How to remove header underline?

How to remove the header underline which comes default
please see my aspx gridview
 
Here Subject @ header shown with underline
Please Help..

<asp:GridView ID="dgvMaildetail" runat="server" Width="100%" AutoGenerateColumns="False"                                                        AllowSorting="True" DataKeyNames="md_id" AllowPaging="True" OnPageIndexChanging="dgvMaildetail_PageIndexChanging"                                                        RowStyle-Height="15px">                                                        <Columns>      <asp:TemplateField HeaderText="Subject" SortExpression="md_subject">                                                                <ItemTemplate>                                                                    <asp:LinkButton ID="lnksubject" CausesValidation="false"                                                                         Text='<%#Eval("md_subject") %>'                                                                        runat="server" Width="50%">asp:LinkButton>                                                                ItemTemplate>                                                            asp:TemplateField>  Columns>                                                    asp:GridView>