Scrollbars not working correctly on my picturebox

I have a windows form with the following basic structure....

Picturebox nested on a Panel nested on a ToolStripContainer.ContentPanel.

I've set the picturebox.sizemode to AutoSize and the Panel.autoscroll property to true.
Next I generate a BMP which exceeds the X and Y bounds of the form and place it as the image on the picturebox.
When I do this the vertical scrollbar shows up but the horizontal scrollbar is missing? What am I missing here??

using System;
using System.Collections.Generic;
using System.Text;
using System.Windows.Forms;
using System.Drawing;
using System.Drawing.Imaging;
using System.IO;

namespace Project1
{
    /* DEMO class for exploring behavior of scrollbars,
     * dockstyles, and anchorstyles.
     */
    class clsGUI : Form
    {
        ToolStripContainer tscMain = new ToolStripContainer();
        ToolStrip tsTop = new ToolStrip();
        ToolStrip tsBottom = new ToolStrip();
        Panel pnlMain = new Panel();
        PictureBox pboxMain = new PictureBox();

        public clsGUI()
        {
            //tool strip container
            tscMain.Anchor = AnchorStyles.Top & AnchorStyles.Left;
            tscMain.Dock = DockStyle.Fill;

            //panel nested on the toolstrip container contentpanel
            pnlMain.Anchor = AnchorStyles.Top & AnchorStyles.Left;
            pnlMain.Dock = DockStyle.Fill;
            pnlMain.AutoScroll = true;
            pnlMain.BackColor = Color.DarkGray;
            tscMain.ContentPanel.Controls.Add(pnlMain);

            //picturebox nested on pnlMain
            pboxMain.Anchor = AnchorStyles.Top & AnchorStyles.Left;
            pboxMain.Dock = DockStyle.Top;

            pboxMain.SizeMode = PictureBoxSizeMode.AutoSize;
            pnlMain.Controls.Add(pboxMain);

            ToolStripButton tsbtn1=new ToolStripButton("push me",null,loadImage);
            tsTop.Items.Add(tsbtn1);

           
            tscMain.TopToolStripPanel.Controls.Add(tsTop);
            tscMain.BottomToolStripPanel.Controls.Add(tsBottom);
            base.Controls.Add(tscMain);

        }

        private void InitializeComponent()
        {
            base.SuspendLayout();
            base.Width = 500;
            base.Height = 500;
            base.CenterToScreen();
            this.ResumeLayout(false);
        }

        private void loadImage(object sender, EventArgs e)
        {

            Bitmap objBMP = new Bitmap(1000, 1000);
            MemoryStream ms=new MemoryStream();
            Graphics GFX=Graphics.FromImage(objBMP);
            Pen objPen = new Pen(Color.Red);
            objPen.Width = 10;
            GFX.DrawEllipse(objPen,0,0,1000,1000);
            objBMP.Save(ms,ImageFormat.Bmp);
            pboxMain.Image = Image.FromStream(ms);
        }

 
    }
}