Subcellular localisation of BETL-1, -2 and -4 in Zea mays L. endosperm

In maize, the endosperm region next to the placento-chalaza is characterised by the presence of cells with extensive cell wall ingrowths (transfer cells). Specific genes, collectively named as BETL (for Basal Endosperm Transfer Layer) genes, are expressed in these cells in a way that coincides with the temporal development of the transfer cell layer. The BETL-1 gene encodes a polypeptide of 7 kDa, exclusively expressed in the transfer cell region and possibly involved in the construction of the cell wall. However, the sequence (defensin- like) is also consistent with an antipathogenic role. We found that BETL-1 is strongly associated with the cell wall ingrowths of the endosperm transfer layer. On the other hand, BETL-2 (an antifungal protein) is not bound to the cell wall of transfer cells. The two BETL-2 forms (the 8 kDa propeptide and the final 4.5 kDa peptide) are detected in different subcellular compartments. The propeptide is mostly associated with the membrane fraction of endosperm, while the low molecular weight form is soluble. Vesicles containing the 8-kDa BETL-2 propeptide are located in association with microtubules present in the transfer cell region. Together with previous evidence on the differential distribution of the BETL-2 forms, these data outline the processing pathway of BETL-2, from synthesis in the transfer cells to the site of utilisation in the placento-chalaza region. BETL-4 (which is weakly related to a family of protease inhibitors) is mainly found in the transfer cell layer, and is absent from adjacent regions. Unlike BETL-1, BETL-4 is not a cell wall protein, even though it is associated with the ingrowths of the transfer cell walls. Therefore, BETL proteins show distinct properties of localisation and distribution, which are related to their differential functions in the transfer cells.