The importance of oxygen availability in two plant-based bioprocesses: hairy root cultivation and malting

The main aim of this work was to examine the role of oxygen availability in two model processes: the cultivation of Agrobacterium rhizogenes-induced hairy roots of the medicinal plant Egyptian henbane (Hyoscyamus muticus), and barley (Hordeum vulgare) malting. This was accomplished by transferring the vhb gene encoding bacterial haemoglobin (VHb) from Vitreoscilla to H. muticus and to barley with the specific goals of improving the growth and alkaloid production of H. muticus hairy roots and the germination rate of barley. In H. muticus hairy roots, heterologous vhb expression and an increase in aeration both improved the growth, but not the specific hyoscyamine production of the hairy roots. This suggests that oxygen availability may not be a limiting factor in hyoscyamine synthesis. The effect of VHb on the alkaloid profile of H. muticus hairy roots was not identical to that of aeration, indicating that the physiological effects of VHb are probably not related directly to its ability to increase the intracellular effective oxygen concentration. Although VHb production did not have a statistically significant effect on hyoscyamine production due to wide somaclonal variation, the highest production levels were found among the VHb-lines. A problem related to vhb expression was the tendency of hairy roots to dedifferentiate to callus. This effect probably counteracted the positive effects of vhb expression, because root dedifferentiation leads to impaired alkaloid production. Barley plants expressing the vhb gene did not react to oxygen deficiency by increasing the alcohol dehydrogenase (ADH) activity in the roots, unlike the control plants. This could be due to the ability of VHb to supplement the role of ADH in the recycling of NADH and maintaining glycolysis. In contrast to previous findings, we found that constitutive vhb expression did not improve the