Transport of the Two Natural Auxins, Indole-3-Butyric Acid and Indole-3-Acetic Acid, in Arabidopsis

Abstract

Polar transport of the natural auxin indole-3-acetic acid (IAA) is important in a number of plant developmental processes. However, few studies have investigated the polar transport of other endogenous auxins, such as indole-3-butyric acid (IBA), in Arabidopsis. This study details the similarities and differences between IBA and IAA transport in several tissues of Arabidopsis. In the inflorescence axis, no significant IBA movement was detected, whereas IAA is transported in a basipetal direction from the meristem tip. In young seedlings, both IBA and IAA were transported only in a basipetal direction in the hypocotyl. In roots, both auxins moved in two distinct polarities and in specific tissues. The kinetics of IBA and IAA transport appear similar, with transport rates of 8 to 10 mm per hour. In addition, IBA transport, like IAA transport, is saturable at high concentrations of auxin, suggesting that IBA transport is protein mediated. Interestingly, IAA efflux inhibitors and mutations in genes encoding putative IAA transport proteins reduce IAA transport but do not alter IBA movement, suggesting that different auxin transport protein complexes are likely to mediate IBA and IAA transport. Finally, the physiological effects of IBA and IAA on hypocotyl elongation under several light conditions were examined and analyzed in the context of the differences in IBA and IAA transport. Together, these results present a detailed picture of IBA transport and provide the basis for a better understanding of the transport of these two endogenous auxins.

Ethylene Modulates Flavonoid Accumulation and Gravitropic Responses in Roots of Arabidopsis

“…The sterilized seeds were sown on control plates: 0.8% (w/v) type M agar (A-4800; Sigma), 1 3 Murashige and Skoog nutrients (macro and micro salts: MSP0501; Caisson Labs), vitamins (1 mg mL 21 thiamine, 1 mg mL 21 pyridoxine HCl, and 0.5 mg mL 21 nicotinic acid), 1.5% (w/v) Suc, 0.05% (w/v) MES, with pH adjusted to 6.0 with 1 N KOH before autoclaving. This medium has higher Murashige and Skoog and Suc concentrations that are optimal for root growth, but because this medium has been used in previous experiments in our laboratory (Rashotte et al, 2000(Rashotte et al, , 2003Brown et al, 2001;Buer and Muday, 2004) we continue to use it for consistency. The unsealed plates were placed vertically in racks and the seedlings were grown under standard light regimes (100 mmol m 22 s 21 ).…”

“…In roots, auxin is transported via two polarities in different tissues ). Basipetal auxin transport occurs in the outer cell layers from the root tip toward the root-shoot junction in the first centimeter of the root, whereas acropetal transport is in the opposite direction over the length of the central cylinder (Rashotte et al, 2003). Basipetal transport is necessary for root gravity responses (Rashotte et al, 2000).…”

“…In the light, there is a dramatic induction in flavonoid accumulation mediated by induced transcription of the genes encoding flavonoid biosynthetic enzymes (Kubasek et al, 1992;Wade et al, 2001;Buer and Muday, 2004). Light levels also regulate auxin transport (Jensen et al, 1998;Rashotte et al, 2003). Flavonoids are quickly and abundantly produced when plants are wounded or during pathogen attacks, conditions that also alter auxin transport (Mathesius et al, 1998;Berleth and Sachs, 2001;Schwalm et al, 2003).…”

Auxin transport is sufficient to generate a maximum and gradient guiding root growth

“…In the cytoplasm, auxin is de-protonated such that free passage outwards is very slow; consequently, transport facilitators are necessary that locally permeate the membrane and facilitate auxin efflux. PIN proteins have cell-type-specific polar localizations in a manner that fully correlates with auxin transport routes as determined by auxin labelling, local auxin production and bioassays 16,25,26 , although other directional transporters may have additional roles 27,28 . Hence, in our model we specifically focus on the localization of the PINs (with permeability P e pin ).…”

Auxin and Ethylene Induce Flavonol Accumulation through Distinct Transcriptional Networks

“…Flavonoids promote gravitropism presumably by regulating auxin movement in the root tip, which modulates differential growth (Buer and Muday, 2004). Finally, factors that regulate flavonoid biosynthesis, such as light levels (Jensen et al, 1998;Rashotte et al, 2003), wounding and pathogen attacks (Mathesius et al, 1998;Berleth and Sachs, 2001), and gravity stimulation (Buer and Muday, 2004;Buer et al, 2006), also affect auxin transport. Flavonoid synthesis is regulated by a variety of developmental and environmental cues (Winkel-Shirley, 2002).…”