The Physiology of Flowering Plants - KHAM PHA MOI

248 PHOTOMORPHOGENESISABbelow that necessary for photosynthesis, in many cases by briefexposure only, and can occur in plants where the photosyntheticapparatus has not yet developed.Light is perceived by a series of photoreceptors, the best studiedof which are the phytochromes. However, plants contain severaldistinct families of photoreceptors in addition to phytochrome –these include the blue/UV-A photoreceptors (cryptochromes),poorly characterized UV-B photoreceptors and the phototropins(involved in phototropic responses – see Chapter 12).10.3 Phytochrome and photomorphogenesisFig: 10:2 Etiolated andde-etiolated growth maize plants(Zea mays). The etiolated maizeplant (A) is pale andachlorophyllous. The leaves haveemerged from the coleoptile butdevelopment is limited. Rootdevelopment is also poor. Incontrast the light-grown maize plant(B) is de-etiolated. The leaves aregreen and have expanded asphotosynthesis fuels growth. Rootdevelopment is extensive.10.3.1 The discovery of phytochromeOur understanding of plant photoreceptors has been revolutionized inrecent years by the use of molecular genetic techniques coupled withcareful biochemical and physiological measurements. By isolating andcharacterizing the genes encoding the different photoreceptors, andby producing mutant and transgenic plants with lesions in photoperception,it has been possible to build a framework describingsome of the mechanisms by which plants respond to light. However,this current revolution is based upon a long, and distinguished, historyof studying plant responses to light.The existence of phytochrome was first deduced from studies ofcertain varieties of lettuce (Lactuca sativa) seeds which require light forgermination. These seeds are induced to germinate by brief (a fewminutes) exposure to red light of low irradiance, whilst a similarexposure to far-red light is inhibitory to germination. Careful measurementsmade at many different wavelengths allowed the actionspectra of these two responses to be established (Fig. 10.3). Thestimulation of germination was found to have a maximum in the redat 660 nm whilst the inhibition of germination by far-red light had amaximum at 730 nm. A key breakthrough came with the discoverythat alternating exposure to red and far-red light can be given for manycycles and whichever wavelength is given last determines theresponse. These observations led Hendricks and Borthwick, workingin Beltsville, Maryland in the 1950s, to postulate the existence of aphotoreceptor named phytochrome, existing in two photointerconvertibleforms. One form, termed P R , absorbs red light and is convertedto the other form P FR . The spectral properties of P FR are such that itabsorbs light with a peak in the far-red and under far-red illuminationit is converted to P R again (Fig. 10.4). P FR also reverts slowly to P R whenplants are placed in complete darkness. Many models of phytochromeaction have proposed that P FR is the active form, whilst P R is inactive.However, as will become apparent later in this chapter, such a simpleinterpretation may not always be entirely accurate.Subsequently phytochrome was isolated from plant tissues. It is alow-abundance, blue-green chromoprotein and, in solution, showsphotoreversibility between the P R and P FR forms with absorption