Light control of shoot regeneration in callus cultures derived from barley (Hordeum vulgare L.) immature embryos

Callus growth and shoot regeneration were examined in cultures of immature barley embryos incubated under various combinations of a week under a 16-h photoperiod and a week under continuous darkness. Incubation in darkness for four weeks, during which calli were formed, enhanced shoot regeneration in 'K-3' and 'Kanto Nijo-5', but inhibited it in 'Lenins'. 'K-3' and 'Kanto Nijo-5', incubated in darkness during the first two weeks of callus induction, followed by two weeks of a 16-h photoperiod (D2L2), showed a higher percentage of shoot regeneration than those incubated for two weeks with a 16-h photoperiod followed by two weeks of darkness (L2D2). Nevertheless, when the light conditions, D2L2 and L2D2, were combined into the same periods of a 16-h photoperiod and continuous darkness, except for the order, light conditions affected shoot regeneration differently. The early stage of callus induction seems to be sensitive to light. The expression ratios of the auxin-responsive gene (AUX/IAA) to the cytokinin-responsive gene (WPK4) were increased in continuous darkness in 'K-3' and 'Kanto Nijo-5'. The effects of light qualities (white, red, farred, and blue) on callus growth and shoot regeneration were also examined. Blue light inhibited shoot regeneration, as did white light, in 'K-3' and 'Kanto Nijo-5', and far-red light in 'Kanto Nijo-5'. Light probably controls shoot regeneration from calli by modifying cytokinin levels and/or response; blue light signals act in photo-inhibition of shoot regeneration in immature barley embryo culture.