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GA regulate its own metabolism the level of active GA is kept within a narrow range (homeostasis) negative feedback regulation: depress biosynthesis positive feed-forward regulation: stimulate catabolism + GA GA20ox, GA3ox GA2ox Low GA mutant GA20ox, GA3ox GA2ox AMO-1618, cycocel (CCC); phosphon D (Commercial name) Paclobutrazol (Bonzi) Ancymidol (A-Rest) Can be genetically engineered BX-112 GA-glycosides, GA-glycosyl ester GA1 and GA4 have intrinsic bioactivity for stem growth, GA1 for pea; GA4 for Arabidopsis (p. 520~1) Environmental conditions: light, photoperiod and temperature affect the transcription of GA biosynthesis genes The wavelength of light is critically important, implying that specific photoreceptor(s) are involved p. 519 Far-red illumination of petiole elongation of Arabidopsis ∵ AtGA20ox2 transcription 40-fold increase AtGA20ox1 transcription minor effect Web Topic 20.5 Plants growing in very-low-light conditions or in complete darkness are characterized by an etiolated growth habit with long, thin stems and small, yellow cotyledons and leaves (see chapter 17). On exposure to normal light conditions, the growth rate of the stem is reduced, the stems thicken, and leaves expand and green. This process is known as de-etiolation (see Figure 17.1, p 418). After years of controversy as to whether or not de-etiolation involves changes in GA content, it has now been clearly established that exposure of etiolated pea seedlings to light results in a rapid, but temporary, reduction in the content of GA1, which is the major bioactive GA in pea stems. The light, which is detected by phytochrome A and possibly a blue light receptor, causes a down-regulation of 3-hydroxylation and an up-regulation of 2-hydroxylation within 30 minutes of exposure. The consequence of this altered metabolism—a transient reduction in the steady-state level of GA1—is thought to be important in establishing the types of growth changes that accompany photomorphogenesis (Reid et al. 2002). More prolonged changes in GA status affect the sensitivity of light-grown tissues to GA1, rather than the actual level of GA1 (O’Neill et al. 2000). GA responses: during germination and early seedling growth p. 528 a barley grain - and -amylase, maltase Aleurone cells: contain many protein body. Cereal grains: 1. Embryo: embryo, scutellum 2. Endosperm: starch endosperm, aleuron layer 3. Testa-pericarp not for -amylase, callus same The sole function of the aleurone layer of the cereal grains appears to be the synthesis and release of hydrolytic enzymes into the starchy endosperm during germination. After completing the synthesis and secretion of hydrolytic enzymes, the aleurone cells undergo programmed cell death. The precise location of GA biosynthesis within the germinating grain Rice grain, 24 imbibition GA biosynthetic enzymes (A and B) and GA signaling molecules (C~E) (A) OsGA3ox1:GUS: embryo only (B) OsGA20ox1:GUS: embryo only (C) G:GUS: embryo and aleurone subunit of G protein (D) SLR:GUS: embryo and aleurone (E) OsGAMYB:GUS: embryo and aleurone [Biosynthesis vs. response site] GA : flowering in long-day plants Photoperiodism: plants have evolved to flower in response to specific day lengths three major categories: Short-day plants long-day plants day-neutral plants the perception of day length occur in the leaves not in the apical meristem “ forigen” microRNAs in the regulation of GAMYBs Auxin promotes GA biosynthesis and signaling p. 537-538 Environmental conditions: light, photoperiod and temperature affect the transcription of GA biosynthesis genes Stratification: cold temperature is required for the germination of certain seeds Vernalization: a prolonged cold treatment is required for the flowering of certain species Cold treatment increase the activity of ent-kaurenoic acid hydroxylase GA can substitute for the cold treatment Shoot tip: the perception site of the cold treatment Cold then to high temperature: ent-kaurenoic acid GA9, the most active GA for stimulating the flowering response