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

Paclobutrazol (Bonzi)

BX-112 Can be genetically engineered

GA-glycosides, GA-glycosyl ester

GA1 and GA4 have intrinsic bioactivity for

stem growth, GA1 for pea; GA4 for

Arabidopsis (p. 520~1)

Ancymidol (A-Rest)

(Commercial name)

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 le

vel of GA1 (O’Neill et al. 2000).

Cereal grains:

1. Embryo: embryo, scutellum

2. Endosperm: starch endosperm, aleuron layer

3. Testa-pericarp

GA responses: during germination and early seedling growth

Aleurone cells:

contain many protein body.

not for -amylase,

callus same

p. 528

a barley grain

- and -amylase,

maltase

The sole function of the aleurone layer of the cereal grai

ns appears to be the synthesis and release of hydrolytic e

nzymes into the starchy endosperm during germination.

After completing the synthesis and secretion of hydrolyti

c enzymes, the aleurone cells undergo programmed cell d

eath.

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 seedsVernalization: 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