Delta(15) fatty acid desaturase capable of converting hexadecadienoic acid into hexadecatrienoic acid (16:3(9Z,12Z,15Z)) (PubMed:17178719). The double bond introduced occurred between carbons 15 and 16 resulting in a terminal double bond aliphatic chain (PubMed:17178719). Can also convert linoleic acid (18:2(9Z,12Z)) into alpha-linolenic acid (18:3(9Z,12Z,15Z)) (PubMed:17178719). Involved in the biosynthesis of the aliphatic side chain of sorgoleone, a phytotoxic secondary metabolite playing a direct role in allelopathic interactions (PubMed:15361534, PubMed:17178719). (9Z,12Z)-hexadecadienoyl-CoA + AH2 + O2 = (9Z,12Z,15Z)-hexadecatrienoyl-CoA + A + 2 H2O Lipid metabolism; polyunsaturated fatty acid biosynthesis. Highly expressed in root hair cells. Barely detected in panicle, shoot apex, stems and leaves. Up-regulated upon auxin treatment (PubMed:22087851). Up-regulated by jasmonate and methyl jasmonate (PubMed:23702703). The histidine box domains may contain the active site and/or be involved in metal ion binding. Belongs to the fatty acid desaturase type 1 family.