Natural Arabidopsis brx Loss-of-Function Alleles Confer Root Adaptation to Acidic Soil

Abstract

Soil acidification is a major agricultural problem that negatively affects crop yield. Root systems counteract detrimental passive proton influx from acidic soil through increased proton pumping into the apoplast, which is presumably also required for cell elongation and stimulated by auxin. Here, we found an unexpected impact of extracellular pH on auxin activity and cell proliferation rate in the root meristem of two Arabidopsis mutants with impaired auxin perception, axr3 and brx. Surprisingly, neutral to slightly alkaline media rescued their severely reduced root (meristem) growth by stimulating auxin signaling, independent of auxin uptake. The finding that proton pumps are hyperactive in brx roots could explain this phenomenon and is consistent with more robust growth and increased fitness of brx mutants on overly acidic media or soil. Interestingly, the original brx allele was isolated from a natural stock center accession collected from acidic soil. Our discovery of a novel brx allele in accessions recently collected from another acidic sampling site demonstrates the existence of independently maintained brx loss-of-function alleles in nature and supports the notion that they are advantageous in acidic soil pH conditions, a finding that might be exploited for crop breeding.

Primary root protophloem differentiation requires balanced phosphatidylinositol-4,5-biphosphate levels and systemically affects root branching

“…This might mean that in both mutants, endogenous auxin levels or activity are already high and are therefore saturated earlier. This notion contrasts with the previous finding that in brx mutants meristematic auxin response is reduced as indicated by the inverse auxin activity reporter 35S::DII-VENUS (Gujas et al, 2012). Investigation of ops and cvp2 cvl1 root meristems with this marker revealed equally decreased auxin activity (Fig.…”

“…An inactive, yet correctly targeted OPS variant would be needed to address this issue. Finally, as auxin is thought to activate proton pumps, the 'auxin traffic jam' scenario would also be consistent with the reported proton pump hyperactivity of brx roots, the proposed adaptive feature that is responsible for the rare occurrence of brx-null mutants in natural Arabidopsis populations (Gujas et al, 2012).…”

“…These so-called 'gap cells' disrupt the continuity of the protophloem sieve element strands, resulting in reduced phloem cargo delivery to the meristem (Depuydt et al, 2013;Rodriguez-Villalon et al, 2014;Truernit et al, 2012). This manifests in, for example, a systemically reduced auxin response throughout the meristem (Gujas et al, 2012;Rodriguez-Villalon et al, 2014), which is likely responsible for its reduced activity and size, and thus the decreased overall root growth observed in the mutants. Recent studies suggest that OPS is a master regulator of the commitment to sieve element fate, because extra OPS dose or an ops gain-of-function allele can rescue brx defects and can overcome hyperactivity of the BARELY ANY MERISTEM 3 (BAM3)-CLAVATA3/EMBRYO SURROUNDING REGION 45 (CLE45) receptor-ligand module (Rodriguez-Villalon et al, 2014), a powerful inhibitor of sieve element specification that is de-regulated in brx roots (Depuydt et al, 2013).…”

“…The cvp2-1, cvl1-1, cvp2 cvl1, ops-2 and brx-2 mutant null alleles used in this study, and the 35S::DII-VENUS, 35S::mDII-VENUS, CVP2::NLS-VENUS, SUC2:: GFP and APL::GFP reporter genes have been previously described (Brunoud et al, 2012;Carland and Nelson, 2009;Gujas et al, 2012;Scacchi et al, 2010;Truernit et al, 2012). The brx-2 cvp2-2 double mutant obtained from the suppressor screen was backcrossed to Col-0 wild type twice before being used in our analyses.…”

Molecular genetic framework for protophloem formation

“…4G), and protophloem differentiation was not affected. This underlines the proposed essential function of the protophloem for root meristem growth in the early seedling and systemic performance of the root system later on (13,25). Systemically reduced auxin activity could also account for the observation that on CLE45 treatment, and in brx or ops loss-of-function mutants, overall cell number was sometimes slightly reduced in the radial dimension beyond the two missing incipient metaphloem cell files (e.g., Fig.…”