It is known for ten years that self-dual Yang-Miffs theory is the effective
field theory of the open N = 2 string in (2 + 2)-dimensional space-time. W
e uncover an infinite set of Abelian rigid string symmetries, corresponding
to the symmetries and integrable hierarchy of the self-dual Yang-Mills equ
ations. The twistor description of the latter naturally connects with the B
RST approach to string quantization, providing an interpretation of the pic
ture phenomenon in terms of the moduli space of string backgrounds.