Transgenic Barley Grain Overexpressing Thioredoxin Shows Evidence That the Starchy Endosperm Communicates with the Embryo and the Aleurone

Homozygous lines of barley overexpressing a wheat thioredoxin h transgene (up to 30-fold) were generated earlier by using a B(1)-hordein promoter with a signal peptide sequence for targeting to the protein body and found to be enriched in starch debranching enzyme (pullulanase). Here, we describe the effect of biochemically active, overexpressed thioredoxin h on germination and the onset of alpha-amylase activity. Relative to null segregant controls lacking the transgene, homozygotes overexpressing thioredoxin h effected (i) an acceleration in the rate of germination and appearance of alpha-amylase activity with a 1.6- to 2.8-fold increase in gibberellin A(1) (GA(1)) content; (ii) a similar acceleration in the appearance of the alpha-amylase activity in deembryonated transgenic grain incubated with gibberellic acid; (iii) a 35% increase in the ratio of relative reduction (abundance of SH) of the propanol soluble proteins (hordein I fraction); and (iv) an increase in extractable and soluble protein of 5-12% and 11-35%, respectively. Thioredoxin h, which was highly reduced in the dry grain, was degraded in both the null segregant and homozygote after imbibition. The increase in alpha-amylase activity and protein reduction status was accompanied by a shift in the distribution of protein from the insoluble to the soluble fraction. The results provide evidence that thioredoxin h of the starchy endosperm communicates with adjoining tissues, thereby regulating their activities, notably by accelerating germination of the embryo and the appearance of alpha-amylase released by the aleurone.