SMALL ACIDIC PROTEIN 1 (SMAP1) and SCF(TIR)(1) ubiquitin proteasome pathway act in concert to induce 2,4-dichlorophenoxyacetic acid-mediated alteration of actin in Arabidopsis roots.

2,4-dichlorophanoxyacetic acid (2,4-D), a functional analogue of auxin, is used as an exogenous source of auxin as it evokes physiological responses like the endogenous auxin, Indole-3-acetic acid (IAA). Previous molecular analyses of the auxin response pathway revealed that IAA and 2,4-D share a common mode of action to elicit downstream physiological responses. However, recent findings with 2,4-D specific mutants suggested that 2,4-D and IAA might also use distinct pathways to modulate Arabidopsis root growth. Using genetic and cellular approaches, we demonstrate that the distinct effects of 2,4-D and IAA on actin filament organization partly dictate the differential responses of roots to these two auxin analogues. 2,4-D but not IAA altered the actin structure in long term and short term assays. Analysis of the 2,4-D specific mutant aar1-1 revealed that Small Acidic Protein 1 (SMAP1) functions positively to facilitate the 2,4-D-induced depolymerization of actin. The ubiquitin proteasome mutants, tir1-1 and axr1-12, which show enhanced resistance to 2,4-D compared with IAA for root growth inhibition, were also found to have less disrupted actin filament networks after 2,4-D exposure. Consistently, chemical inhibitor of the ubiquitin proteasome pathway mitigated the disrupting effects of 2,4-D on the organization of actin filaments. Roots of the double mutant aar1-1 tir1-1 also showed enhanced resistance to 2,4-D-induced root growth inhibition and actin degradation compared with their respective parental lines. Collectively, these results suggest that the effects of 2,4-D on actin filament organization and root growth is mediated through synergistic interactions between SMAP1 and SCF(TIR)(1) ubiquitin proteasome components. This article is protected by copyright. All rights reserved.

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