Regulation Of Chlorophyll Biosynthesis Research Articles

Mitigating Sun-Seeking Behavior

The moment the growing seedling emerges, the Sun acts as both power and poison. Precursors of chlorophyll lie ready to be converted to functioning chlorophyll, but the precursor form can do more harm than good if exposed to light. Huq et al. have now identified a transcription factor, PIF1, that interacts with light-sensing phytochromes and regulates chlorophyll biosynthesis. PIF1 negotiates the balance between chlorophyll synthesis and the sensing of light such that emergent seedlings become photoautotrophic before they would otherwise burn up. E. Huq, B. Al-Sady, M. Hudson, C. Kim, K. Apel, P. H. Quail, PHYTOCHROME-INTERACTING FACTOR 1 is a critical bHLH regulator of chlorophyll biosynthesis. Science 305 , 1937-1941 (2004). [Abstract] [Full Text]

Phytochrome-interacting factor 1 is a critical bHLH regulator of chlorophyll biosynthesis.

Photosynthetic organisms must achieve a delicate balance between the light energy absorbed by chlorophyll and their capacity to channel that energy into productive photochemical reactions. Release of excess absorbed energy in the cell can cause lethal photooxidative damage. We identified a basic helix-loop-helix (bHLH) transcription factor, designated PHYTOCHROME-INTERACTING FACTOR 1 (PIF1), that negatively regulates chlorophyll biosynthesis. pif1 mutant seedlings accumulate excess free protochlorophyllide when grown in the dark, with consequent lethal bleaching upon exposure to light. PIF1 interacts specifically with the photoactivated conformer of phytochromes A and B, suggesting a signaling pathway by which chlorophyll biosynthetic rates are tightly controlled during the critical initial emergence of seedlings from subterranean darkness into sunlight.

Characterization of Glutamate-1-Semialdehyde Aminotransferase and Its Activity Throughout Tomato Fruit Development and Ripening

Glutamate-1-semialdehyde aminotransferase (GSAT) is a key enzyme in the regulation of chlorophyll biosynthesis and a plausible site to regulate chlorophyll loss during fruit development via a decrease in its activity. The primary objective of this research was to characterize GSAT from tomato fruit by varying substrate concentration and pH and to measure GSAT in various organs of the tomato plant. A secondary objective was to measure GSAT activity and chlorophyll concentration at 5-day intervals from 10 d to 60 d postanthesis during tomato fruit development and ripening. An optimum pH of 7.0 was found, and a maximum velocity (Vmax) of 2.3 nmol·g–1 of tissue and a Km of 119 mmol·L–1 were calculated for GSAT. Activity among organs was greatest at 3.84 nmol·g–1 tissue in leaves of 4-week-old seedlings, 1.67 nmol·g–1 tissue in fruits 15 days postanthesis, and below 0.70 nmol·g–1 tissue in roots, stems, and overripe fruits (65 d postanthesis). During fruit development, GSAT activity was greatest at 10 d postanthesis and declined to day 25. Activity increased from day 25 to a peak at day 40 and then decreased again during ripening to 60 d postanthesis. Total chlorophyll and chlorophyll a concentrations in tomato fruit declined throughout development and ripening with the greatest decrease between 10 d and 25 d postanthesis. Thus, GSAT activity may contribute to the control of chlorophyll concentration in tomato fruit from 10 to 25 d postanthesis when chlorophyll loss mirrors the decline in GSAT activity.

Regulation of Chlorophyll Biosynthesis in Angiosperms.

Light controls plant growth and development in a pleiotropic manner. For example, light dictates a pattern of gene expression that leads to photomorphogenesis (Kendrick and Kronenberg, 1994). In the absence of light, an alternative developmental process, skotomorphogenesis, takes place. Photomorphogenesis and skotomorphogenesis are controlled by several different photoreceptors, such as Pr and Pfr and blue-light-absorbing photoreceptors. During skotomorphogenesis, the germinating seedling utilizes all of the nutrients contained in the seed to establish conditions that allow the plant to harvest even traces of light. The hypocotyl elongates dramatically just to place the cotyledons above the soil. Within the cotyledons, proplastids differentiate into etioplasts, and a large supply of the tetrapyrrole pigment precursor Pchlide is built up in the prolamellar bodies of the etioplasts. Together with NADPH, Pchlide is bound to the PORA. This ternary complex is photoactive and thus able to immediately photoreduce the Pchlide to Chlide as soon as the cotyledons are exposed to light (for review, see Ryberg and Sundqvist, 1991). During photomorphogenesis, hypocotyl elongation is suppressed, but cotyledon unfolding and expansion proceed normally. Chloroplast development as well as Pchlide reduction occur in parallel and collectively lead to the rapid greening of the plant. Pioneering experiments performed in Granick's group almost 40 years ago shed some light on the regulatory mechanisms that govern Chl biosynthesis in higher plants. When angiosperm plants were germinated in the dark, their cotyledons appeared pale yellow. If such etiolated seedlings were incubated with ALA, a common precursor -of all tetrapyrrole pigments, they turned greenish (Granick, 1959). This apparent greening was not caused by the synthesis of Chl but was due to the accumulation of excess Pchlide, the immediate precursor of Chlide. Similar to leaf tissues, isolated plastids were found in later studies to accumulate excess Pchlide when they were fed ALA in the dark (Kannangara and Gough, 1977). Taken together, these basic experiments showed that all of the enzymes necessary for the conversion of ALA to Pchlide must be present in dark-grown angiosperm plants, that the entire pathway is likely to operate in the plastid, and that the only known light-requiring reaction for the synthesis of Chl is the reduction of Pchlide to Chlide. It is the aim of this review to summarize our current knowledge on key regulatory steps of Chl biosynthesis in angiosperms.

