etiolated etiolated Pr of Pfr form?

Phytochrome is a dimer composed of two polypeptides Native phytochrome is a soluble protein with MW about 250 kDa. It occurs as a dimer made up of two equivalent subunits. Each subunit consists of two components: light absorbing pigment: chromophore a polypeptide chain: apoprotein (125 KDa)

Structure of the Pr and Pfr forms of the chromophore

Structure of the phytochrome dimer

Phytochromobilin is synthesized in plastids Transported into cytosol by a passive process Assembly of the phytochrome apoprotein (autocatalytic) 4. Consists of holoprotein hy1 and hy2 mutant: fail to supress hypocotyl elongation. chromophore biosynthesis mutant, normal apoprotein.

Both chromphore and protein undergo conformational changes Absorption of light The Pr chromophore undergoes cis-trans isomerization The protein moiety of phytochrome holoprotein also undergoes a subtle conformational change ( both N-terminal and C-terminal regions)

Two types of phytochromes have been identified Dark 9 : 1 Light 5 : 5 Existence Etiolated plant Green plant and seed

Phytochrome is encoded by a multigene family 5 structurally related phytochrome genes in Arabidopsis (PHYA, PHYB, PHYC, PHYD, PHYE)

PHY genes encode two types of phytochrome Phytochrome gene family (PHY) PHYA PHYB PHYC PHYD PHYE Type I Type II

PHYA PHYB PHYC PHYD PHYE red mRNA Pr Pfr Response far-red ubiquitin + ATP ubiquitin degradation degradation PHYB PHYC red mRNA Pr Pfr Response PHYD far-red PHYE