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Planta

, Volume 243, Issue 6, pp 1397–1406 | Cite as

Functional redundancy in the control of seedling growth by the karrikin signaling pathway

  • John P. Stanga
  • Nicholas Morffy
  • David C. NelsonEmail author
Original Article
Part of the following topical collections:
  1. Strigolactones

Abstract

Main conclusion

SMAX1 and SMXL2 control seedling growth, demonstrating functional redundancy within a gene family that mediates karrikin and strigolactone responses.

Strigolactones (SLs) are plant hormones with butenolide moieties that control diverse aspects of plant growth, including shoot branching. Karrikins (KARs) are butenolide molecules found in smoke that enhance seed germination and seedling photomorphogenesis. In Arabidopsis thaliana, SLs and KARs signal through the α/β hydrolases D14 and KAI2, respectively. The F-box protein MAX2 is essential for both signaling pathways. SUPPRESSOR OF MAX2 1 (SMAX1) plays a prominent role in KAR-regulated growth downstream of MAX2, and SMAX1-LIKE genes SMXL6, SMXL7, and SMXL8 mediate SL responses. We previously found that smax1 loss-of-function mutants display constitutive KAR response phenotypes, including reduced seed dormancy and hypersensitive growth responses to light in seedlings. However, smax1 seedlings remain slightly responsive to KARs, suggesting that there is functional redundancy in karrikin signaling. SMXL2 is a strong candidate for this redundancy because it is the closest paralog of SMAX1, and because its expression is regulated by KAR signaling. Here, we present evidence that SMXL2 controls hypocotyl growth and expression of the KAR/SL transcriptional markers KUF1, IAA1, and DLK2 redundantly with SMAX1. Hypocotyl growth in the smax1 smxl2 double mutant is insensitive to KAR and SL, and etiolated smax1 smxl2 seedlings have reduced hypocotyl elongation. However, smxl2 has little or no effect on seed germination, leaf shape, or petiole orientation, which appear to be predominantly controlled by SMAX1. Neither SMAX1 nor SMXL2 affect axillary branching or inflorescence height, traits that are under SL control. These data support the model that karrikin and strigolactone responses are mediated by distinct subclades of the SMXL family, and further the case for parallel butenolide signaling pathways that evolved through ancient KAI2 and SMXL duplications.

Keywords

Leaf morphology MAX2 rac-GR24 Seed germination Seedling photomorphogenesis Strigolactone 

Abbreviations

KAR

Karrikin

SL

Strigolactone

KL

KAI2 ligand

SMAX1

SUPPRESSOR OF MAX2 1

SMXL

SMAX1-LIKE

D3 (14, 53)

DWARF3 (14, 53)

PIN1

PIN-FORMED1

KAI2

KARRIKIN INSENSITIVE2

MAX2

MORE AXILLARY GROWTH2

DLK2

D14-LIKE2

KUF1

KAR-UP F-BOX1

IAA1

INDOLE-3-ACETIC ACID INDUCIBLE1

Notes

Acknowledgments

Funding from the National Science Foundation (IOS-1350561) to D.C.N. and NIGMS National Institutes of Health Award T32GM007103 to N.M. supported this work.

Supplementary material

425_2015_2458_MOESM1_ESM.docx (101 kb)
Supplementary material 1 (DOCX 100 kb)

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Copyright information

© Springer-Verlag Berlin Heidelberg 2016

Authors and Affiliations

  • John P. Stanga
    • 1
  • Nicholas Morffy
    • 1
  • David C. Nelson
    • 1
    Email author
  1. 1.Department of GeneticsUniversity of GeorgiaAthensUSA

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