GLSL : Access sampler2Drect as monochrome texture

I have a YUV420P frame encoded as a monochrome texture in OF (one byte per pixel). I want to render the image using GLSL making the conversion to RGB in the GPU.

I’m using the texture2DRect( ) function to get the values from the texture. Which gives me a vec4.

Is there a way to retreive a single float from a texture, knowing that is a monochromatic texture? This way I can access directly the x, y pixels and set them to the gl_FragColor.