Vascular Pattern Initiation at the Shoot Apex

The plant vascular system is a continuous network of highly differentiated cells, connecting leaves and other parts of the shoot with the root, enabling transport of water and assimilates. The vascular tissue is primarily composed of two different types of conducting tissue, the xylem and the phloem. Whereas the former is responsible for the acropetal transport of water and mineral salts, the latter provides transport of organic compounds to sink tissue and organs. The development of the plant vascular tissue is initiated at the meristem, a tissue type of undifferentiated but actively dividing cells. This meristematic tissue, referred to as cambium, gives rise to xylem (proximal to the pith) and phloem (distal to the pith) cells, by forming vascular bundles. These bundles are arranged along a vascular ring throughout the inflorescence shoot, separated by the interfascicular region. The developmental process of the vascular tissue is regulated by various plant hormones. The most important one regarding plant development is auxin, a key player in the formation and differentiation of vascular tissue. Within the inflorescence shoot auxin maxima are formed specifically in vascular bundles, and therein in procambial and differentiating xylem cells, indicating a correlation of auxin maxima and vascular bundles. Indeed the number of auxin maxima determines the number of vascular bundles, established in the inflorescence stem. In addition to auxin, also cytokinins, brassinosteroids and many more plant hormones have been implicated to have regulatory impact onto plant development. Brassinosteroids have long been known in promoting cell expansion. In addition mutants deficient in brassinosteroid signalling showed a reduced number of vascular bundles as well as in auxin maxima, indicating a contributing role in vascular bundle formation, by serving as a kind of promoting signal in the patterning process. To periodically arrange vascular bundles in the inflorescence stem a coordinated action of auxin and brassinosteroids is required. Whereas auxin maxima seem to be the crucial element in the establishment of vascular bundles, brassinosteroids play an important role in promoting their formation. Determining the dynamics of the genetic network controlling vascular patterning in general, and its modulation by brassinosteroids in particular, are the main objectives of the applied project.