Before the photosynthetic apparatus develops sufficiently in the early stages of germination, the stored energy reserves of starch nourish the seedling. Usually in germination, the breakdown of starch to glucose in the endosperm begins shortly after the seed is exposed to water. Gibberellic acid. The formation of the bioactive end products of the pathway from GA 12 involving mainly 2-ODD enzymes in plants is illustrated in Fig.
Although vegetative tissues of rice contain predominantly hydroxylated GAs Kobayashi et al. This finding prompted Magome et al. This anomaly may be related to differences in the efficiency of transport, or inactivation between the GAs when applied in bioassays. Arabidopsis contains two members of the CYPA subfamily, which are expressed in developing seeds Zhang et al.
He et al. They determined the function of CYP72A family members in Arabidopsis and other species and showed that some have related activities, although usually with a more restricted substrate range.
Indeed, many of the enzymes for which activity could be shown acted only on ent -kaurenoic acid. Overexpression of CYP72A9 , but not its paralogs, in Arabidopsis resulted in strong dwarfism, while seeds of cyp72a9 mutants germinated more rapidly than those of the wild type without stratification, suggesting that CYP72A9, in common with other GA-inactivating enzymes in seeds, may have a role in promoting dormancy He et al.
Phylogenetic analysis showed that the T. The gene, named GIM2 Xiong et al. It was shown by both groups that the enzyme expressed in Escherichia coli acts on GA 12 , but in one case it produced an unidentified hydroxyGA 12 Xiong et al. As both groups were working with the same gene, the discrepancy in their results is difficult to explain. There must still be some uncertainty about the function of this enzyme and whether GA 12 is its only substrate.
In the reaction sequence, the C methyl is oxidized to the alcohol and then to the aldehyde, from which C is lost. The alcohol and aldehyde intermediates accumulate and are efficiently converted further, which is consistent with the mechanism of 2-ODD enzymes, in which the reaction products are released from the enzyme active site before the substrates are rebound for the next round of oxidation Myllyla et al.
The alcohol intermediates, i. While oxidation of the C alcohol by GA20ox involves stereospecific loss of the pro R H, the spinach enzyme removed the pro S H from the lactone, in which the pro R H is fixed in a sterically hindered and poorly accessible position Ward et al. The mechanism for the loss of C from the aldehyde is not well understood. The oic acid is normally a minor by-product of the reaction Lange et al.
However, this carboxylic acid is not an intermediate in C 19 -GA biosynthesis Kamiya and Graebe Nevertheless, C is lost as CO 2 rather than as formic acid, which would be the case if it was lost directly from the aldehyde Kamiya et al.
Ward et al. The mechanism by which this radical is formed is unclear. In contrast to the alcohol and aldehyde intermediates, which accumulate often to relatively high concentrations, no intermediate between the aldehyde and C 19 -GA final product has been identified, prompting Ward et al. It has not been possible to study the fungal enzyme in vitro, but work with cultures of the F. Through the use of 18 O-labeled substrates, it was also established that both O atoms in the 19,lactone originate from the oic acid Bearder et al.
In contrast to the fungal enzyme, CYP from the bacterium Erwinia tracheiphila , prepared by expression in E. On the basis of incubations in 18 O 2 , Nagel and Peters confirmed that C is lost as CO 2 and proposed a mechanism in which the aldehyde, GA 24 , present as the C geminal diol or as the lactol, is oxidized to the C—20 anhydride with C present as the geminal diol.
Further oxidation of this intermediate would release C as CO 2 forming the 19,lactone by rearrangement via the C radical. While it is possible that hydrolysis of the anhydride to the dicarboxylic acid could account for the small amounts of oic acid formed as a by-product of C 19 -GA formation in plants, it is also noteworthy that C,20 dioic acids readily form the anhydride in solution but are not substrates for GA20ox in plants, fungi or bacteria Kamiya and Graebe , Tudzynski et al.
Seed plants contain a family of GA20ox genes, with members differing in their developmental, environmental and tissue expression patterns. For example, Arabidopsis contains five GA20ox genes encoding functionally similar enzymes, except for AtGA20ox5, which produces the aldehyde without further conversion to the C 19 -GA Plackett et al.
AtGA20ox1 and AtGA20ox2 act partially redundantly in plant development, with AtGA20ox3 having a minor role, while the physiological function of the other two genes is unclear Rieu et al.