Light Regulation of Chlorophyll Biosynthesis at the Level of 5-Aminolevulinate Formation in Arabidopsis

Light Regulation of Chlorophyll Biosynthesis at the Level of 5-Aminolevulinate Formation in Arabidopsis

Components of chlorophyll biosynthesis in a barley albina mutant unable to synthesize ?-aminolevulinic acid by utilizing the transfer RNA for glutamic acid

Components of chlorophyll biosynthesis were investigated in the plastid-ribosome-deficient albostrians mutant of barley (Hordeum vulgare L.). Compared with green leaves, white leaves lacked chloroplast tRNA(Glu) and 16S ribosomal RNA, but contained a much higher level of the mRNA for glutamate 1-semialdehyde aminotransferase. Substantial amounts of protochlorophyllide were accumulated when the mutant was incubated in a solution of δ-aminolevulinic acid. The level of protochlorophyllide oxidoreductase mRNA (PCOR, EC 1.6.99.1.) in etiolated albostrians plants reached only about 50% of the level in wild-type plants. In addition the content of PCOR protein and the activity of chlorophyll synthetase were distinctly lower than in the wild-type. Mutant and wild-type barley seedlings which were grown under a daily light/dark regime and were therefore nonetiolated both possessed PCOR mRNA. The data presented may help explain the albino phenotype of this mutant. The results are discussed in relation to biosynthesis of tetrapyrrols in higher plants, regulation of chlorophyll biosynthesis and the action of a plastidderived signal involved in the expression of certain nuclear genes.

Stimulation of protochlorophyllide oxidoreductase by thioredoxin

Protochlorophyllide oxidoreductase [E.C.1.3.1.33] catalyses the reduction of protochlorophyllide to chlorophyllide. Pigment mutant C-2A′ of the alga Scenedesmus obliquus possesses this enzyme when grown in darkness or light. The mutant accumulates protochlorophyllide and a trace of chlorophyll in darkness. In the light it synthesizes chlorophyll in amounts comparable with that of the wild type. Reduced plastidal thioredoxin f of Scenedesmus obliquus, which is formed in this mutant during greening prior to chlorophyll synthesis, stimulates protochlorophyllide oxidoreductase activity in vitro in light and darkness. The cytoplasmic thioredoxins and E. coli thioredoxin are less active. This system is considered to be a new case of thiorexodin-mediated control of chloroplastic enzymes, and a new factor in the regulation of chlorophyll biosynthesis.

Photosynthetic Functions and Induction of Etiolation in Chloroembryos of Dolichos lablab L.

Summary The regulation of chlorophyll biosynthesis was blocked in chloroembryos of Dolichos lablab through blockage of light to the fruit resulting in the transformation of chloroembryos (green embryos) into etiolated embryos. Aminolevulinic acid (ALA) was not blocked from synthesis in etiolated embryos as evidenced by a comparable level of ALA in etiolated embryos as in green embryos. Therefore, it may be concluded that chlorophyll biosynthesis in the etiolated embryos was not blocked at ALA formation but at the protochlorophyll level as evidenced by resynthesis of pigments upon removal of the mask. Photosynthetic CO 2 uptake and Hill activity in green embryos reveals that photosystems I and II are functional in the chloroplasts of green embryos. Respiration in green embryos of stage I was twice as high as the photosynthetic rate.

Enzymic capacities for chlorophyll biosynthesis. Activation and de novo synthesis of enzymes.

A previously published working model for the regulation of chlorophyll formation has been tested studying early steps of chlorophyll and porphyrin biosynthesis in developing cotyledons of Helianthus annuus. The activities of delta-aminolevulinate synthetase (ALAS), delta-aminolevulinate dehydratase (ALAD), and the porphobilinogenase complex (PBGase) at any given time have been found to be strongly associated with endogenous developmental processes. Highest activities in darkness have been observed at times when maximum chlorophyll formation would have occurred had the plants been exposed to light. Only in the case of ALAS was the maximum activity in light much greater than that observed in the dark. Density labeling experiments and other data suggest that enzyme synthesis is mediated both by development and by illumination. Moreover, ALAS activity appears to be subject to inhibition, presumably by products of the porphyrin biosynthesis, as indicated by halflife experiments. Rapid enzyme degradation in the absence of light seems to be less probable. Slight ALAS activity in darkness is present as long as the plastids are not fully developed. In contrast to findings with cell cultures of tobacco, in Helianthus cotyledons ALAS certainly plays the main role in the regulation of chlorophyll biosynthesis. Nevertheless, increasing activities of the succeeding enzymes, located in the plastids, ensure that increased concentrations of delta-aminolevulinate (ALA) are drawn into the chlorophyll biosynthetic pathway. The experiments corroborate the suggestion that chlorophyll biosynthesis is controlled by different but interdependent mechanisms. The dominant regulatory mechanism is dependent on the stage of development.