Gibberellins aleurone layer

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. 

Musgrave, A. Kays, S. E. Kende, H. Uptake and metabolism of radioactive gibberellins by barley aleurone layers. 

Morre et al. (1970) detected the synthesis of PC from CDP-choline in extracts from onion stem. The activity was associated with a particulate fraction. Devor and Mudd (1971b) found that the 100,000-g pellet from spinach leaves had the highest total amount and specific activity of this enzyme. The metal ion requirement could be satisfied by Mn + (saturating at 0.3 mM) or Mg + (saturating at 13 mM). The for CDP-choline was 10 pM, and the pH optimum was 7.5-8.0. Only modest stimulation could be achieved by the addition of DG. Johnson and Kende (1971) measured the enzyme in particulate fractions (44,000-g pellet) from barley (Hordeum vulgare L.) aleurone layers and found the activity greatly stimulated when the aleurone layers were exposed to gibberellin before extraction and assay of the enzyme. 

When applied to barley aleurone layers, gibberellic acid stimulated the incorporation of [met/iy/- CJcholine into PC (Evins and Varner, 1971). Absci-sic acid prevented this effect of gibberellic acid. Johnson and Kende (1971) showed that gibberellin stimulated both PC cytidyltransferase and choline phosphotransferase and that these stimulations could be prevented by absci-sic acid, cycloheximide, and actinomycin D. 

Ho THD, Shih S, Kleinhofs A (1980) Screening for barley mutants with altered hormone sensitivity in their aleurone layers. Plant Physiol 66 153-157 Ho THD, Nolan RC, Shute DE (1981) Characterization of a gibberellin-insensitive dwarf wheat, D 6899 Evidence for a regulatory step common to many diverse responses to gibberellins. Plant Physiol 67 1026-1031... 

Translocation of synthesized gibberellin to aleurone layer (although see text). Coleorhiza ruptures coat... 

Events associated with gibberellin-induced formation and release of hydrolytic enzymes from the aleurone layer have received much attention. These will be considered separately, and in depth, in Chapter 7. We will simply reiterate here that gibberellin (GA) released from the embryo induces the aleurone layer to undergo a series of metabolic changes which result in the release of a-amylase (along with a number of other hydrolases) into the endosperm, where they degrade the reserves stored therein. 

We saw in Chapter 6 how the reserves in the endosperm of cereals are mobilized largely as a result of the activity of enzymes secreted by the aleurone layer. We also referred to the stimulation of aleurone layer activity by gibberellin coming from the embryo. Herein lies the basis of the control exerted by the embryo over food mobilization, which we will now examine more closely. 

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