AUX1 Promotes Lateral Root Formation by Facilitating Indole-3-Acetic Acid Distribution between Sink and Source Tissues in the Arabidopsis Seedling

Abstract

Arabidopsis root architecture is regulated by shoot-derived signals such as nitrate and auxin. We report that mutations in the putative auxin influx carrier AUX1 modify root architecture as a result of the disruption in hormone transport between indole-3-acetic acid (IAA) source and sink tissues. Gas chromatography-selected reaction monitoring-mass spectrometry measurements revealed that the aux1 mutant exhibited altered IAA distribution in young leaf and root tissues, the major IAA source and sink organs, respectively, in the developing seedling. Expression studies using the auxin-inducible reporter IAA2 :: uidA revealed that AUX1 facilitates IAA loading into the leaf vascular transport system. AUX1 also facilitates IAA unloading in the primary root apex and developing lateral root primordium. Exogenous application of the synthetic auxin 1-naphthylacetic acid is able to rescue the aux1 lateral root phenotype, implying that root auxin levels are suboptimal for lateral root primordium initiation in the mutant.

Apyrases (Nucleoside Triphosphate-Diphosphohydrolases) Play a Key Role in Growth Control in Arabidopsis

“…Observations consistent with this interpretation are that APY1:GUS and APY2:GUS assays show the highest expression in cells where the PIN auxin efflux facilitators are also highly expressed, and plants suppressed in apyrase expression have decreased lateral root formation, just like both mutants in aux1 (Marchant et al, 2002) and wild-type plants that have been treated with auxin transport inhibitors (Casimiro et al, 2001). …”

EXORDIUM– a gene expressed in proliferating cells and with a role in meristem function, identified by promoter trapping in Arabidopsis

“…Hormones such as auxin and cytokinin are known to influence cell division activity at the meristems, to modulate plant architecture (Cockcroft et al, 2000;Laskowski et al, 1995;Marchant et al, 2002;Su and Howell, 1992). The influence of exogenous auxins and cytokinins, of the polar auxin transport inhibitors NPA and 1-NOA and of auxindefective mutant backgrounds (aux1, axr1, pin1, rooty, this paper and emb30; Topping and Lindsey, 1997) on EXO expression together suggest that endogenous hormones are influential and may define the pattern of expression.…”

“…The influence of exogenous auxins and cytokinins, of the polar auxin transport inhibitors NPA and 1-NOA and of auxindefective mutant backgrounds (aux1, axr1, pin1, rooty, this paper and emb30; Topping and Lindsey, 1997) on EXO expression together suggest that endogenous hormones are influential and may define the pattern of expression. There is compelling evidence that auxin accumulates in root tips to activate genes and regulate cellular patterning (Kerk and Feldman, 1995;Marchant et al, 2002;Sabatini et al, 1999;Swarup et al, 2001), and may be at least partially responsible for regulating the level of EXO expression in the meristem. Interestingly, the EXO and DR5 promoters (Sabatini et al, 1999) show identical shifts in activity in response to the auxin transport inhibitor NPA, suggesting some common regulatory features by auxin.…”

Arabidopsis phosphatidylinositol monophosphate 5-kinase 2 is involved in root gravitropism through regulation of polar auxin transport by affecting the cycling of PIN proteins

“…Among the signals that determine lateral root formation, the plant hormone auxin is widely considered a key regulator [24,25]. Arabidopsis mutants with high levels of endogenous auxin as well as seedlings treated with exogenous auxin display increased lateral roots [26][27][28], whereas mutants defective in auxin accumulation, transport and signaling produce fewer lateral roots [29][30][31].…”