Required for endosperm development and polar nuclei fusion (PubMed:15634699). Mediates oxidation of cinnamyl alcohol and of p-hydroxylated derivatives of cinnamyl alcohol (i.e. the monolignols p-coumaryl-, coniferyl-, and sinapyl alcohol) to their corresponding aldehydes. Can use cinnamyl alcohol and derivatives, as well as beta-O-glycosylated form of coniferyl alcohol (coniferin) as substrate (PubMed:26037923). (E)-cinnamyl alcohol + NADP(+) = (E)-cinnamaldehyde + H(+) + NADPH (E)-coniferol + NADP(+) = (E)-coniferaldehyde + H(+) + NADPH (E)-coniferol + NADP(+) = (E)-coniferaldehyde + H(+) + NADPH Binds 1 FAD per subunit in a bicovalent manner. Expressed in sepals and stamen. The FAD cofactor is bound via a bicovalent 6-S-cysteinyl, 8alpha-N1-histidyl FAD linkage. Endosperm development arrest and impaired polar nuclei fusion. Belongs to the oxygen-dependent FAD-linked oxidoreductase family.