Transcription activator. Involved in the regulation of secondary wall biosynthesis in fibers and vessels (PubMed:17890373). Transcription activator of the mannan synthase CSLA9 that recognizes and binds to the DNA consensus sequence 5'-[AG][GT]T[AT]GGT[GA]-3' cis-regulatory element of CSLA9 promoter (PubMed:24243147). Transcription factor that acts as molecular switch in the NAC012/SND1-mediated transcriptional network regulating secondary wall biosynthesis. Is directly activated by NAC012/SND1. Functions redundantly with MYB83 in the transcriptional regulatory cascade leading to secondary wall formation in fibers and vessels (PubMed:19808805). Transcription activator that binds to the DNA consensus sequence 5'-ACC[AT]A[AC][TC]-3', designated as the secondary wall MYB-responsive element (SMRE). Regulates directly numerous transcription factors and a number of genes involved in secondary wall biosynthesis that contain SMRE elements in their promoters (PubMed:22197883). Is an obligate component of the transcriptional regulatory complex toward the commitment of secondary wall cellulose synthesis. Is required for functional expression of the three secondary wall CESA genes, CESA4, CESA7 and CESA8 (PubMed:23726771). Expressed at low levels in stems and siliques, specifically in xylem. Slightly induced by salicylic acid (SA). Positively regulated by SND1 and homolog proteins. No visible phenotype under normal growth conditions, but the double mutant plants myb48 and myb83 nearly lack secondary wall thickening, stop growing after developing one to two pairs of small leaves and subsequently die. Plants over-expressing MYB46 show a significant increase in mannan synthase activity and mannan content in plant.