Abstract
Summary Targeted cellular auxin distribution is required for morphogenesis and adaptive responses of plant organs. In Arabidopsis thaliana (Arabidopsis), this involves the prototypical auxin influx facilitator AUX1 and its LIKE‐AUX1 (LAX) homologs, which act partially redundantly in various developmental processes. Interestingly, AUX1 and its homologs are not strictly essential for the Arabidopsis life cycle. Indeed, aux1 lax1 lax2 lax3 quadruple knock‐outs are mostly viable and fertile, and strong phenotypes are only observed at low penetrance.Here we investigated the Brachypodium distachyon (Brachypodium) AUX1 homolog BdAUX1 by genetic, cell biological and physiological analyses.We report that BdAUX1 is essential for Brachypodium development. Bdaux1 loss‐of‐function mutants are dwarfs with aberrant flower development, and consequently infertile. Moreover, they display a counter‐intuitive root phenotype. Although Bdaux1 roots are agravitropic as expected, in contrast to Arabidopsis aux1 mutants they are dramatically longer than wild type roots because of exaggerated cell elongation. Interestingly, this correlates with higher free auxin content in Bdaux1 roots. Consistently, their cell wall characteristics and transcriptome signature largely phenocopy other Brachypodium mutants with increased root auxin content.Our results imply fundamentally different wiring of auxin transport in Brachypodium roots and reveal an essential role of BdAUX1 in a broad spectrum of developmental processes, suggesting a central role for AUX1 in pooideae.
Highlights
Modulation of auxin activity through differential auxin distribution plays a central role in developmental and adaptive growth processes (Benjamins & Scheres, 2008; Zazimalova et al, 2010)
Arabidopsis thaliana (Arabidopsis) mutants in the prototypical auxin influx facilitator AUX1 have been identified because of their root agravitropism (Maher & Martindale, 1980), which can be rescued by addition of the lipophilic auxin analog 1-naphthylacetic acid (1-NAA) (Swarup et al, 2001)
We report that Brachypodium distachyon AUX1 (BdAUX1) loss-of-function results in counter-intuitive root phenotypes and reveals its essential role in a broad spectrum of developmental processes, suggesting a more central and diversified role for AUX1 in pooideae
Summary
Modulation of auxin activity through differential auxin distribution plays a central role in developmental and adaptive growth processes (Benjamins & Scheres, 2008; Zazimalova et al, 2010) It is largely achieved through plasma membrane-integral auxin efflux carriers, the PIN-FORMED (PIN) proteins, whose polar cellular localization can lead to asymmetric auxin secretion. (LAX) proteins that accelerate auxin uptake have been identified (Maher & Martindale, 1980; Bennett et al, 1996; Marchant et al, 2002; Yang et al, 2006; Peret et al, 2012) Their differential expression, as well as often polar localization, can modulate polar auxin transport to reinforce or attenuate local auxin accumulations. Corresponding multiple mutants reveal (partially) redundant roles of AUX1 and LAX1-3, for instance in phyllotaxis (Bainbridge et al, 2008) and embryogenesis (Robert et al., 1216 New Phytologist (2018) 219: 1216–1223 www.newphytologist.com
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