Poaceae grasses , including the cereals, typically contain four GA20ox genes, with the expression of one of them, GA20ox3 , although relatively very high, restricted to the endosperm of developing grain, which produces large amounts of GAs of uncertain function Pearce et al.
It should be noted that gene annotation numbers, which usually relate to their order of discovery, do not denote orthology, except within plant families, as GA-oxidase gene multiplication and divergence seems to have occurred relatively late in evolution Han and Zhu , Huang et al. GA3ox-like enzymes present in developing seeds of both eudicots and monocots may have quite diverse activities.
For example, a GA3ox from C. GA 54 and GA are the major GAs present in developing seeds of wheat and barley, respectively MacMillan , although their function in seeds is unknown. GA biosynthesis is often very strong in developing seeds, which can produce a wide array of structures, reflecting the functional diversity of the enzymes involved, although in many cases the nature of these enzymes is still unknown. GA3ox genes are present as small families, with Arabidopsis containing four members and rice and barley only two.
A second rice enzyme OsGA3ox1, which contributes particularly toward reproductive development, does not have close orthologs in barley and wheat, in which, apart from GA3ox2, the other GA3ox-like genes are mainly expressed in developing seeds. Production of bioactive GAs by fungal and bacterial plant pathogens is proposed to facilitate infection by suppressing jasmonic acid signaling that promotes immunity, whereas in the symbiotic N 2 -fixing rhizobia, in which GA has a role in nodule formation, the ability of the plant to regulate GA production must be advantageous Nagel and Peters Inactivation, i.
A number of inactivating reactions have been described illustrated in Fig. While these families are phylogenetically not closely related, there is functional overlap, with some enzymes belonging to the C 19 -GA2ox family acting on C 20 -GAs, usually as a minor activity, and vice versa Lange et al.
The C 19 -GA2ox family is the largest of the GA 2-ODD families and, on the basis of sequence, falls into two sub-families, this division preceding the divergence of the monocots and eudicots Kawai et al.
While GA2ox genes are present in seed plants Niu et al. The exceptions are symbiotic GA-producing bacteria for which GA production may need to be more tightly regulated, but as noted above, they produce the precursor, GA 9 , allowing the plant host to regulate the synthesis of bioactive GA. It is likely that the catabolites are artifacts of isolation and that the product in planta is the unrearranged ketone.
Different enzyme types are indicated by color: brown, 2-oxoglutarate-dependent dioxygenases; green, cytochrome Ps enzymes present in rice in dark green and in Arabidopsis light green ; red, methyltransferases; blue, glucosyltransferases; and purple, epoxide hydrolase. GA 12 hydroxylation, which initiates the biosynthesis of GA 1 , results in slight reduction in bioactivity and is consequently included in B.
The involvement of EUI2 in dihydrodiol formation is assumed, but has not been demonstrated. The recent determination of the X-ray crystal structure of the rice C 19 -GA2ox OsGA2ox3 revealed that it formed a tetramer in the presence of its substrate GA 4 , with the monomers linked via two GA 4 molecules and two disulfide bridges Takehara et al.
The tetramer was shown to be more active than the monomer, exhibiting a lower Km for GA 4 by enabling an energetically more favorable pathway to the active site. Thus, increasing GA 4 concentration promotes multimer formation and enhances enzyme activity, providing an allosteric feedforward mechanism to maintain GA homeostasis.
Remarkably, Takehara et al. GA2ox enzymes have an essential function in regulating GA concentration during normal plant development, and also in response to changes in environmental conditions. The wheat reduced height alleles Rht18 and Rht14 cause increased expression of the C 20 -GA2ox gene TaGA2oxA9 in stems, while some loss-of-function taga2ox9 mutants generated from Rht18 exhibited overgrowth phenotypes compared with the tall parent of Rht18 , which suggests a potential role for this gene in the control of stem height Ford et al.
The reduction in the bioactive GA content in leaves as they mature was shown in pea to be due to high rates of 2-oxidation rather than reduced biosynthesis Ross et al. In developing seeds, GA2ox activity may increase to high levels as the seed approaches maturity Albone et al. This is illustrated by the slender sln mutant of pea, which has an overgrowth phenotype during early seedling development due to a mutation in the PsGA2ox1 gene that allows GA 20 to accumulate in the mature seed Lester et al.
