Regulation of auxin transport by aminopeptidases and endogenous flavonoids

Abstract

The 1-N-naphthylphthalamic acid (NPA)-binding protein is a putative negative regulator of polar auxin transport that has been shown to block auxin efflux from both whole plant tissues and microsomal membrane vesicles. We previously showed that NPA is hydrolyzed by plasma-membrane amidohydrolases that co-localize with tyrosine, proline, and tryptophan-specific aminopeptidases (APs) in the cotyledonary node, hypocotyl-root transition zone and root distal elongation zone of Arabidopsis thaliana (L.) Heynh. seedlings. Moreover, amino acyl-beta-naphthylamide (aa-NA) conjugates resembling NPA in structure have NPA-like inhibitory activity on growth, suggesting a possible role of APs in NPA action. Here we report that the same aa-NA conjugates and the AP inhibitor bestatin also block auxin efflux from seedling tissue. Bestatin and, to a lesser extent, some aa-NA conjugates were more effective inhibitors of low-affinity specific [3H]NPA-binding than were the flavonoids quercetin and kaempferol but had no effect on high-affinity binding. Since the APs are inhibited by flavonoids, we compared the localization of endogenous flavonoids and APs in seedling tissue. A correlation between AP and flavonoid localization was found in 5- to 6-d-old seedlings. Evidence that these flavonoids regulate auxin accumulation in vivo was obtained using the flavonoid-deficient mutant, tt4. In whole-seedling [14C]indole-3-acetic acid transport studies, the pattern of auxin distribution in the tt4 mutant was shown to be altered. The defect appeared to be in auxin accumulation, as a considerable amount of auxin escaped from the roots. Treatment of the tt4 mutant with the missing intermediate naringenin restored normal auxin distribution and accumulation by the root. These results implicate APs and endogenous flavonoids in the regulation of auxin efflux.

Ethylene Modulates Flavonoid Accumulation and Gravitropic Responses in Roots of Arabidopsis

“…In vivo studies in Arabidopsis seedlings with mutations in genes encoding flavonoid biosynthetic enzymes demonstrated a role for flavonoid regulation of auxin transport. These tt4 mutants had elevated auxin transport in young seedlings, roots, or inflorescences, consistent with the absence of an endogenous negative auxin transport regulator (Murphy et al, 2000;Brown et al, 2001;Buer and Muday, 2004;Peer et al, 2004). A comparison of the root gravitropic responses of the wild type and the chalcone synthase (CHS) null mutant, tt4(2YY6), identified a lag in gravitropic curvature in tt4(2YY6).…”

“…Reversible protein phosphorylation, which may modulate the activity, abundance, or localization of auxin transport proteins, is a second mechanism (Deruère et al, 1999;Christensen et al, 2000;Benjamins et al, 2001;Rashotte et al, 2001;DeLong et al, 2002;Friml et al, 2004). Finally, the localized synthesis of small molecules that regulate auxin transport modulates the rate of gravity response (Murphy et al, 2000;Brown et al, 2001;Buer and Muday, 2004).…”

Hormonal and Stress Induction of the Gene Encoding Common Bean Acetyl-Coenzyme A Carboxylase

“…4). Amazingly, the same root regions with GUS activity accumulate both flavonoids and auxins (Murphy and Taiz, 1999;Murphy et al, 2000;Peer et al, 2001;Ljung et al, 2005). In this context, it should be stressed that, in Arabidopsis roots, ChS and chalcone isomerase (Saslowsky and Winkel-Shirley, 2001) are located in sites where the PvACCase gene promoter is active (Fig.…”

“…It is noteworthy that PvACCase::GUS transgene expression during development has a strong correlation to the tissues where auxins are accumulated in Arabidopsis. For example, several studies have shown high auxin concentrations in Arabidopsis hydathodes, stipules, and root tips; furthermore, flavonoids colocalize with zones of auxin accumulation (Murphy et al, 2000;Peer et al, 2001;Aloni et al, 2003). To determine auxin responsiveness of the PvACCase gene promoter, 8-dold roots were treated with IAA.…”

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

“…This is of interest because flavonols, such as quercetin and kaemferol, have been shown to inhibit plant [6,26,43] and even mammalian ABCB activities [113], and are considered to be plant endogenous auxin transport modulators or inhibitors [101,114,115].…”