Plants maintain higher levels of phytochrome at their growing tips where phytochrome plays important roles in growth responses to light. Phytochrome levels are also high in areas of the plant that are "greening". What does this suggest about the role of phytochrome in these "greening" zones?

Explanation:

Phytochrome is a protein with kinase activity present in plant organisms, whose function is to act as a photoreceptor mainly of red light (600-700nm) and distant red light (700-800nm), thanks to its chromophore. The phytochrome depending on the type of light detected can trigger different responses in the plant, such as flowering, germination, growth as an escape response to the shadow -development of epicotyls at night and cotyledons during the day-, regulation of expression of metabolic activity during day and night (circadian rhythms).

Phytochromes were discovered in the fifties as part of an investigation on the effect of light on the germination of lettuce seeds. It was observed that the seeds that germinated in the dark did not reach 20%; On the contrary, the germination percentage was maximum when the seeds were irradiated with a pulse of red light (R). It was also found that subsequent irradiation with a pulse of distant red light (RL) nullified the inducing effect of red light, preventing germination.

Alternating irradiation with light R and RL (R, R + RL, R + RL + R, R + RL + R + RL, etc.) showed that the last color applied determined the germination of the seeds and that the light red was the stimulating factor of the process and, its inhibitor, the distant red light.

In search of an explanation of such phenomena, the existence of a pigment was proposed, which they called phytochrome, which absorbed the red light. The phytochrome in question, after absorbing red light, became a way capable of absorbing distant red radiation; form that turned to its initial condition after performing said absorption. The hypothesis found experimental support in the early sixties with the purification, from extracts of cereal seedlings, of a protein endowed with the predicted characteristics. Phytochromes are soluble proteins found in the seeds, leaves, stems, roots and other organs of the plant.

Biochemically, phytochrome is a protein with a Bilin chromophore.

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