Species differences in ligand specificity of auxin-controlled elongation and auxin transport: comparing Zea and Vigna

Abstract

The plant hormone auxin affects cell elongation in both roots and shoots. In roots, the predominant action of auxin is to inhibit cell elongation while in shoots auxin, at normal physiological levels, stimulates elongation. The question of whether the primary receptor for auxin is the same in roots and shoots has not been resolved. In addition to its action on cell elongation in roots and shoots, auxin is transported in a polar fashion in both organs. Although auxin transport is well characterized in both roots and shoots, there is relatively little information on the connection, if any, between auxin transport and its action on elongation. In particular, it is not clear whether the protein mediating polar auxin movement is separate from the protein mediating auxin action on cell elongation or whether these two processes might be mediated by one and the same receptor. We examined the identity of the auxin growth receptor in roots and shoots by comparing the response of roots and shoots of the grass Zea mays L. and the legume Vigna mungo L. to indole-3-acetic acid, 2-naphthoxyacetic acid, 4,6-dichloroindoleacetic acid, and 4,7-dichloroindoleacetic acid. We also studied whether or not a single protein might mediate both auxin transport and auxin action by comparing the polar transport of indole-3-acetic acid and 2-naphthoxyacetic acid through segments from Vigna hypocotyls and maize coleoptiles. For all of the assays performed (root elongation, shoot elongation, and polar transport) the action and transport of the auxin derivatives was much greater in the dicots than in the grass species. The preservation of ligand specificity between roots and shoots and the parallels in ligand specificity between auxin transport and auxin action on growth are consistent with the hypothesis that the auxin receptor is the same in roots and shoots and that this protein may mediate auxin efflux as well as auxin action in both organ types.

Nitric oxide mediates humic acids-induced root development and plasma membrane H+-ATPase activation

“…These inhibitory eVects were more pronounced in IAA-treated plants than on the HA-treated ones, suggesting that part of the HA eVects could activate other pathways beyond that of auxins. Although, IAA and HA induced only a non-statistically signiWcant increase (20-30%) of PR length, this tendency is in agreement with previous data, which showed that auxin concentrations as low as 10 ¡10 M could induce this trait in maize roots (Zhao et al 2002;Zandonadi et al 2007). On the other hand, the LR density was clearly enhanced in plants treated with IAA and HA, indicating that the average number of LR emergence events per area of the parent root increased.…”

“…Further, the inhibitory eVect of vanadate on maize root elongation induced by 1 nM IAA was reported (Gouvêa et al 1997). Auxin can enhance PR elongation in maize seedlings at very low concentrations (Zhao et al 2002), possibly by H + -pump activation (Zandonadi et al 2007).…”

Auxin-Dependent Cell Division and Cell Elongation. 1-Naphthaleneacetic Acid and 2,4-Dichlorophenoxyacetic Acid Activate Different Pathways

“…This means that the auxin receptor driving cell division shows a different ligand pattern than the receptor controlling cell elongation. If two receptors differ in their ligand pattern, they must be different receptors (Zhao et al, 2002).…”

“…In fact, the response of cell division to auxin has been shown recently to require the activity of a putative heterotrimeric G-protein, whereas the response of cell elongation was not dependent on this G-protein (Ullah et al, 2001(Ullah et al, , 2003. When different receptors are involved, they are expected to differ in their affinity for different ligands (Zhao et al, 2002). Different auxins should therefore affect cell division and cell elongation with different dose response relations.…”

Two Distinct Signaling Pathways Participate in Auxin-Induced Swelling of Pea Epidermal Protoplasts

“…Hertel (1995) argued that ABP1 is not the auxin receptor for elongation growth. An alternative candidate discussed by Hertel et al was the plasma membrane-located auxin efflux carrier (Zhao et al, 2002). Recent studies indicated that soluble auxin receptors distinct from ABP1 participate in auxin-induced responses, at least at the plasma membrane (Kim et al, 1998(Kim et al, , 2000(Kim et al, , 2001 and for transcriptional regulation (Dharmasiri et al, 2003).…”