Endosperm gibberellin synthesis

In the germination of cereal seeds, it was long known by brewers that if the embryo was excised (or dead) the endosperm would not be hydrolyzed and sugars would not be released. In 1960, Paleg showed that amylolytic activity in the embryo-less half could be fully restored in the presence of gibberellin. In other words, the substance that passed from the embryo to the endosperm (or rather, to the living cells of the aleurone layer that encloses the dead endosperm) induces there the synthesis of a-amylase which is responsible for hydrolysis of the stored starch reserves held in the endosperm. The extent to which the a-amylase was induced became another bioassay for gibberellin. 

Attempts have been made to determine why a-amylase synthesis eventually declines in the intact grain (e.g. see the decline in rate of a-amylase synthesis in Figure 7.4A and B after three and two days respectively). One possible reason is that production of the stimulus (i.e. GA) for a-amylase synthesis decreases, perhaps because hormonal synthesis is itself inhibited by the products of starch degradation. Unequivocal evidence supporting this possibility is lacking. In any case, sufficient gibberellin is released into the endosperm during the first one or two days after the start of imbibition to support a-amylase synthesis for at least four to five days thereafter (Fig. 7.4 C). Thus any limitation in GA synthesis after one or two days is unlikely to affect enzyme production. 

At first glance, it is difficult to recognize the gibberellins as diter-penes. However, any element of doubt about this has been dispelled by isotope experiments in vivo and in a cell-free system. In the fluid endosperm of the wild cucumber Echinocystis macrocarpa the following synthetic pathway has been demonstrated (Fig. 85) geranylgeranyl pyrophosphate—(—) kaurene—(—) kauren-19-ol —gibberellin A5. Presumably the synthesis then proceeds via Aj to gibberellic acid. 

Gibberellins are mainly responsible for triggering the synthesis of key enzymes that will degrade nntrients stored in the first and second reserve tissues of the germ and endosperm, respectively. Two gibberellic acids (GA, and GA3) are the major hormones produced by the anbryo. These hormones are later transported along with the absorbed water to the endosperm through the tube cells, and induce aleurone cells... 

---