A protective role for the embryo surrounding region of the maize endosperm, as evidenced by the characterisation of ZmESR-6, a defensin gene specifically expressed in this region

A Zea mays cDNA clone, ZmESR-6, was isolated as a gene specifically expressed at the basal region of immature kernels. ZmESR-6 cDNA encoded for a small (11.1 kDa) protein homologous to plant defensins. As for other defensins, the protein contained an N-terminal signal peptide signature and a C-terminal acidic peptide, the mature peptide has a molecular mass of 5.5 kDa. ZmESR-6 was highly expressed in developing kernels but the transcript could not be detected in any other maize tissue. The recombinant ZmESR-6 protein, purified from E. coli, showed strong in vitro inhibitory activity against bacterial and fungal plant pathogens, suggesting a role for ZmESR-6 in plant defence. The distribution of the transcripts was restricted to the embryo surrounding region (ESR) of the kernel. Immunolocalisation experiments revealed, however, that at the grain filling phase ZmESR-6 was accumulated in the placentochalaza-cells, rather than in the ESR cells that produce it. Our results suggest that the ESR has a role in protecting the embryo at the very early stages of seed development, whilst contributes to the general defence mechanism of the kernel at later developmental stages.