Auxin transport at cellular level: new insights supported by mathematical modelling

Abstract

The molecular basis of cellular auxin transport is still not fully understood. Although a number of carriers have been identified and proved to be involved in auxin transport, their regulation and possible activity of as yet unknown transporters remain unclear. Nevertheless, using single-cell-based systems it is possible to track the course of auxin accumulation inside cells and to specify and quantify some auxin transport parameters. The synthetic auxins 2,4-dichlorophenoxyacetic acid (2,4-D) and naphthalene-1-acetic acid (NAA) are generally considered to be suitable tools for auxin transport studies because they are transported specifically via either auxin influx or efflux carriers, respectively. Our results indicate that NAA can be metabolized rapidly in tobacco BY-2 cells. The predominant metabolite has been identified as NAA glucosyl ester and it is shown that all NAA metabolites were retained inside the cells. This implies that the transport efficiency of auxin efflux transporters is higher than previously assumed. By contrast, the metabolism of 2,4-D remained fairly weak. Moreover, using data on the accumulation of 2,4-D measured in the presence of auxin transport inhibitors, it is shown that 2,4-D is also transported by efflux carriers. These results suggest that 2,4-D is a promising tool for determining both auxin influx and efflux activities. Based on the accumulation data, a mathematical model of 2,4-D transport at a single-cell level is proposed. Optimization of the model provides estimates of crucial transport parameters and, together with its validation by successfully predicting the course of 2,4-D accumulation, it confirms the consistency of the present concept of cellular auxin transport.

The allelochemical MDCA inhibits lignification and affects auxin homeostasis

“…In contrast to NAA, the transport of 2,4-dichlorophenoxy acetic acid (2,4-D) into tobacco BY-2 cells is largely dependent on auxin uptake carriers making 2,4-D an excellent marker for the auxin influx activity. When added to BY-2 cell cultures 2,4-D rapidly accumulated in the cells, and MDCA had a mild, but positive effect on this profile; however, the mild accumulation likely reflects the partial inhibition of the active cellular efflux of 2,4-D under these conditions as a similar profile was described for the auxin efflux blocker NPA as well (Hosek et al, 2012). In Arabidopsis cell cultures, 2,4-D seems to be a good substrate for both influx and efflux auxin carriers (Seifertová et al, 2014) and, accordingly, the positive effect of MDCA on the accumulation of 2,4-D is more pronounced here than in BY-2 cells (Supplemental Fig.…”

Computational Models of Auxin-Driven Development

“…With a few exceptions (Steinacher et al 2012;Hošek et al 2012), the terms in Eqs. (15.5), (15.7), and (15.10) have been assumed constant in simulation models.…”

“…Models of auxin-driven patterning range from those directly rooted in biochemistry (Renton et al 2012;Steinacher et al 2012;Hošek et al 2012) to more abstract constructs that aim at deducing morphogenetic characteristics of molecular-level process from the observed patterns and forms. In some cases, several hypotheses have been proposed to explain the same phenomenon, for example, the formation of phyllotactic and vascular patterns (Merks et al 2007;Stoma et al 2008;Bayer et al 2009).…”

Auxin metabolism and homeostasis during plant development

“…α-NAA), and some are more slowly metabolised (e.g. 2,4-D) (Hosek et al, 2012).Besides IAA, other endogenous auxins have been identified in plants (reviewed by Simon and Petrášek, 2011). Phenylacetic acid (PAA) has been found in different plant species as well as in bacteria, although PAA has a much weaker auxin effect than IAA and its role during plant development is unclear.…”