A new inhibitor of carotenoid synthesis in higher plants: 4-chloro-5-(dimethylamino)-2- , , ,(trifluoro-m-tolyl)-3(2H)-pyridazinone

Development of the Casparian strip is delayed by blue light in pea stems

To understand the regulatory mechanisms involved in tissue development by light, the kinetics of regulation of Casparian strip (CS) development in garden pea stems was studied. We found that short-term irradiation with white light delayed the development of the CS and used this delay to assess the quantitative effect of light on CS development. We examined the effect of the duration and fluence rates of white light treatment on CS development and observed a significant relationship between fluence and the delay in CS development indicating that the Bunsen-Roscoe law of reciprocity holds for this response. The effect of white light irradiation was not inhibited in the presence of a photosynthetic inhibitor, DCMU, or a carotenoid biosynthesis inhibitor, Norflurazon, indicating that the delay in CS development by light is a photomorphogenetic response rather than a subsidiary effect mediated by photosynthetic activity. An action spectrum for the response displayed a major peak in the blue-light region, suggesting a dominant role for blue-light receptors. A minor peak in the red-light region also suggested the possible involvement of phytochromes. Although phytochromes are known to contribute to blue-light responses, phytochrome-deficient mutants showed a normal delay of CS development in response to blue light, indicating that the response is not mediated by phytochrome and suggesting a role for one or more specific blue-light receptors.

Phototropism: mechanism and outcomes

Plants have evolved a wide variety of responses that allow them to adapt to the variable environmental conditions in which they find themselves growing. One such response is the phototropic response - the bending of a plant organ toward (stems and leaves) or away from (roots) a directional blue light source. Phototropism is one of several photoresponses of plants that afford mechanisms to alter their growth and development to changes in light intensity, quality and direction. Over recent decades much has been learned about the genetic, molecular and cell biological components involved in sensing and responding to phototropic stimuli. Many of these advances have been made through the utilization of Arabidopsis as a model for phototropic studies. Here we discuss such advances, as well as studies in other plant species where appropriate to the discussion of work in Arabidopsis.

Light perception and the role of the xanthophyll cycle in blue-light-dependent chloroplast movements in lemna trisulca L

In most higher plants, chloroplasts move towards the periclinal cell walls in weak blue light (WBL) to increase light harvesting for photosynthesis, and towards the anticlinal walls as an escape reaction, thus avoiding photo-damage in strong blue light (SBL). The photo- receptor(s) triggering these responses have not yet been identified. In this study, the role of zeaxanthin as a blue-light photoreceptor in chloroplast movements was investigated. Time-lapse 3D confocal imaging in Lemna trisulca showed that individual chloroplasts responded to local illumination when one half of the cell was treated with light of different intensity or spectral quality to that received by the other half, or was maintained in darkness. Thus the complete signal perception, transduction and effector system has a high degree of spatial resolution and is consistent with localization of part of the transduction chain in the chloroplasts. Turnover of xanthophylls was determined using HPLC, and a parallel increase was observed between zeaxanthin and chloroplast movements in SBL. Ascorbate stimulated both a transient increase in zeaxanthin levels and chloroplast movement to profile in physiological darkness. Conversely, dithiothreitol blocked zeaxanthin production and responses to SBL and, to a lesser extent, WBL. Norflurazon preferentially inhibited SBL-dependent chloroplast movements. Increases in zeaxanthin were also observed in strong red light (SRL) when no directional chloroplast movements occurred. Thus it appears that a combination of zeaxanthin and blue light is required to trigger responses. Blue light can cause cis-trans isomerization of xanthophylls, thus photo-isomerization may be a critical link in the signal transduction pathway.

