Simple SummaryDepending on their habitat conditions, plants can greatly change the patterns of biomass allocation between different organs. Thus, low light inhibits the growth of roots, but at the same time, plants maintain elongation of their shoots. Phytohormones ensure the coordination of processes occurring in different plant organs due to the transmission of signals regarding changes in the environment. They are synthesized both in roots and shoots. Their transport from roots to shoots and vice versa occurs through the xylem and phloem, respectively. It is believed that long distance transport of auxins is important for the control of the growth of roots under conditions of low light; however, the mechanism of these effects remains unclear. The inhibition of lateral root emergence in barley under reduced levels of irradiance is due to a disturbance of auxin transport through the phloem and down-regulation of the genes responsible for auxin transport in plant roots. A comparison of results obtained for wheat and barley shows that mechanisms controlling the auxin signal sent from shoots to roots can be different and species specific.AbstractDepending on their habitat conditions, plants can greatly change the growth rate of their roots. However, the mechanisms of such responses remain insufficiently clear. The influence of a low level of illumination on the content of endogenous auxins, their localization in leaves and transport from shoots to roots were studied and related to the lateral root branching of barley plants. Following two days’ reduction in illumination, a 10-fold reduction in the emergence of lateral roots was found. Auxin (IAA, indole-3-acetic acid) content decreased by 84% in roots and by 30% in shoots, and immunolocalization revealed lowered IAA levels in phloem cells of leaf sections. The reduced content of IAA found in the plants under low light suggests an inhibition of production of this hormone under these conditions. At the same time, two-fold downregulation of theLAX3gene expression, facilitating IAA influx into the cells, was detected in the roots, as well as a decline in auxin diffusion from shoots through the phloem by about 60%. It was suggested that the reduced emergence of lateral roots in barley under a low level of illumination was due to a disturbance of auxin transport through the phloem and down-regulation of the genes responsible for auxin transport in plant roots. The results confirm the importance of the long distance transport of auxins for the control of the growth of roots under conditions of low light. Further study of the mechanisms that control the transport of auxins from shoots to roots in other plant species is required.