Theory of plasma oscillation developed by Bohm and Pines is extended to involve the coupling between longitudinal and transverse oscillations. The coupling arises from the quantum fluctuation of electrons interacting with electromagnetic field. The coupling energy is found to be ħkv approximately, where ħk represents the recoil momentum of an electron due to the absorption (emission) of a longitudinal quantum with wave number k and v the velocity of the electron. This coupling may account for the solar outburst of radio waves, if the plasma oscillation is excited in a stream of electrons injected into corona with high velocity.