A small acidic protein 1 (SMAP1) mediates responses of the Arabidopsis root to the synthetic auxin 2,4‐dichlorophenoxyacetic acid

Abstract

Summary 2,4-dichlorophenoxyacetic acid (2,4-D), a chemical analogue of indole-3-acetic acid (IAA), is widely used as a growth regulator and exogenous source of auxin. Because 2,4-D evokes physiological and molecular responses similar to those evoked by IAA, it is believed that they share a common response pathway. Here, we show that a mutant, antiauxin resistant1 (aar1), identified in a screen for resistance to the anti-auxin p-chlorophenoxyisobutyric acid (PCIB), is resistant to 2,4-D, yet nevertheless responds like the wild-type to IAA and 1-napthaleneacetic acid in root elongation and lateral root induction assays. That the aar1 mutation alters 2,4-D responsiveness specifically was confirmed by analysis of GUS expression in the DR5:GUS and HS:AXR3NT-GUS backgrounds, as well as by real-time PCR quantification of IAA11 expression. The two characterized aar1 alleles both harbor multi-gene deletions; however, 2,4-D responsiveness was restored by transformation with one of the genes missing in both alleles, and the 2,4-D-resistant phenotype was reproduced by decreasing the expression of the same gene in the wild-type using an RNAi construct. The gene encodes a small, acidic protein (SMAP1) with unknown function and present in plants, animals and invertebrates but not in fungi or prokaryotes. Taken together, these results suggest that SMAP1 is a regulatory component that mediates responses to 2,4-D, and that responses to 2,4-D and IAA are partially distinct.

Mutation ofE1-CONJUGATING ENZYME-RELATED1Decreases RELATED TO UBIQUITIN Conjugation and Alters Auxin Response and Development

“…In vitro, however, TIR1 displays a marked preference for the natural auxin IAA over 2,4-D (Dharmasiri et al, 2003a;Kepinski and Leyser, 2005), suggesting that discriminatory mutant phenotypes do not necessarily reflect receptor affinity differences. In addition, the antiauxin resistant1 mutant displays resistance to 2,4-D and p-chlorophenoxyisobutyric acid, but not IAA or 1-napthaleneacetic acid (Rahman et al, 2006). Moreover, a gain-of-function mutation in PDR9, which encodes a pleiotropic drug resistance transporter, reduces responsiveness to 2,4-D, but not IAA, IBA, or p-chlorophenoxyisobutyric acid (Ito and Gray, 2006).…”

New Insights to Auxin Signaling through The Use of Small Molecules

“…Seedlings with longer roots were recovered as mutants and were named aar (anti-auxin resistant). In addition to PCIB resistance, the aar mutants show resistance to the synthetic auxin 2,4-D in root elongation and lateral root induction assays (Rahman et al, 2006, Biswas et al, 2007, Nakasone et al, 2009. Among these aar mutants, aar1 and aar3 were defective for previously uncharacterized genes.…”

“…SMAP1 contains phenylalanine and aspartic acidrich domain (F/D-rich domain) that consists of 18 amino acids at the C-terminus. The putative genes encoding SMAP1-like protein with F/D-rich domain are conserved in divergent animal and plant species, implying that the protein plays an important role that is common in many species even though its function is unknown (Rahman et al, 2006;Nakasone et al, 2009). The AAR3 gene encodes a nuclear protein that had not previously been implicated in auxin signaling and that shares homology with the DCN-1 (defective in cullin neddylation-1) protein identified in yeast and mammals through the conserved PONY domain.…”

“…Among these aar mutants, aar1 and aar3 were defective for previously uncharacterized genes. The causal gene responsible for the aar1 phenotype encodes SMAP1 (small acidic protein) that functions upstream of the Aux/IAA-protein degradation step in auxin signaling (Rahman et al, 2006). SMAP1 contains phenylalanine and aspartic acidrich domain (F/D-rich domain) that consists of 18 amino acids at the C-terminus.…”

“…The screening for new small molecules having auxin activity or antagonistic activity from chemical libraries turned out to be a powerful tool to dissect auxin response mechanisms (reviewed in Tóth & Hoorn, 2010). Further use of these new auxin-related molecules for genetic screening resulted in identification of novel signaling components in the auxin response pathway (Cheng et al, 2004;Rahman et al, 2006;Walsh et al, 2006;Biswas et al, 2007).…”