Alpha is Luminance

There is a difference between mental ray and Maya software render when reading the alpha
For example using a black and white image that doesn’t contain an alpha channel embedded as a transparency map, may not render correctly in mental ray.
 You can override that behavior by doing the following

• A- Windows > setting and preference > preference
  Under the rendering section , check Use Maya style alpha detection
• B- Open the Attribute editor for the file texture and under Color Balance Alpha is Luminance