Conversion of GA 20 to GA 1 following seed imbibition promotes the excessive seedling growth. It has been reported for several species that GA2ox expressed at the base of the shoot apical meristem limits the influx of bioactive GA to the meristem to control meristem function Sakamoto et al.
Induction of GA2ox expression by stress is a common mechanism for growth control and enhanced stress tolerance with different GA2ox genes being targeted according to the stress reviewed in Colebrook et al. Of particular significance, EUI1 CYPD1 has an important developmental function in rice by restricting culm height, acting particularly on the upper internodes Zhu et al.
Introduction of eui1 mutant alleles into male sterile rice to allow adequate panicle exsertion was an important development for hybrid rice production Liang et al. GA 16,dihydrodiols are widely distributed in plants suggesting that double bond epoxidation and hydrolysis may be a common activity. EUI1 is highly expressed in the nodes and intercalary meristem of the upper internodes, as well as in the flowering spikelets of the young panicle Zhu et al.
Unlike vegetative tissues of rice, reproductive tissues contain predominantly H GAs, with mature anthers containing very high concentrations of GA 4 Hirano et al. As EUI is active only against H GAs, it is tempting to suggest that it regulates the levels of GA GA 4 or precursors reaching the upper internodes by movement from the panicle, a process that serves to coordinate panicle exsertion with anthesis.
GA inactivation by methylation, catalyzed by members of the SABATH family of methyl transferases, has been reported for Arabidopsis, in which the enzymes are present in developing seed Varbanova et al. Although they have been described only for Arabidopsis, GA methyl transferases are more widely distributed in seed plants and are also present in some fern species, in which methyl GAs are present as antheridiogens Yumane et al.
Sugar conjugation, particularly reversible esterification with glucose Schneider et al. There has been a hiatus in research on GA conjugates since the s, but their relevance to GA metabolism and transport merits renewed investigation.
The sites of GA synthesis and their relationship to the sites of action are of major relevance to any consideration of function. There is renewed interest in GA distribution stemming from the identification of GA transporters and the development of in vivo methods to determine GA distribution and movement at the cellular level Rizza et al.
The topic has been reviewed recently Lacombe and Achard , Binenbaum et al. The sites of GA synthesis are usually inferred from the expression of biosynthesis genes on the basis of reporter activity, in situ hybridization or, in the case of Arabidopsis roots, transcript analysis in combination with cell isolation and sorting Birnbaum et al. However, this does not allow for differences in translational efficiency or enzyme stability.
Treatment of spinach with GA biosynthesis inhibitors resulted in elevated levels of SoGA20ox1 protein, measured by Western blotting, in the petioles and shoot tip, but no change in the transcript level Lee and Zeevaart , emphasizing the need to consider posttranscriptional regulation.
The location of GA biosynthesis has been investigated more directly from the application of radioactively labeled GAs, e. Normal development under non-stressful conditions depends on appropriate coordination of GA biosynthesis and inactivation.
Reinecke et al. Ectopic expression of PsGA3ox1 , which is rate limiting for GA biosynthesis in pea, resulted in strong upregulation of the GA-catabolic gene PsGA2ox1 , which the authors suggested would be normally segregated from cells responsible for GA biosynthesis.
In vegetative organs, GAs are synthesized mainly in growing regions, such as elongating stems and leaves, and root tips. Very high rates of synthesis occur in certain tissues, including anthers Hirano et al.
While there is evidence based on transcript localization for GA biosynthesis occurring at or close to the site of action, e. Examples are the cereal embryo scutellum as a source of GA for the aleurone Appleford and Lenton , the suspensor as a GA source for the embryo in several species reviewed in Jacob and Brian and GA or precursors from the anther tapetum being required for filament elongation and petal growth Weiss and Halevy , Silverstone et al.
In female cucumber flowers, GA 9 produced in ovaries moves to the petals and sepals where it is converted to GA 4 , which promotes the expansion of these organs Lange and Lange In these cases, the hormone acts to coordinate the growth and development of neighboring, physiologically related organs.
As suggested above, GA from the anthers in cereals may also stimulate peduncle elongation to ensure adequate emergence of the spike. There are also examples of long distance GA transport, such as from leaves to induce the transition to flowering at the shoot apex in Arabidopsis Eriksson et al.
