During differentiation,the Lepidium perfoliatum seed coat epidermal cells produce copious amounts of mucilage that is extruded from the seed coat upon imbibition.Mucilage is composed primarily of pectin,a polysaccharide that is a main component of the cell wall.To validate the function of a mucilage related gene MUM4 (MUCILAGE-MODIFIED4,encodes a putative NDP-l-rhamnose synthase in Arabidopsis),LpMUM4 from L.perfoliatum,which is myxospermous,was characterized in the present study.Sequence analysis revealed that the protein is highly homologous to AtMUM4 from Arabidopsis.Analysis of qRT-PCR showed that LpMUM4 was expressed in various parts of L.perfoliatum,and the transcription level was relatively high in silicles and root and the expression level gradually increased with the development of silicles.Moreover,the immunolocalization analysis showed that LpMUM4 was strongly expressed in both outer and inner integument layers at the earlier stage,then was observed more strongly in the epidermal and subepidermal layers of outer integument,finally was concentrated in between the epidermal and subepidermal layers,which corresponded to the process of the outer integument (testa) differentiation and the mucilage production of seed in L.perfoliatum.Overexpression of LpMUM4 in Arabidopsis showed a remarkable inhibition in the transcription level of AtTTG1 gene (which locates upstream of the AtMUM4 in the mucilage synthesis pathway) in silicles.However,the inspection of the morphology and mucilage release behavior of seed coat revealed no evident difference between LpMUM4 overexpressed transgenic line and WT.The possible explanation may be that the differentiation of the seed coat and production of the mucilage are complex processes in L.perfoliatum,and which is regulated by an interaction network of numerous genes with function redundancy.Therefore,the increasing expression of one gene (LpMUM4) during development may not cause evident phenotype change.