Most of the biochemistry of pterins is known from work with bacteria, insects and vertebrates. Great progress has been made in the elucidation of the chemistry and function of pterins in these organisms (for review see: Nichol et al., 1985; Ziegler, 1987). The conspicuous absence of plant material in the 1986 Proceedings of the Eighth International Symposium on Pterines (Cooper and Whitehead, 1986) demonstrates that the interest of plant physiologists and plant biochemists in this area has been rather modest in comparison. The decision to review nonetheless the role of pterins in plants has been largely motivated by our conviction that pterins are relevant to the problem of blue-light reception in plants and microorganisms including fungi. While the manyfold effects of near-UV-blue light (30&500 nm) on phototropism, phototaxis, photomorphogenesis and the metabolism of plants and microorganisms are well known, the nature of the photoreceptor(s) mediating these responses is still a matter of dispute. Flavoproteins are presently considered the most likely candidates (Galland and Senger, 1988; Schmidt, 1980; Senger and Briggs, 1981; Senger and Schmidt, 1986). however, carotenoproteins are still being discussed as possible bluelight receptor pigments (Shropshire, 1980). Little attention has so far been paid to pterins, although their physiochemical properties d o meet the requirements for a receptor pigment acting in the nearultraviolet. Pterins share with flavins properties such as radical formation, participation in redox chains, photosensitizing capacity and absorption of nearultraviolet light. These features make them suitable candidates as receptor or coreceptor pigments in the near-ultra-violet. They could also function as redox partners in flavin-sensitized electron transport chains. In this contribution we review the literature pertinent to the possible role of pterins in photoreception and extend our discussion to their role in metabolism, because of the interrelations between the two functions.

The role of pterins in the photoreception and metabolism of plants

(1990). Circadian Rhythms in Neurospora crassa: Biochemistry and Genetics. Critical Reviews in Microbiology: Vol. 17, No. 5, pp. 365-416.

Circadian rhythms in Neurospora crassa: biochemistry and genetics.

It is generally believed that enzymes are probable candidates for regulation within the sequence light perception — transduction — metabolic response Therefore it is not surprising that in recent years several reviews have dealt with the effect of light on enzymes [2, 8, 23, 24, 25, 41, 42, 68, 73, 89] However, strangely enough, the specific influence of the blue light (BL) photoreceptors has been neglected to some extent In some cases this was certainly due to the fact that the reviews had been intentionally restricted to specific problems, eg, to the action of phytochrome [68] or to chloroplast enzymes [8] Only general reviews on BL responses contain short chapters on the action of BL on enzymes [31, 70, 71, 84, 85], and, of course, some information about this topic is included in the proceedings of the first Blue Light Conference [69] It might be of some value, therefore, to present a review specifically on the effect of BL on enzymes In this review, the present status of knowledge will be summarized not only to inform, but also to encourage further work in this interesting and important field

Effects of Blue Light on Enzymes

Fluorescence excitation- and emission spectra indicate the presence of pterin(s) and flavin(s) in isolated flagella of the phytoflagellate Euglena gracilis. These compounds appear to bind at least in part non-covalently to the molecular framework of the paraflagellar body, which is the presumed photoreceptor organelle and which is attached to the isolated flagella. A compound with pterin-like fluorescence excitation and emission spectrum could he extracted with methanol from isolated flagella and could he recovered on thin-layer silica gels. Besides the previously assumed photoreceptor function of flavins, our results suggest also a role for pterins in the photosensory transduction chain of Euglena gracilis.

PTERIN- AND FLAVIN-LIKE FLUORESCENCE ASSOCIATED WITH ISOLATED FLAGELLA OF Euglena gracilis.

ion (upper part of the diagram: An electron is donated to triplet oxygen, creating superoxide anion, which via formation of hydrogen peroxide and hydroxide radical abstracts hydrogen from the lipid Also in this case the lipid is degraded to lipid peroxide and a chain reaction is initiated 2. PHOTOTOXICITY IN PLANT DEFENCE The most important defences of plants against parasites and grazers are of a chemical nature, and among chemical defences phototoxicity plays an important role, especially among flowering plants. The phototoxic substances employed by plants can also affect people when they appear in food, perfumes and other cosmetic products, and even if we just touch certain plants. Downum (1992) estimates that 75-100 different phototoxic molecules have been isolated from flowering plants. Phototoxins or phototoxic activity has been reported for about 40 of more than 100 angiosperm families, representing all subclasses except Alismatidae and Arecidae. Many plants have several phototoxic substances. From Ammi majus as well as from Angelica arch angelica the following ones are reported: angelicin, bergapten, 8-methoxypsoralen, and pimpinellin; from the former one in addition furocoumarin and from the latter one psoralen. The plant family Apiace (former name Umbelliferae) dominates the cases of phototoxicity of most

Photobiology:The Science of Light and Life

It has been proposed by Klemm and Ninnemann [29] that nitrate reductase (NR) may act as a photoreceptor in blue light-stimulated conidiation of Neurospora crassa (carotenoid-less mutant al-2 bd). Nitrate reduction and conidiation appear to be correlated in some specific ways: Hochberg and Sargent [19] and Weiss and Turian [61] reported that conidiation of Neurospora is maximally supported by nitrate as sole nitrogen source in the medium, whereas the mycelium grows optimally with ammonium nitrate or diammonium citrate. Weiss and Turian [61] also described suppression of conidiation on diammonium citrate (+ 2% sucrose). The same amino acids (e.g., try, ala, his) appear to be favorable for conidiation [47] and induction of NR [51]. We found [26] that hyphae grown on nitrate formed conidia under starvation conditions (nitrogen and carbon source depleted, possibly partial anaerobiosis) whereas conidia occurred much later if at all when the mycelium was starved after growth on medium containing NH4NO3 (Fig. 1A and B). Also light-induced absorbance changes at 423 and 557 nm — regarded as an assay system for a primary process in the vicinity of the photoreceptor — could be seen in vivo in Neurospora mycelium only at a time when light stimulated conidiation [28]. We have done preliminary experiments which show that the light requirement for promotion of conidiation decreased in somatic fusion products nit 1 + 3 made from protoplasts of nitrate reductase-deficient nit-1 and nit-3 mutants of Neurospora as compared with the parental strains [37].

Helga Ninnemann

Papers

The Nitrate Reductase System