In some cases, long distance movement of precursors rather than the active hormone has been noted Proebsting et al. It is unclear what specifies the structure of the mobile molecules, but it may be determined by the properties of transmembrane transporters.
So far only influx transporters have been identified and these lack specificity, transporting other hormones as well as unrelated molecules reviewed in Binenbaum et al. According to the ion-trap hypothesis, the high pH environment of the cytosol would deter efflux of GAs by passive diffusion through the cell membrane, while influx from the more acidic apoplast would be more favored Kramer However, passive diffusion across biological membranes, which are rich in proteins and other molecules that can interact with mobile signals, may be limited Kell , such that both influx and efflux transporters are necessary for effective mobility.
The identification and properties of GA transporters is likely to remain an active field of research. The concentration of biologically active GAs in GA-responsive tissues is tightly regulated through biosynthesis, inactivation and transport. The mechanisms involved in regulating the expression of GA biosynthesis and inactivation genes in higher plants in response to developmental and environmental signals are active areas of research. The literature on this topic has been reviewed in detail Hedden and Thomas , Hedden , Magome and Kamiya and will be summarized here, as well as highlighting some recent findings see Fig.
CPS catalyzes the first committed step and is suggested to be the gateway to the GA-biosynthetic pathway with a role in developmental regulation Silverstone et al. Members of the 2-ODD gene families, which differ in their spatial and temporal expression patterns, are major sites of regulation.
Expression of OsEUI1 was shown also to be promoted directly by the homeodomain-leucine zipper transcription factor HOX12 to regulate panicle exsertion Gao et al.
Regulation of GA biosynthesis and inactivation, highlighting the GA signal transduction pathway that enables GA homeostasis. The figure summarizes the data for Arabidopsis. Some pathways for the regulation of GA metabolism in response to environmental signals are also indicated.
Thomas et al. Briefly, binding of GA to its receptor GID1 results in a conformational change in the receptor that promotes its interaction with DELLA proteins, which then through association with the F-box component of an E3 ubiquitin ligase are targeted for degradation via the ubiquitin-proteasome pathway. Fukazawa et al. This transcription factor was also shown to specify the expression of AtGA20ox2 in the shoot apex and root tip Fukazawa et al.
As discussed above, feedback regulation may also occur at the protein level Lee and Zeevaart and, although it is more difficult to study, it warrants further investigation. Crosstalk between hormone signaling pathways is well established with DELLA proteins acting as a major hub.
The evidence for other hormone signaling pathways targeting GA metabolism is conflicting Ross et al. A major function of GA is to mediate growth and developmental responses to environmental changes, which can cause rapid modification in GA concentration through altered metabolism. Environmental factors, including temperature, mechanical stimulation, abiotic and biotic stress and the duration, intensity and quality of light, have all been shown to affect GA biosynthesis and inactivation, acting primarily on the expression of the 2-ODD genes.
In many cases, the transcription factors mediating these responses have been identified reviewed in Hedden and Thomas , Hedden , Magome and Kamiya Promotion of Arabidopsis seedling growth by transfer to higher temperature is associated with increased expression of AtGA20ox1 and AtGA3ox1 and decreased expression of AtGA2ox1 in the hypocotyl Stavang et al.
Recently, it was shown that microRNA regulation of an AP2 protein promoted stem elongation in barley, but the authors propose that AP2 acts through the jasmonate, rather than GA pathway to restrict internode elongation in this case Patil et al.
In common with other secondary metabolites, GA production by the fungus F. Factors affecting the expression of the GA-biosynthetic operon in bacteria are less well understood, although in symbiotic rhizobia GA production is highly dependent on the developmental stage of the host plant Mendez et al.
Gibberellin biosynthesis is a mature field, which began in the lates with work on GA 3 biosynthesis in the fungus F. Progress in the field was initially slow but accelerated with the development of sensitive methods for compound identification, the increasing availability of mutants and, more recently, of full genome sequences. These advances led first to the establishment of the metabolic pathways and then to the identification of the enzymes and finally of the relevant genes, in plants, fungi and recently bacteria.
