Impairment in karrikin but not strigolactone sensing enhances root skewing in Arabidopsis thaliana.

Stéphanie M Swarbreck,Julia M Davies,Fiona J C Jamieson,Elsa Matthus,Yannick Guerringue

doi:10.1111/tpj.14233

Stéphanie M Swarbreck, Julia M Davies + Show 3 more

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https://doi.org/10.1111/tpj.14233

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Abstract

SummaryRoots form highly complex systems varying in growth direction and branching pattern to forage for nutrients efficiently. Here mutations in the KAI2 (KARRIKIN INSENSITIVE) α/β‐fold hydrolase and the MAX2 (MORE AXILLARY GROWTH 2) F‐box leucine‐rich protein, which together perceive karrikins (smoke‐derived butenolides), caused alteration in root skewing in Arabidopsis thaliana. This phenotype was independent of endogenous strigolactones perception by the D14 α/β‐fold hydrolase and MAX2. Thus, KAI2/MAX2 effect on root growth may be through the perception of endogenous KAI2‐ligands (KLs), which have yet to be identified. Upon perception of a ligand, a KAI2/MAX2 complex is formed together with additional target proteins before ubiquitination and degradation through the 26S proteasome. Using a genetic approach, we show that SMAX1 (SUPPRESSOR OF MAX2‐1)/SMXL2 and SMXL6,7,8 (SUPPRESSOR OF MAX2‐1‐LIKE) are also likely degradation targets for the KAI2/MAX2 complex in the context of root skewing. In A. thaliana therefore, KAI2 and MAX2 act to limit root skewing, while kai2's gravitropic and mechano‐sensing responses remained largely unaffected. Many proteins are involved in root skewing, and we investigated the link between MAX2 and two members of the SKS/SKU family. Though KLs are yet to be identified in plants, our data support the hypothesis that they are present and can affect root skewing.

Highlights

Roots grow in complex patterns that are highly relevant to their adaptation to different soil conditions and yet very difficult to investigate in this complex medium
Horizontal growth index (HGI; ratio of root tip displacement along the x-axis to root length; Grabov et al, 2004; Vaughn and Masson, 2011) was significantly higher in kai2-1, kai2-2, max2-7 and max2-8 compared with wild-type (Figure 1d; Tukey HSD, P < 0.01), supporting the skewing angle data and showing increased deviation from vertical by mutant roots
The vertical growth index (VGI; ratio of root tip displacement along the y-axis to root length; Grabov et al, 2004; Vaughn and Masson, 2011) was significantly smaller for kai2-1, kai2-2, max2-7 and max2-8 compared with wild-type (Figure 1e; Tukey HSD, P < 0.01)

Summary

Introduction

Roots grow in complex patterns that are highly relevant to their adaptation to different soil conditions and yet very difficult to investigate in this complex medium. Arabidopsis thaliana roots grown vertically on solid medium produce specific surface-dependent growth patterns described as skewing (deviation from vertical) and waving (Roy and Bassham, 2014). While gravitropism and negative phototropism can essentially be described in two dimensions, a third dimension must be considered (z), which corresponds to the distance away from the growth surface (Figure 1a). This allows for root movement or circumnutation along the z-axis (Migliaccio and Piconese, 2001; Simmonds et al, 2005), and for this movement to be impaired when the roots touch the surface of the solid medium (Thompson and Holbrook, 2004). While root movement in the z-dimension can affect root skewing, the rootskewing response is measured as the deviation of root growth along the x-axis that can only be seen when roots

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