Sponsel, V. M. (1995). "Gibberellin biosynthesis and metabolism". Plant Hormones: Physiology, Biochemistry and Molecular Biology. Dordrecht: Kluwer. pp. 66-97.
Furthermore, evidence will be presented which indicates that dark-grownstock cultures exposed to light for 48 h (80 ft.-candles) can perceivethe light stimulus and then promote gibberellin biosynthesis indark-grown shake cultures.
In this chapter we will provide data to show that actually the activity of geranyl diphosphate synthase (GPS) is required in both tomato (Solanum lycopersicum) and Arabidopsis thaliana for the biosynthesis of gibberellins.
We will also argue that the pool of GGPP that is used for the biosynthesis of gibberellins is different from those GGPP pools that are used for other terpenoid-based molecules.
Through analysis of Arabidopsis microarray data we attempt to predict which member of the GGPS gene family is actually involved in gibberellin precursor biosynthesis.
Biosynthesis and Metabolism
ABA is a naturally occurring compound in plants. It is a sesquiterpenoid (15-carbon) which is partially produced via the mevalonic pathway in chloroplasts and other plastids. Because it is sythesized partially in the chloroplasts, it makes sense that biosynthesis primarily occurs in the leaves. The production of ABA is accentuated by stresses such as water loss and freezing temperatures. It is believed that biosynthesis occurs indirectly through the production of carotenoids. Carotenoids are pigments produced by the chloroplast which have 40 carbons. Breakdown of these carotenoids occurs by the following mechanism: Violaxanthin is a carotenoid which has forty carbons. It is isomerized and then split via an isomerase reaction followed by an oxidation reaction. One molecule of xanthonin is produced from one molecule of violaxanthonin and it is uncertain what happens to the remaining biproduct. The one molecule of xanthonin produced is unstable and spontaneously changed to ABA aldehyde. Further oxidation results in ABA. Activation of the molecule can occur by two methods. In the first method, an ABA-glucose ester can form by attachment of glucose to ABA. In the second method, oxidation of ABA can occur to form phaseic acid and dihyhdrophaseic acid. The transport of ABA can occur in both xylem and phloem tissues. It can also be translocated through paranchyma cells. The movement of abscisic acid in plants does not exhibit polarity like auxins. ABA is capable of moving both up and down the stem (Walton and Li, 1995; Salisbury and Ross).
Köhler hasdemonstrated that pea seedlings grown in red light possess more than tentimes as much gibberellin activity as plants grown in the dark and thatthe biosynthesis is influenced by the phytochrome system4,5.
The Relationship of Different Gibberellin Biosynthetic Pathways in Cucurbita maxima Endosperm and Embryos and the Purification of a C-20 Oxidase from the Endosperm
Previous studies suggested that the semi-dwarf stature of d35Tan-Ginbozu is caused by a defective early step of gibberellin biosynthesis, which is catalyzed by ent-kaurene oxidase (KO).
GA3ox1 encodes a Gibberellin 3-oxidase, which catalyses the last step in the biosynthesis of bioactive gibberellins (GA1 and GA4) (). GAs are important regulators of plant growth through both cell division and cell elongation (; ; ). It has been shown that the expression pattern of GA3ox1 (and other members of the GA3ox family) corresponds to the sites where active GAs are produced (; ). To confirm that endogenous IND participates in establishing the GA3ox1 expression pattern, we used the AtGA3ox1 TC-GUS reporter, which has been validated previously in a detailed analysis of GA3ox1 expression in seedlings (; ). Examination of stage 15 gynoecia showed expression of AtGA3ox1 TC-GUS in valve margins and the septum (; stages defined in ), overlapping with the expression pattern of IND (). Importantly, this specific aspect of AtGA3ox1 TC-GUS expression depended on IND: In the ind-1 mutant, expression was significantly reduced in valve margins but remained comparable in the gynophore (at the base of the developing fruit) (). In conclusion, the IND-dependent AtGA3ox1 TC-GUS expression, the microarray, and the ChIP results together indicate that IND directly activates GA3ox1 in medial tissues of the fruit, which include valve margins.
Together, all of the results described above support the following model for GA-mediated specification of SL in Arabidopsis (). We propose that IND directly activates the expression of GA3ox1 and, consequently, the production of bioactive GAs in developing valve margins. Prior to SL specification, we hypothesize that ALC is bound to DELLA proteins, preventing activation of its targets. Local gibberellin synthesis leads to degradation of DELLA proteins, releasing ALC to modulate the expression of its target genes and direct the differentiation of the SL. In this model, activation of ALC by GA biosynthesis occurs post-transcriptionally and not through regulation of ALC gene expression. To test if this is the case, we analyzed the ALC expression pattern in the ga4-1 mutant (Supplemental Fig. S5). No alterations in ALC expression were observed in the ga4-1 mutant background, consistent with our hypothesis that GAs control ALC at a post-transcriptional level.