The availability of genome sequences has prompted interest in the evolution of GA metabolism. Current information indicates that in plants GA biosynthesis evolved with vascularization, emphasizing the importance of GAs as a mobile signal. It is remarkable that the ability to produce these complex molecules has evolved three times in different kingdoms: plants, fungi and bacteria. There are reports of GAs occurring in algae, but as nonvascular land plants had not evolved the capability to synthesize GAs, it is unclear where algae fit in the evolutionary scheme.
The reports need to be confirmed and the relevant enzymes identified. Current work with flowering plants is focused on the mechanisms involved in regulating GA concentrations in response to developmental and environmental cues. While there is particular emphasis on the expression of GA biosynthesis and catabolism genes, it is also necessary to determine the sites of GA biosynthesis and action, ideally at the cellular level, and the mechanisms involved in linking them. Indeed, GA localization and movement is currently attracting considerable interest, e.
Locating the sites of GA accumulation at the cellular levels is an important goal that is being addressed through in vivo methods.
Currently, for practical reasons, this has been restricted to locating the biologically active compounds, but determining the location of precursors can provide important information on how GA production is regulated.
This will need to be addressed, and although the quantification of GAs and their precursors and catabolites at the cellular level by physicochemical methods is challenging, it is becoming more realistic with the increasing sensitivity of methods, such as UPLC—MS.
I am grateful to colleagues at Rothamsted Research for continued support and to Prof. Aach H. Planta : — Google Scholar. Albone K. Plant Physiol. Appleford N. Beale M. Phytochemistry 21 : — Bearder J. Part XIV. Metabolic pathways from ent -kaurenoic acid to fungal gibberellins in mutant Ba of Gibberella fujikuroi.
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Frigerio M. Frisse A. Fujioka S. Fukazawa J. In recent years, a number of studies have revealed less apparent roles for GA in a surprisingly broad set of developmental as well as cell biological processes.
The diterpenoid substances of the gibberellin GA class were first described and isolated in the s and s based on the property of a compound isolated from the rice pathogenic fungus Gibberella fujikuroi to induce strong elongation growth and other disease symptoms in rice. After their identification as causative agent of this so-called bakanae foolish seedling disease, it was later discovered that GAs are also synthesized by plants where they promote a number of important developmental processes besides elongation such as germination and flowering.
In the following decades, GA biology gained particular attention because it was recognized that interfering with GA signaling by chemical or genetic means could be used to modulate plant growth and most importantly to control crop yield and quality Peng et al.
The mechanisms that underlie GA action in plant growth control have mainly been revealed through studies conducted in rice, Arabidopsis and other model species such as pea and tomato.
There, the analysis of mutants with defects in GA biosynthesis and signaling as well as the availability of chemical GA biosynthesis inhibitors has allowed the identification of the molecular components that control GA response during germination Lee et al.
More recently, less apparent roles for GAs could be elucidated such as roles in cell proliferation Achard et al. In order to keep the complexity of the present minireview to an appropriate level, this review almost exclusively summarizes molecular results from rice and Arabidopsis thaliana , where the wealth of genetic resources has enabled gaining the most valuable insights into GA biology and signaling.
The discovery of the GA receptor based on the rice gibberellin insensitive dwarf1 gid1 mutant in represented a major breakthrough in the understanding of the signaling pathway of this hormone Ueguchi-Tanaka et al. Figure 1. In species with multiple DELLA proteins, such as Arabidopsis , the latter statement is complicated by the fact that homeostasis mechanisms are in place that regulate the overall abundance of the functionally redundant DELLA proteins via negative feedback mechanisms that at least in part function via GA biosynthesis Peng et al.
At the same time it is known that the expression of GA biosynthesis genes is upregulated in such backgrounds and that reduced RGA levels are the consequence of increased GA-dependent protein turnover. DELLA proteins are typically but not exclusively inactivated by protein degradation. This inactivating mechanism allows to explain the comparatively mild phenotypes of rice gid2 and Arabidopsis sly1 mutants, E3 ubiquitin ligase subunit mutants that accumulate very high levels of DELLA proteins.
Biochemical and physiological data suggest that the substitution of a specific proline residue in GID1, e. These studies revealed that the hypothesis about their predicted role as transcriptional regulators was correct, while at the same time, the hypothesis about them functioning as DNA-binding proteins most likely is incorrect.
