PHOTOMORPHOGENESIS IN PLANT STEMS

Summary The large morphological differences between plants grown in the dark and those exposed to light appear to result from an effect of light on accelerating the developmental process and causing it to proceed further. Light accelerates the early phase of expansion of stems but inhibits their final growth in length. At least two photochemical processes are involved in morphogenetic responses, the red/far-red reversible reaction of the pigment phytochrome and a second process which only becomes important when irradiation is prolonged. This second process is particularly sensitive to blue light and frequently also to far-red. In many cases prolonging the exposure to red light increases the magnitude of the response, even though the phytochrome equilibrium is established within minutes of the beginning of the irradiation. Nothing is known with certainty about the pigments involved in these responses to prolonged irradiation nor what kind of relationship exists between them and the reactions mediated by phytochrome. Some evidence is presented to show that the red/far-red reaction acts earlier in the growth process than the prolonged light reaction and that, in stems, the latter acts only to inhibit elongation. There is also some indication that blue and far-red radiation do not always act identically in a prolonged light reaction. Effective auxin levels, as measured by bioassays, are lowered after transfer to red light; the depression may be only temporary, the levels rising again later. Both co-factors and inhibitors of IAA oxidase have been shown to be affected by light. Red light causes the production of an inhibitor of IAA oxidase. The effect of a natural inhibitor of IAA oxidase is prevented by blue light together with riboflavine. The co-factors and inhibitors are phenolic compounds. Anthocyanin synthesis is also dependent on light and may be related to the morpho-genetic responses. In some species both the low-energy phytochrome reaction and a prolonged light reaction, sensitive to blue and far-red, affect anthocyanin and growth. A number of flavonoid compounds and their precursors, especially the substituted cinnamic acids, affect the elongation of stems and roots, so that a change in the type and quantity of these compounds may be the cause of some of the light effects on growth; this may be particularly so in the case of the prolonged light inhibition of stem elongation. The compounds act as inhibitors and co-factors of the IAA oxidase system but they may affect growth in other ways than as regulators of in vivo auxin levels. Gibberellic acid acts by promoting stem elongation more in the light than in the dark, and it has been suggested that the light inhibition of stem elongation is caused by a lowering of the level of endogenous gibberellins. However, gibberellic acid promotes elongation whether light is promoting or inhibiting, and both the red/far-red reversible reaction and the blue light inhibition continue to be shown in the presence of saturating doses of gibberellins. Neither the prolonged light reaction nor the phytochrome reaction, therefore, appear to lower the levels of endogenous gibberellins and the light effects are probably related only distantly to gibberellin levels. In dwarf peas, however, there is some evidence that a third photoreaction acts in reducing the effectiveness of gibberellins, perhaps by causing the synthesis of a compound interfering with gibberellin activity. The low-energy red response probably concerns the activation of an enzyme by the formation of an isomer when a photon is absorbed; this activity is maintained in the dark for a while but the enzyme slowly reverts to an inactive form. The reactions catalysed by this enzyme are unknown, but it is probably a pacemaker enzyme acting at some key point in metabolism. The involvement of CoA metabolism has been suggested but there is as yet little evidence to support this. Phosphorylative capacity is enhanced by red light but there is no clear indication how directly this effect is related to the photochemical step. In the prolonged light reaction, enzyme activation or synthesis is probably involved. Protein synthesis and the formation of certain enzymes have been shown to require light. Nucleic acid metabolism has been shown to be concerned in some morphogenetic responses and in anthocyanin synthesis.