Post-phloem protein trafficking in the maize caryopsis: zmTRXh1, a thioredoxin specifically expressed in the pedicel parenchyma of Zea mays L., is found predominantly in the placentochalaza.

The pedicel of the maize (Zea mays L.) caryopsis is a complex structure consisting of layers of specialized cell-types involved with solute transfer into the developing kernel. A molecular marker for one of these cell layers, the phloem parenchyma, has been obtained by differential screening of a cDNA library from 7 days after pollination (DAP) kernels. The clone encodes a novel processed type of thioredoxin, ZmTRXh1, with a variant active site sequence. The transcript is exclusively present in phloem parenchyma cells of the pedicel. The protein accumulates predominantly in the adjacent placentochalazal layer up to 21 DAP, declining thereafter until, at 31 DAP, only traces remain. ZmTRXh1, which is catalytically active, is present in the cytosol, and is restricted to a fraction of the placentochalazal cells at 12 DAP, where it accumulates to high levels. ZmTRXh1 represents the first example of post-phloem protein trafficking in the pedicel. The reasons for this transfer and possible functions for the protein in the placentochalaza are discussed.