Phytochrome interacting factors PIFs are a subfamily of basic helix-loop-helix bHLH transcription factors that are characterized by the AP domain, a domain for the binding of the light-activated Pfr conformer of phytochromes Leivar and Quail, Through these interactions, DELLAs seemingly prevent PIFs from binding to their cognate promoter binding sites, and they thereby interfere with the transcriptional activity of PIFs and ultimately their biological function, e.
Importantly, PIFs also integrate light signaling through their interaction with activated phytochrome, and this is followed by their proteasomal degradation. Since it is known that GA levels decline when dark-grown seedlings are transferred to the light Achard et al.
In the light, PIF proteins — albeit present at low levels — are expected to retain functionality in the light-grown seedling and plant. Figure 2. Auxin transport positively feeds back on GA biosynthesis. SPT was identified based on its role in septum, style, and stigma development in the flower Alvarez and Smyth, SPT was subsequently also shown to control germination in response to cold temperature in Arabidopsis seeds and to repress cotyledon expansion Penfield et al.
At present, it remains to be seen whether such a regulation requires the interaction between DELLAs and SPT and which other molecular players contribute to this interaction. Gene expression studies and pathobiological assays had suggested that GA controls JA-responsive gene expression Cao et al. In an analysis of the underlying molecular causes it was found that GA attenuates the JA-induced expression of a number of JA-responsive genes Hou et al. Most of the DELLA-dependent regulatory events that have been identified so far involve a cascade of — typically repression — events that ultimately lead to gene expression changes that — somehow — are responsible for GA-mediated growth control.
Few of these signaling events allow hypothesizing about how GA controls growth at the cell biological level. At least two reports link GA signaling to auxin transport and these findings help at least in part to explain how the GA signal is influenced by or is influencing transport of the plant hormone auxin and ultimately plant growth.
Auxin transport controls organ initiation and development as well as tropic responses: auxin is synthesized in the shoot apex and transported toward the root tip via the activity of PIN-FORMED PIN auxin efflux carriers. When the shoot and consequently the shoot-derived auxin source is removed, roots cease to grow and fail to respond properly to GA. This reduced GA-responsiveness can be suppressed when the auxin indoleacetic acid IAA is applied to the site of shoot removal.
GA promotes the degradation of the auxin efflux carriers PIN1 and PIN2, at least in root tips and inflorescence stems, possibly by increased targeting of these transporter proteins for degradation in the vacuole Vieten et al. At the physiological level, the downregulation of PIN proteins as observed in GA and GA signaling-deficient mutants correlates with a reduction in auxin transport in inflorescence stems, defective embryo development and reduced root gravitropism.
This review provides an overview of the current knowledge on the mechanisms governing GA signaling. Certainly, it has become clear that especially in recent years major advances have been made in understanding the molecular mode of action of the DELLA proteins as key repressors of the pathway.
A number of obvious questions remain to be answered, some of which have already been phrased in the above paragraphs. SPYs repression function in Arabidopsis GA signaling was recapitulated in other plant species including tomato and rice Greb et al. A proof for this or an alternative mode of action urgently needs to be brought about. Although it had initially been claimed that this modification is essential for the proteasomal degradation of DELLAs Sasaki et al.
On the other side, in vitro data using pharmacological inhibitors suggest that phosphoregulation may be involved in DELLA protein turnover Wang et al. The mutation of predicted phosphorylation sites in Arabidopsis RGL2 impairs GA-induced protein degradation but whether or not this effect is ultimately due to changes in RGL2 phosphorylation rather than RGL2 functionality could not be fully clarified Hussain et al.
Further experiments will bring light into the identity of the in vivo targets of this kinase and may identify further kinases that interfere with DELLA activity. The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Achard, P. The plant stress hormone ethylene controls floral transition via DELLA-dependent regulation of floral meristem-identity genes.
CrossRef Full Text. DELLAs contribute to plant photomorphogenesis. Plant Physiol. Integration of plant responses to environmentally activated phytohormonal signals. Science , 91— Gibberellin signaling controls cell proliferation rate in Arabidopsis. Plant DELLAs restrain growth and promote survival of adversity by reducing the levels of reactive oxygen species. Plant Cell 15, — Alabadi, D.
Gibberellins repress photomorphogenesis in darkness. Alvarez, J. Development , — Pubmed Abstract Pubmed Full Text. Ariizumi, T. Plant Cell 20, —
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