TheSAC51Family Plays a Central Role in Thermospermine Responses in Arabidopsis

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 (b-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.

Salt hypersensitivity is associated with excessive xylem development in a thermospermine‐deficient mutant of Arabidopsis thaliana

“…On the other hand, growth in the presence of xylemin, a competitive inhibitor of thermospermine synthesis (Yoshimoto et al ., ), enhanced the sensitivity to NaCl in wild‐type seedlings (Figure c). Because growth with thermospermine drastically represses the development of xylem vessels in the root (Cai et al ., ) while that with xylemin has an opposite effect, the degree of xylem development appears to be correlated with the increase in salt sensitivity.…”

“…acl5 sac51‐d restored normal sensitivity to salt (Figure a). SAC51 is one of four members of the SAC51 gene family, and sacs represents a quadruple T‐DNA insertion mutant of these genes, among which sacl1‐1 may be a weak allele but the other three are null, and partially mimics the excess xylem phenotype of acl5 while accumulating increased levels of thermospermine (Cai et al ., ). sacs also showed increased salt sensitivity (Figure a).…”

“…In addition to sac51‐d , we have identified sac52‐d , sac53‐d , sac56‐d and sac57‐d as genetic suppressors that reverse the dwarf phenotype of acl5 . sac52‐d , sac53‐d and sac56‐d represent dominant alleles of ribosomal protein genes, RPL10 , RACK1 and RPL4 , respectively, and may result in the enhanced translation of the SAC51 mRNA without thermospermine (Imai et al ., ; Kakehi et al ., ), while sac57‐d has a point mutation in a conserved uORF of SACL3 and its main ORF translation was shown to be enhanced like that of the SAC51 mRNA in sac51‐d (Vera‐Sirera et al ., ; Cai et al ., ). These suppressor mutants in the acl5 mutant background also recovered normal sensitivity to NaCl in the absence of thermospermine (Figure S6).…”

“…Although the sodium level in xylem sap remains to be determined, the excess of xylem vessel development in acl5 might result in higher sodium accumulation in the xylem sap. In agreement with this notion, 24‐h supplementation with thermospermine did not alter sodium sensitivity, but a 5‐day growth with thermospermine, which has a potent suppressive effect on xylem development (Cai et al ., ; Yoshimoto et al ., ), reduced sodium uptake and, probably consequently, alleviated the sensitivity in acl5 and even in the wild‐type (Figure a,b). The sodium content is also low in pao5 seedlings treated with NaCl for 12 h compared with that in the wild‐type (Sagor et al ., ).…”

Realizing pipe dreams – a detailed picture of vascular development

“…Nevertheless all three models support the interaction of auxin and cytokinin as a plausible system for generating early vascular pattern (Mellor et al , 2017). While these hormone interactions are responsible for generating pattern, they are underpinned by a network of interacting factors, dominated by helix-loop-helix transcription factors, TARGET OF MONOPTEROS 5 (TMO5) (Schlereth et al , 2010), LONESOME HIGHWAY (LHW) (Ohashi-Ito and Bergmann, 2007), SUPRESSOR OF ACAULIS51 (SAC51) (Imai et al , 2006), and SAC51-LIKE (SACL) (Cai et al , 2016), that act downstream of auxin signalling and influence cytokinin activity (reviewed in Campbell and Turner, 2017). …”