Structure and biogenesis of Chlamydomonas reinhardtii photosystem I

The photosystem I complex of the green alga Chlamydomonas reinhardtii was isolated and fractionated into its two subcomplex components: the core complex (CC I), which contained the reaction center (P-700) and had four polypeptide subunits, and the light-harvesting complex (LHC I) which contained four polypeptides of about 22, 25, 26 and 27 kDa. The 22-kDa apoprotein was isolated as a chlorophyll a and b binding protein. In the isolated photosystem I holocomplex, about ten copies of the 22-kDa LHC I apoprotein are present for each CC I unit. The 22-kDa polypeptide as well as the other three polypeptides of this complex and the subunit II of CC I are translated on 80S cytoplasmic ribosomes, and therefore are coded in the nucleus. During the greening process of the Chlamydomonas reinhardtii y-1 mutant the 22-kDa LHC I polypeptide, which cross-reacts with polyclonal antibodies raised against the Lemna gibba 20-kDa LHC I apoprotein, accumulates in thylakoids at a late stage of their development, and about 2-3 h after the LHC II and CC I subunit II polypeptides have accumulated. Accumulation of the 22-kDa protein during greening is inhibited by cycloheximide but not by chloramphenicol.

Photoregulation of psbA transcript levels in mustard cotyledons

We have investigated the photoreceptors potentially involved in the light regulation of the transcript levels of the psbA gene coding for D1, the 32 kD QB-binding protein of PSII. In cotyledons of 4 day old mustard seedlings, increasing fluence rates of continuous white light from ca. 0.1 to 250 μmol m(-2)s(-1) (400-700 nm) lead to a five-fold increase in transcript level from ca. 0.7 to 2.8 mg/g total RNA. The blue (<500 nm) component of this light did not contribute substantially to this effect, thus ruling out cryptochrome as the receptor responsible. Although phytochrome involvement was apparent from red/far-red reversibility, even multiple red pulses failed to elicit a comparable increase in transcript level to that seen under continuous white light. Although DCMU successfully inhibited delayed fluorescence quenching, it had no effect on transcript levels, thus ruling out photoregulation via electron transport and later components of the photosynthetic system. By contrast, Norflurazon, which leads to photobleaching of chlorophyll and hence disruption of thylakoid membrane assembly, completely abolished the light effect on psbA transcript level. We infer that photoregulation of the psbA transcript is principally related to thylakoid development, which is in turn critically dependent on photoconversion of protochlorophyllide to chlorophyll, but also associated with other processes such as phytochrome-regulated LHCP availability. Photocontrol of psbA expression is discussed in relation to that of the nuclear cab and rbcS genes.

Carotenoid synthesis and pleiotropic effects in carotenoid-deficient seedlings of maize

Plastid-envelope membranes from seedlings ofZea mays L. made carotenoid-deficient by either norflurazon treatment or mutation lack an activity permitting conversion of phytoene to β-carotene. This activity in membrane fractions was measured by coincubation in vitro with a soluble system from spinach chloroplasts capable of converting [(14)C]isopentenyl pyrophosphate into phytoene. When grown in light, the carotenoid-deficient seedlings lack many soluble chloroplast proteins, including NADP-dependent malic enzyme (EC 1.1.1.40), pyruvate phosphate dikinase (EC 2.7.9.1), and ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39), but apparently still contain the soluble activities permitting synthesis of phytoene.

The implication of a plastid-derived factor in the transcriptional control of nuclear genes encoding the light-harvesting chlorophyll a/b protein

In carotenoid-deficient albina mutants of barley and in barley plants treated with the herbicide Norflurazon the light-dependent accumulation of the mRNA for the light-harvesting chlorophyll a/b protein (LHCP) is blocked. Thus, the elimination of a functional chloroplast, either as a result of mutation or as a result of herbicide treatment, can lead to the specific suppression of the expression of a nuclear gene encoding a plastid-localized protein. These results confirm and extend earlier observations on maize [Mayfield and Taylor (1984) Eur. J. Biochem. 144, 79-84]. The inhibition of mRNA accumulation appears to be specific for the LHCP; the mRNAs encoding the small subunit of ribulose-1,5-bisphosphate carboxylase and the NADPH: protochlorophyllide oxidoreductase are relatively unaffected. The failure of the albina mutants and of Norflurazon-treated plants to accumulate the LHCP mRNA is not exclusively caused by an instability of the transcript but rather by the inability of the plants to enhance the rate of transcription of the LHCP genes during illumination. Several chlorophyll-deficient xantha mutants of barley, which are blocked after protoporphyrin IX or Mg-protoporphyrin, and the chlorophyll-b-less mutant chlorina f2 accumulate the LHCP mRNA to almost normal levels during illumination. Thus, if any of the reactions leading to chlorophyll formation is involved in the control of LHCP mRNA accumulation it should be one between the formation of protochlorophyllide and the esterification of chlorophyllide a. While the nature of the regulatory factor(s) has not been identified our results suggest that, in addition to phytochrome (Pfr), plastid-dependent factors are required for a continuous light-dependent transcription of nuclear genes encoding the LHCP.

