THE ALTERED GRAVITROPIC RESPONSE OF THE LAZY-2 MUTANT OF TOMATO IS PHYTOCHROME REGULATED

Shoots of the lazy-2 (lz-2) gravitropic mutant of tomato (Lycopersicon esculentum Mill.) have a normal gravitropic response when grown in th e dark, but grow downward in response to gravity when grown in the lig ht. Experiments were undertaken to investigate the nature of the light induction of the downward growth of lz-2 shoots. Red light was effect ive at causing downward growth of hypocotyls of lz-2 seedlings, wherea s treatment with blue light did not alter the dark-grown (wild-type) g ravity response. Downward growth of lz-2 seedlings is greatest 16 h af ter a 1-h red light irradiation, after which the seedlings begin to re vert to the dark-grown phenotype. lz-2 seedlings irradiated with a far -red light pulse immediately after a red light pulse exhibited no down ward growth. However, continuous red or far-red light both resulted in downward growth of lz-2 seedlings. Thus, the light induction of downw ard growth of lz-2 appears to involve the photoreceptor phytochrome. F luence-response experiments indicate that the induction of downward gr owth of lz-2 by red light is a low-fluence phytochrome response, with a possible high-irradiance response component.