Plant Strategies To Control Growth And Development: Integration Of Both Signal Molecules, Auxin And Nitric Oxide

Abstract

The term auxin is derived from the Greek word ?auxein,? which means to grow or to expand and 3 was sealed by Charles Darwin more than a century ago. In ?The Power of Movement in Plants? 4 (1880), Darwin first described the effects of light on the movement of canary grass 5 coleoptiles. He demonstrated that the tip of the seedling was responsible for producing some 6 signal, namely auxin, which was transported to the lower part of the coleoptile, where the 7 physiological response of bending following the light occurred. Auxin is probably the most 8 intensely-studied molecule in plants as it impacts virtually every aspect of growth and 9 development during their life cycle. 10The role of auxin is warranted by the coordination of its synthesis, metabolism, transport, and 11 perception. The plant cell traduces the auxin signal through a well-characterized nuclear 12 signaling pathway, triggering transcriptional responses depending on a specific cell, tissue, or 13 organ. 14Auxin signaling pathway initiates once the hormone moves into the nucleus and is bound by a 15 coreceptor system by the E3 ubiquitin ligase SCFTIR1/AFBs and its degradation substrates 16 AUX/IAAs transcriptional repressors. Upon auxin binding SCFTIR1/AFBs trigger ubiquitylation 17 and further AUX/IAA turnover by the proteasome. AUX/IAAs block the expression of auxin-18 responsive genes, their degradation is essential for auxin pathway activation. 19Since plants are sessile organisms unable to escape changes in the environment, the degradation 20 of pre-synthesized AUX/IAA repressor proteins instead of the de novo synthesis of activation 21 proteins constitutes a more rapid and efficient strategy for the activation of molecular pathways 22 required to adapt to new situations. Thus, the ubiquitin proteasome system via the exquisite 23 action of specific E3 ubiquitin ligases, such as the SCFTIR1/AFBs recruit directly proteins 24 degradation substrates. SCF-type E3 ligases are the most abundant substrate recognition 25 complexes in eukaryotic cells and have been implicated in every major phytohormone signaling 26 pathway. Each individual SCF E3 ligase is a multimer consisting of a scaffold protein Cullin 1, a 27 RING RBX1 for binding an E2 conjugating enzyme loaded with ubiquitin, and a substrate 28 binding module build by the adaptor protein, SKP1 (in Arabidopsis ASK1) and, an 29 interchangeable substrate-recognition unit F-box Protein (FBP). 30In the last decade we have gained tremendous knowledge of how the signal auxin is perceived 31 and transmitted, and now we are starting to unveil a new level of regulation of the system at the 32 level of SCFTIR1/AFB stability. Since the Arabidopsis genome encodes hundreds of FBPs, and 33 ASK is able to associate with diverse FBPs to form multiple SCF complexes, the challenge of 34 regulating SCF assembly is particularly relevant. The SCF complex is therefore an exceptional 35 core in which different levels of post-translational modifications might take place. In addition to 36 auxin, nitric oxide (NO) is considered a ubiquitous signal in plants which contributes to 37 determining the morphology and developmental pattern of roots, in part by the modulation of 38 auxin response. Previously, we gained evidence on the role of the second messenger NO for the 39 regulation of the FBP TIR for auxin sensing. We wondered further whether NO might play a 40 broader role regulating the SCFTIR/AFB and its functionality in the plant cell.