Carotenoid-deficient maize seedlings fail to accumulate light-harvesting chlorophyll a/b binding protein (LHCP) mRNA

Yellow leaves of chlorophyll-deficient seedlings and white leaves of carotenoid-deficient seedlings contain no detectable light-harvesting chlorophyll a/b binding proteins (LHCP). Chlorophyll-deficient leaves contain plastids which are arrested in development prior to chloroplast formation [Mascia, P.N. and Robertson, D.S. (1978) Planta (Berl.) 143, 207-211] while carotenoid-deficient leaves contain plastids which are arrested in development at a rudimentary stage [Bachmann, M. D., Robertson, D.S., Bowen, C.C., and Anderson, I.C. (1967) J. Ultrastruc. Res. 21, 41-60]. Chlorophyll-deficient leaves have normal levels of nuclear-encoded LHCP mRNA while carotenoid-deficient leaves contain only trace amounts of LHCP mRNA. Similar results were obtained with carotenoid deficiencies caused by nuclear gene mutations and by treatment with the herbicide norflurazon which blocks carotenoid biosynthesis. We conclude that events at early stages of plastid development influence the accumulation of a nuclear-encoded mRNA.

Inhibition of carotenoid biosynthesis by the herbicide SAN 9789 and its consequences for the action of phytochrome on plastogenesis

Treatment of the mustard (Sinapis alba L.) seedling with the herbicide SAN 9789 inhibits synthesis of colored carotenoids and interferes with the formation of plastid membrane lipids without affecting growth and morphogenesis significantly. In farred light, which is hardly absorbed by chlorophyll, development of plastid ultrastructure, synthesis of ribulosebisphosphate carboxylase and synthesis of chlorophyll are not affected by SAN 9789. It is concluded that normal phytochrome actions on plastid structural development, protein and chlorophyll syntheses are not affected by the absence of carotenoids provided that there is no significant light absorption in chlorophyll. The findings show that the inhibition of synthesis of one set of plastid membrane components (the carotenoids) does not stop synthesis of other components such as chlorophyll and does not halt membrane assembly. Supplementary experiments with the closely related compound SAN 9785, which affects the amount and composition of plastid lipids but not carotenoid and chlorophyll syntheses, suggest that the effect of the herbicide SAN 9789 is due exclusively to its inhibition of synthesis of colored carotenoids. In the presence of SAN 9789 white or red light at high fluence rate causes photodestruction of chlorophyll and ribulosebisphosphate carboxylase and photodecomposition of thylakoids. These effects are interpreted as resulting exclusively from the self-photooxidation and photosensitizing action of chlorophyll once the protection by carotenoids of chlorophyll against self- and sensitized photooxidation is lost.

Spectrophotometric phytochrome measurements in light-grown Avena sativa L

Phytochrome was studied spectrophotometrically in Avena sativa L. seedlings that had been grown for 6 d in continous white fluorescent light from lamps. Greening was prevented through the use of the herbicide San 9789. When placed in the light, phytochrome (Ptot) decreased with first order kinetics (τ1/2 ≈ 2 h) but reached a stable low level (≈2.5% of the dark level) after 36 h. This concentration of phytochrome remained constant in the light and during the initial hours of a subsequent dark period, but increased significantly after a prolonged dark period. Evidence suggests that the constant pool of phytochrome in the light is achieved through an equilibrium between synthesis of the red absorbing (Pr) and destruction of the far-red absorbing form (Pfr) of phytochrome. It is concluded that the phytochrome system in light-grown oat seedlings is qualitatively the same as that known from etiolated monocotyledonous seedlings, but different than that described for cauliflower florets.