‚Ä¶endosperm harvested 18 d after pollination and stored at 808C were cut into small pieces and treated with 2% (w/v) formaldehyde in ice-cold PBS buffer for 4 h. Cross-linking was quenched by addition of 2 M glycine for 1 h. Total proteins were then extracted as described by Boehlein et al. (2005).