A novel putative auxin carrier family regulates intracellular auxin homeostasis in plants

Abstract

The phytohormone auxin acts as a prominent signal, providing, by its local accumulation or depletion in selected cells, a spatial and temporal reference for changes in the developmental program [1][2][3][4][5][6][7] . The distribution of auxin depends on both auxin metabolism (biosynthesis, conjugation and degradation) [8][9][10] and cellular auxin transport [11][12][13][14][15] . We identified in silico a novel putative auxin transport facilitator family, called PIN-LIKES (PILS). Here we illustrate that PILS proteins are required for auxin-dependent regulation of plant growth by determining the cellular sensitivity to auxin. PILS proteins regulate intracellular auxin accumulation at the endoplasmic reticulum and thus auxin availability for nuclear auxin signalling. PILS activity affects the level of endogenous auxin indole-3-acetic acid (IAA), presumably via intracellular accumulation and metabolism. Our findings reveal that the transport machinery to compartmentalize auxin within the cell is of an unexpected molecular complexity and demonstrate this compartmentalization to be functionally important for a number of developmental processes.Prominent auxin carriers with fundamental importance during plant development are PIN-FORMED (PIN) proteins [1][2][3]6,9,15 . PIN1-type auxin carriers regulate the directional intercellular auxin transport at the plasma membrane. In contrast, atypical family member PIN5 regulates intracellular auxin compartmentalization into the lumen of the endoplasmic reticulum and its role in auxin homeostasis was recently identified 15,16 . PIN proteins have a predicted central hydrophilic loop, flanked at each side by five transmembrane domains. We screened in silico for novel PIN-like putative carrier proteins with a predicted topology similar to PIN proteins ( Fig. 1a and Supplementary Fig. 2) and identified a protein family of seven members (Fig. 1b) in Arabidopsis thaliana, which we designated as the PILS proteins. In contrast to the similarities in the predicted protein topology, PIN and PILS proteins do not show pronounced protein sequence identity (10-18%), which limits the identification of PILS proteins by conventional, reciprocal basic local alignment search tool (BLAST) approaches. However, the distinct PIN and PILS protein families contain both the InterPro auxin carrier domain which is an insilico-defined domain, aiming to predict auxin transport function (http://www.ebi.ac.uk/panda/InterPro.html). The PILS putative carrier family is conserved throughout the whole plant lineage, including unicellular algae (such as Ostreococcus tauri and Chlamydomonas reinhardtii) (Supplementary Fig. 3) where PIN proteins are absent 16 , indicating that PILS proteins are evolutionarily older.PILS genes are broadly expressed in various tissues (Fig. 1c) and PILS2-PILS7 were transcriptionally upregulated by auxin application in wild-type seedlings (Fig. 1d-f and Supplementary Fig. 4), indicating a role in auxin-dependent processes. To investigate the potential function of the putative PILS auxi...

Understanding the role of Oryza sativa OsPILS (PIN Like) genes in auxin signaling

“…In the present study, it has found that, all the OsPILS (OsPILS1, OsPILS2, OsPILS6a, OsPILS6b, OsPILS7a and OsPILS7b) proteins contains short hydrophilic loop and hence supposed to be endomembrane localized (Table 3). Finding presented in this manuscript also closely matches with finding of Barbez et al, (2012). In the phylogenetic tree, the short transmembrane domain containing proteins OsPIN5 and AtPIN5 are present very closely with OsPILS proteins (Figure 6).…”

“…Maximum of OsPILS genes were found to be subcellularly localized either in endoplasmic reticulum and vacuoles (Table 3). Presence of OsPILS proteins in endoplasmic reticulum/vacuoles that facilitates auxin accumulation inside the cells seems to contribute the possibility of compartmentalized regulation of auxin metabolism (Barbez et al 2012). It could be period in auxin treated root tissues (Figure 7) respectively.…”

“…Besides vascular transport system present in plants, cell to cell transport system also present in the tissues which covers long and short distance auxin distributions across the plant (Zazímalová et al 2010). Polar auxin efflux carrier (PIN) genes plays important roles in auxin distribution and homeostasis in plants in polarized manner (Friml and Palme 2002;Abas et al 2006;Barbez et al 2012). AtPIN1gene was the first PIN gene to be cloned from plant Arabidopsis thaliana (Gälweiler et al 1998).…”

“…Based on the divergence of central hydrophilic loop, PIN proteins are divided into different groups (Křeček et al 2009). Although several PIN genes from different species and their function has been reported to date, there are only few reports been available regarding the roles of PIN likes (PILS) genes in plants (Barbez et al 2012). As PILS genes are described as PIN like genes, therefore I tried to analyze their roles of OsPILS by treating them with auxin and cytokinin.…”

Master and servant: Regulation of auxin transporters by FKBPs and cyclophilins

“…By contrast, short PINs (PIN5, 6,8) are localized in the endomembrane system (most likely the endoplasmic reticulum (ER)) and appear to function in intracellular auxin transport and homeostasis [32,33]. Recently, a second family of ER-localized auxin transporters, called PILS (for PIN-LIKES), was also shown to regulate auxin homeostasis [34], although their role in auxin transport is less clear. AUX1/LAX and ABCB proteins are plasma membrane localized like long PINs, but are most often apolar (reviewed in Ref.…”