ZmES genes encode peptides with structural homology to defensins and are specifically expressed in the female gametophyte of maize

All four members of a gene family, which are highly expressed in the cells of the female gametophyte (ZmES1-4: Zea mays embryo sac), were isolated fro m a cDNA library of maize egg cells. High expression of ZmES genes in the s ynergids around the micropylar region was detected in thin sections of maiz e ovaries. Single-cell RT-PCR analyses with the various cells of the female gametophyte confirmed the expression in synergids and also showed expressi on in the egg cell and central cell, and low expression in the antipodals. The expression of the whole gene family is suppressed after fertilization o f the embryo sac, and expression in two-cell or later embryo stages or othe r tissues of maize could not be detected. In order to investigate ZmES mRNA gradients in the highly polarized and vacuolized cells of the maize embryo sac, a whole-mount in situ protocol with isolated single cells was develop ed: as for total RNA, ZmES transcripts are uniformly distributed in the cyt oplasm of egg cell, synergids and central cell. ZmES genes encode small, cy steine-rich proteins with an N-terminal signal peptide, probably for transl ocation into the embryo sac cell wall. The four ZmES proteins display high sequence identity with each other, and the proposed tertiary structure of t he mature peptides is similar to that of plant and animal defensins. The fu nction of ZmES1-4 during the fertilization process is discussed.