Land plants are sessile organisms, and as a consequence they have acquired a flexible developmental program that allows them to adapt to sudden changes in their environment. One of the well-studied examples of plant developmental plasticity is the growth of seedling shoots towards the light. Already by the end of the 19th century, Darwin's observations on the bending of canary grass coleoptiles to unidirectional light led him to conclude that some matter in the upper part of the coleoptile is acted on by light, and then transmits its effects to the lower part of this tissue (Darwin and Darwin 1881). Around 1930, this matter was identified as indole-3-acetic acid (IAA) and named after the Greek word "auxein" meaning "to grow" (Went 1926; Kogl and Haagen-Smit 1931). Detailed observations by Went and Cholodny on the auxin-mediated orientation of plant growth to unidirectional light (phototropism) or gravity (gravitropism) led to the Cholodny and Went hypothesis (Went 1926; Cholodny 1927; Went F. and Thimann 1937). This model states that tropic growth is the result of predominant distribution of auxin to the dark or lower side upon light or gravity stimulation, and that, due to differences in sensitivity to auxin, these elevated auxin concentrations enhance cell elongation in the shoot, and inhibit cell growth in the root, ultimately leading to bending of the organ. In support of this hypothesis, more recent experiments demonstrated asymmetric auxin distribution leading to expression of auxin responsive genes in light and gravity-induced shoots (Li et al. 1991; Friml et al. 2002; Esmon et al. 2006) and roots (Larkin et al. 1996; Luschnig et al. 1998; Ottenschlager et al. 2003).
The tropic growth experiments clearly demonstrate that auxin action is a result of the interplay between the local auxin concentration—which is determined by biosynthesis, transport, and inactivation—and the sensitivity or responsiveness of cells to this plant hormone. Currently, auxin is recognized as a central regulator of plant development that not only controls elementary processes such as cell division and elongation, but also directs complex developmental and patterning processes such as embryogenesis, vascular differentiation, phyllotaxis and fruit development (Tanaka et al. 2006). Herein we will analyze the signaling pathways that regulate auxin-dependent plant development.
A canonical signaling pathway consists of a phosphorelay system of transmembrane receptor-activated protein kinases, enzymes that mediate reversible phosphorylation of substrate proteins thereby altering the activity, stability and/or subcellular localization of these proteins. Protein kinases control a great variety of cellular functions, and their catalytic domain is extremely conserved among different organisms. Database searches show that in bacteria, in general, less than 10 genes encode protein kinases, whereas 130 have been identified in yeast, 251 in flies, 411 in worms, approximately 520 in humans and more than 1000 in Arabidopsis thaliana (Leonard et al. 1998; Manning et al. 2002; Champion et al. 2004; Milanesi et al. 2005) (PlantsP Database: http://plantsp.sdsc.edu/). Considering that the complexity of the cellular signaling in part reflects the extent to which an organism senses and reacts to environmental changes, the greater number of kinases coded by plants reflects their need for developmental plasticity and robust defense mechanisms against pathogens and other stresses. As we will discuss below, in spite of their abundance in plants, kinases unexpectedly do not play a central, but rather an accessory role in auxin-signaling. In contrast, a particular class of plant specific kinases appears to have an important role in regulating the transport, thereby controlling local levels of auxin.
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