Cytoplasmic male sterility in barley: Evidence for the involvement of cytokinins in fertility restoration

The hypothesis of the association between an increase in cytokinin activity and restoration of anther fertility inmsm1 cytoplasm was tested. The following barley lines withHordeum vulgare cv. Adorra nuclear gene background were studied: Adorra cytoplasm without nuclear restorer gene (fertile), Adorra cytoplasm homozygous for nuclearRfm1a gene (fertile),msm1 cytoplasm without restorer gene (male sterile),msm1 cytoplasm homozygous for nuclearRfm1a gene (fertile). Ethanolic extracts of root exudate were fractionated and bioassayed for cytokinins. Both the biological activity and the total quantity of cytokinins appeared lowest in the unrestored male sterile line. The total biological activities of cytokinins in the three fertile lines were similar, but the quantities in the restorer gene carriers appeared lower. On the other hand, the restorer gene carriers, independent of the cytoplasm, showed 8-9 times more of a bound cytokinin. Because the bound form is evidently underestimated by the bioassay, the increase in the bound cytokinin fraction may mean even a higher total content in theRfm1a gene carriers than in Adorra without the gene. The bound cytokinin may be translocated more readily to distal organs (e.g., the anthers) compared with unbound cytokinins. Because cytokinins are associated with various ecophysiological processes, the rise in a particular form may explain the heterogeneous distribution of the restorer gene in wild barley populations in different regions of Israel.