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Plant Cell Physiol. 2016 Aug;57(8):1583-92. doi: 10.1093/pcp/pcw113. Epub 2016 Jul 7.

The SAC51 Family Plays a Central Role in Thermospermine Responses in Arabidopsis.

Author information

1
Graduate School of Natural Science and Technology, Okayama University, Okayama, Japan perfect@cc.okayama-u.ac.jp.
2
Graduate School of Biological Sciences, Nara Institute of Science and Technology, Ikoma, Japan perfect@cc.okayama-u.ac.jp.
3
Graduate School of Natural Science and Technology, Okayama University, Okayama, Japan.

Abstract

The acaulis5 (acl5) mutant of Arabidopsis thaliana is defective in the biosynthesis of thermospermine and shows a dwarf phenotype associated with excess xylem differentiation. SAC51 was identified from a dominant suppressor of acl5, sac51-d, and encodes a basic helix-loop-helix protein. The sac51-d mutant has a premature termination codon in an upstream open reading frame (uORF) that is conserved among all four members of the SAC51 family, SAC51 and SACL1-SACL3 This suggests that thermospermine cancels the inhibitory effect of the uORF in main ORF translation. Another suppressor, sac57-d, has a mutation in the conserved uORF of SACL3 To define further the function of the SAC51 family in the thermospermine response, we analyzed T-DNA insertion mutants of each gene. Although sacl1-1 may not be a null allele, the quadruple mutant showed a semi-dwarf phenotype but with an increased level of thermospermine and decreased sensitivity to exogenous thermospermine that normally represses xylem differentiation. The sac51-1 sacl3-1 double mutant was also insensitive to thermospermine. These results suggest that SAC51 and SACL3 play a key role in thermospermine-dependent negative control of thermospermine biosynthesis and xylem differentiation. Using 5' leader-GUS (β-glucuronidase) fusion constructs, however, we detected a significant enhancement of the GUS activity by thermospermine only in SAC51 and SACL1 constructs. Furthermore, while acl5-1 sac51-1 showed the acl5 dwarf phenotype, acl5-1 sacl3-1 exhibited an extremely tiny-plant phenotype. These results suggest a complex regulatory network for the thermospermine response in which SAC51 and SACL3 function in parallel pathways.

KEYWORDS:

Arabidopsis; Thermospermine; Translation; Xylem differentiation; uORF

PMID:
27388339
DOI:
10.1093/pcp/pcw113

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