Practice question

Gibberellins are tetracyclic diterpenoid hormones with GA1 GA3 GA4 most bioactive functions centered on stem elongation, seed germination and floral transition. Biosynthesis proceeds from geranylgeranyl diphosphate via ent kaurene ent kaurenoic acid to GA12 catalyzed by terpene synthases and cytochrome P450 monooxygenases GA20 oxidase and GA3 oxidase. Signaling involves soluble receptor GID1 binding GA causing conformational change enabling interaction with DELLA repressors which are then ubiquitinated by SCF SLY1 GID2 complex and degraded via proteasome allowing transcription of growth genes. Physiologically gibberellins induce alpha-amylase and protease expression in barley aleurone via GAMYB transcription factor mobilizing starch reserves during germination antagonizing ABA. They promote internode elongation by enhancing cell division in intercalary meristem and wall extensibility through xyloglucan endotransglycosylase and expansin induction. In vitro GA3 at 0.1-1 mg per L elongates dwarf shoots, breaks bud dormancy, induces precocious flowering, though excessive level inhibits rooting and somatic embryo maturation requiring careful titration within medium composition.