https://orcid.org/0000-0001-7129-805X
Since Charles Darwin studied how seedlings move toward a directional light source (a phenomenon known as phototropism), a mobile signal transmitted from a site of perception to a distal site of the plant that influences plant organ growth kinetics was hypothesized. This signal was later identified as the growth regulator auxin, an integral player in plant development and physiological responses, including leaf primordia patterning, vascular development, primary and lateral root growth, and gravitropism (reviewed by Geisler, 2021). Primary root growth is tightly controlled and requires specific spatial distributions of auxin regulated by a suite of membrane-localized auxin influx/efflux carriers, many of which have been identified: PIN-FORMED (PIN) family transporters, which famously exhibit an asymmetric cellular localization that can drive polarized auxin transport throughout an organ via a chemiosmotic mechanism; ATP-binding cassette transporters of the B sub-family (ABCBs), whose cellular localization is more symmetric, possesses family members capable of auxin transport both with and against auxin gradients; and AUX1/LAX auxin importers.
Despite the current knowledge of these transporters, understanding how their localizations and activities are coordinated to control auxin dynamics has been complicated by transport activity results obtained in heterologous expression systems. Electrophysiological studies of ABCB4 and PIN2 expressed simultaneously in HEK293 cells suggested that certain ABCB–PIN pairs could act cooperatively to have higher auxin flux rates over those possible when expressing PINs or ABCBs independently (Deslauriers and Spalding, 2021); however, other studies on different ABCB–PIN interaction pairs showed auxin transport antagonism. To resolve the nature of ABCB–PIN interactions in planta, Nathan Mellor, Ute Voß and colleagues (Mellor et al., 2022) developed an updated multicellular computational model of auxin distributions in root tips integrating confocal data of ABCB localization in growing Arabidopsis roots. This model simulates auxin fluxes and distributions based on changes in ABCB and PIN expression to test competing hypotheses addressing the putative synergism between PINs and ABCBs in regulating root tip auxin dynamics.
The authors used confocal microscopy of ABCB1-, ABCB4-, and ABCB19–GFP fusion lines to obtain subcellular localization data for ABCBs expressed in the root. These expression data were used to define new rules to incorporate into their previous computational model built on the influence of AUX1/LAX, PINs, and plasmodesmata on auxin dynamics in root tips (Mellor et al., 2020). The updated model was then used to test five hypotheses regarding how different ABCB–PIN interactions could influence auxin dynamics. Transgenic seedlings expressing the degron-based biosensor DII-VENUS (where VENUS signal strength is inversely related to auxin concentration) were used to observe auxin distribution scenarios in planta to check the hypotheses being tested. In comparison to the observed wild-type data, the model predicted that ABCBs can transport auxin independent of PINs, which was verified by testing the observed DII-VENUS expression in different abcb and pin mutants. Surprisingly, abcb1 abcb19 double knockout mutants exhibited an auxin distribution predicted by one of the ABCB–PIN scenarios, in which ABCBs efflux auxin both independently and co-dependently with PINs (Figure). Co-dependent efflux was inferred by localized auxin distributions that were elevated beyond what would be predicted by the concomitant independent efflux activities of ABCBs and PINs alone. This scenario was supported further by the predicted and observed pattern of auxin in pin2 roots, which exhibit elevated auxin in the outer root layers—a phenomenon consistent with independent ABCB transport activity.
Predicted and observed DII-VENUS expression in abcb1 abcb19 mutant Arabidopsis root. The predicted DII-VENUS expression pattern for hypothetical scenario IV (ABCBs transport auxin independently and cooperatively with PINs) qualitatively matches best with the observed DII-VENUS spatial distribution in the root tip of abcb1 abcb19 mutants. Adapted from Mellor et al. (2022), Figures 1 and 3.
Although computational studies have addressed the roles of AUX1/LAX importers and PIN efflux transporters in regulating spatiotemporal auxin dynamics, none have yet accounted for the complicated influence of ABCBs. This work showed the power of a systems biology approach using predictive modeling to infer phenotypes that would otherwise be difficult to understand via classic reverse genetics approaches. Follow-up studies, possibly using a cell-free system to negate the influence of a heterologous system, should address whether there is a direct interaction between ABCB and PIN auxin efflux carriers that could explain this synergistic mode of efflux.
