Two types of photochrome

Photochrome is an element present in plants, generally distributed in the root tips of various plants. Most photochromes can only be synthesized in the dark, while some can be synthesized in both light and darkness, thus distinguishing the two types of photochromes. Photosensitive pigments play a very important role in the growth of plants. You can learn more about them in detail.

1. Photosensitive pigments

Photoreceptors (pigment proteins) that absorb red light and reversibly convert it to far-red light are called photosensitive pigments (phytochrome).

Photosensitive pigments are distributed in various organs of plants, and the content of photosensitive pigments in etiolated seedlings is 20 to 100 times more than that in green seedlings. The tip of the coleoptile of Poaceae, the hook of etiolated pea seedlings, the meristem and root tips of various plants contain high levels of photosensitive pigments. Generally speaking, protein-rich meristems contain more photosensitive pigments. In cells, photosensitive pigments are found in both the cytosol and the nucleus.

2. Type

There are at least two types of phytochromes in plants (Furuya, 1993): one type is present in higher concentrations in etiolated seedlings, can only be synthesized in the dark, and is unstable in the light, called etiolate tissue phytochome (Phy Ⅰ), with an absorption peak at 666nm; the other type is mainly found in green tissues, is relatively stable in the light, and can be synthesized in both the light and the dark, called green tissue phytochome (Phy Ⅱ), with an absorption peak at 652nm. Molecular biology experiments have shown that there is a family of photosensitive pigment genes (called PHY) in angiosperms. For example, the number of PHYs in corn is 2 to 4, while the number of PHYs in oats is more than 4. Five different photosensitive pigment genes were found in Arabidopsis seedlings, named PHYA, PHYB, PHYC, PHYD, and PHYE. The protein phyA encoded by PHYA is a PhyⅠ type photosensitive pigment, which receives continuous far-red light with a wavelength of 700~750nm and is unstable to light. The activity of its mRNA is inhibited under light. The proteins phyB, phyC, phyD, and phyE encoded by the other four genes are photosensitive PhyⅡ-type photopigments, which are highly photostable, unaffected by light, receive 600-700nm red light, and are constitutively expressed. It has been proven that the proteins encoded by the PHYA and PHYB genes can be assembled into the full-photosensitive pigments phyA and phyB. PhyA mainly controls the elongation effect of far-red light on the hypocotyl of seedlings, while phyB mainly controls the inhibitory effect of red light on the hypocotyl of seedlings.

3. Function

The physiological effects of photosensitive pigments are very extensive. They affect the morphological construction of plants throughout their life, from seed germination to flowering, fruiting and aging.

Some reactions controlled by phytochromes in higher plants

1. Seed germination 6. Leaflet movement 11. Photoperiod 16. Leaf abscission

2. Hook opening 7. Membrane permeability 12. Flower induction 17. Tuber formation

3. Internode elongation 8. Phototropism 13. Cotyledon opening 18. Sex expression

4. Root primordium initiation 9. Anthocyanin formation 14. Fleshy 19. Leaf opening (single)

5. Leaf differentiation and expansion 10. Plastid formation 15. Epistasis 20. Rhythmic phenomenon

The time it takes for photosensitive pigments to respond to light stimulation can be fast or slow. Fast reactions are measured in seconds, such as the Tanada effect and the movement of chloroplasts in algae. The terraced field effect refers to the fact that the isolated mung bean root tip induces a small amount of positive charge in the membrane under red light, so it can adhere to the negatively charged glass surface, while far-infrared light reverses this adhesion phenomenon. Slow responses are measured in hours and days; for example, red light promotes seed germination and induces yellowing in lettuce seedlings.