Sugar and Light Effects on the Condition of the Photosynthetic Apparatus of Arabidopsis thaliana Cultured in vitro

Light and sugars are fundamental elements of plant metabolism and play signaling roles in many processes. They are also critical factors determining the condition of plants cultured in vitro. The aim of this work was to investigate the simultaneous influence of irradiance and sugar content in the medium on the growth and photosynthetic apparatus condition of Arabidopsis thaliana in vitro. Plants were grown on media containing 1 or 3% of sucrose or glucose at three irradiances: 25, 100, and 250 mu mol m(-2) s(-1) (weak, medium, and strong light). Media without sugar were used for control plants. Plant growth parameters were measured and the following physiological processes were investigated: photosynthesis, blue light-induced chloroplast relocations, and xanthophyll cycle activity. The expression of genes related to these processes was analyzed. The presence of sugar in the medium was found to be essential for the growth of Arabidopsis in vitro. Weak light significantly limited growth and the capacity to acclimate to changing light conditions. Strong light was a source of stress in some cases. Contrary to earlier reports, exogenous sugars showed a positive effect on photosynthesis. At higher concentration they acted as photoprotectants, overcoming the negative influence of strong light on photosynthesis and the xanthophyll cycle. The expression of all investigated genes was influenced by irradiance and sugar presence. In many cases differential effects of sugar type and concentration could be observed. The specific effects of some irradiance/sugar concentration combinations point to possible interactions between sugar- and light-induced signaling pathways.