Endoplasmic reticulum: the rising compartment in auxin biology

Plants have evolved outstanding capacities to adapt their development and physiology to changes in the environment. The availability, distribution, and activity of endogenous signals, plant hormones, underlie and coordinate these responses (Santner and Estelle, 2009). Auxin, one such signaling molecule, affects a multitude of developmental processes (Woodward and Bartel, 2005; Leyser, 2006; Vanneste and Friml, 2009). The differential distribution of auxin within tissues is essential for many adaptive responses, including embryo and leaf patterning, root and stem elongation, lateral root initiation, leaf expansion, tropisms, regenerative growth, and vascular tissue formation (Vanneste and Friml, 2009). The best characterized auxin signaling pathway operates in the nucleus (Mockaitis and Estelle, 2008). In this pathway, auxin is perceived by and promotes the interaction of TIR1/AFB and Aux/IAA coreceptor proteins (Dharmasiri et al., 2005; Kepinski and Leyser, 2005; Tan et al., 2007), and downstream signaling is mediated by ubiquitination and proteosome-mediated degradation of Aux/IAA transcriptional repressors (Gray et al., 2001; dos Santos Maraschin et al., 2009), which themselves regulate the activity of ARF transcription factors (Tiwari et al., 2004; Guilfoyle and Hagen, 2007). Ultimately, this mechanism leads to complex transcriptional reprogramming in response to auxin. The large number of different Aux/IAA and ARF proteins means that different combinations can be expressed in different cell types, allowing the pathway to produce a multitude of different developmental and physiological responses to auxin (Mockaitis and Estelle, 2008; Vanneste and Friml, 2009). In addition to this nuclear signaling pathway, the observation of rapid cellular responses to auxin suggests the existence of another, presumably nontranscriptional, signaling pathway that might be related to the action of auxin-binding protein1 (ABP1). ABP1 was identified on the basis of its ability to bind to natural and synthetic auxins with high affinity, and has been found in various monocot and dicot species (Napier et al., 2002). However, despite decades of studies, its mode of action remains unclear (Badescu and Napier, 2006; Schenck et al., 2010). The majority of ABP1 is detected in the endoplasmic reticulum (ER), but its physiological effects, mainly relating to cell expansion and division, have been attributed to the smaller apoplastic pool of the protein (Napier et al., 2002; Perrot-Rechenmann, 2010). In theory, auxin could be perceived at several different subcellular destinations, including the nucleus, ER, and extracellular compartment. The final output of auxin signaling is therefore likely to be influenced not only by the overall cellular auxin content, but also by the relative distribution of auxin among different subcellular compartments. Thus subcellular auxin distribution may represent an important regulatory level in the complex action of auxin