Light and dark adaptation in Phycomyces phototropism.

Area of Science:

• * Plant biology
• * Biophysics
• * Photobiology

Background:

• * Phototropism in Phycomyces sporangiophores is a well-studied model for sensory adaptation.
• * The Delbrück-Reichardt model has been a leading framework for understanding this adaptation process.
• * Previous studies have focused on the model's ability to explain observed phenomena.

Purpose of the Study:

• * To rigorously test the validity of the Delbrück-Reichardt model for light and dark adaptation in Phycomyces.
• * To elucidate the distinct mechanisms underlying light and dark adaptation.
• * To genetically dissect the adaptation process using behavioral mutants.

Main Methods:

• * Analysis of phototropism in Phycomyces sporangiophores across a range of light intensities (10⁻⁷–6 W m⁻²).
• * Measurement of adaptation kinetics using the phototropic delay method.
• * Genetic analysis of four distinct behavioral mutants (L82, madA, madB, madC).

Main Results:

• * The Delbrück-Reichardt model accurately predicts adaptation only for small, short light intensity changes.
• * Dark adaptation exhibits biphasic decay with two time constants (b1 ≈ 1-2 min, b2 ≈ 6.5-10 min).
• * Light adaptation kinetics are complex, showing overshooting and dependence on light pulse height and intensity range, inconsistent with the model.
• * Phycomyces possesses distinct input and output adaptation mechanisms.
• * Mutants reveal genetic control: madA alters constants, madB affects fast dark adaptation and causes light adaptation overshooting, madC shows abnormal responses to light steps.
• * madB and madC mutants suggest receptor pigment involvement in adaptation.

Conclusions:

• * The Delbrück-Reichardt model is insufficient for fully explaining Phycomyces phototropic adaptation, particularly light adaptation.
• * Light and dark adaptation are regulated by separate mechanisms.
• * Phycomyces employs at least two adaptation mechanisms: input and output.
• * Genetic evidence points to the receptor pigment playing a role in mediating adaptation processes.