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Gene Review

SSI2  -  acyl-[acyl-carrier-protein] desaturase

Arabidopsis thaliana

Synonyms: AtSSI2, F18O19.18, FAB2, FATTY ACID BIOSYNTHESIS 2, LDW1, ...
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High impact information on SSI2

  • A mutation in the Arabidopsis gene ssi2/fab2, which encodes stearoyl-acyl carrier protein desaturase (S-ACP-DES), results in the reduction of oleic acid (18:1) levels in the mutant plants and also leads to the constitutive activation of NPR1-dependent and -independent defense responses [1].
  • Glycerol application also lowered the 18:1 content in SSI2 plants and converted these into ssi2-mimics [2].
  • We previously showed that stearoyl-acyl-carrier-protein-desaturase mutants ssi2/fab2 carrying a loss-of-function mutation in the plastidial glycerol-3-phosphate (G3P) acyltransferase (act1) have elevated 18:1 levels and are restored in their altered defense signaling [2].
  • Our previous work with the Arabidopsis (Arabidopsis thaliana) ssi2/fab2 mutant and its suppressors demonstrated that a balance between glycerol-3-phosphate (G3P) and 18:1 levels is critical for the regulation of salicylic acid (SA)- and jasmonic acid-mediated defense signaling in the plant [3].
  • Based on these results, SSI2 or an SSI2-generated signal appears to modulate signaling of an SA-dependent, NPR1-independent defense pathway, or an SA- and NPR1-independent defense pathway [4].

Biological context of SSI2

  • In addition to SSI2, the Arabidopsis genome encodes six S-ACP-DES-like enzymes, the native expression levels of which are unable to compensate for a loss-of-function mutation in ssi2 [5].
  • Both ssi2 and fab2 plants show spontaneous cell death, express PR genes constitutively, accumulate high levels of SA, and exhibit enhanced resistance to bacterial and oomycete pathogens [6].

Associations of SSI2 with chemical compounds

  • Restoration of defective cross talk in ssi2 mutants: role of salicylic acid, jasmonic acid, and fatty acids in SSI2-mediated signaling [6].
  • This is evident in the Arabidopsis ssi2/fab2 mutant, which encodes a defective stearoyl-acyl carrier protein-desaturase (S-ACP-DES) and consequently accumulates high levels of stearic acid (18:0) and low levels of 18:1 [5].

Other interactions of SSI2

  • Our results suggest that interaction between an SSI2-dependent factor or factors and an EDS5- and PAD4-dependent mechanism or mechanisms modulates defense to B. cinerea [7].


  1. Plastidial fatty acid signaling modulates salicylic acid- and jasmonic acid-mediated defense pathways in the Arabidopsis ssi2 mutant. Kachroo, A., Lapchyk, L., Fukushige, H., Hildebrand, D., Klessig, D., Kachroo, P. Plant Cell (2003) [Pubmed]
  2. Oleic acid levels regulated by glycerolipid metabolism modulate defense gene expression in Arabidopsis. Kachroo, A., Venugopal, S.C., Lapchyk, L., Falcone, D., Hildebrand, D., Kachroo, P. Proc. Natl. Acad. Sci. U.S.A. (2004) [Pubmed]
  3. Role of salicylic acid and fatty acid desaturation pathways in ssi2-mediated signaling. Kachroo, P., Venugopal, S.C., Navarre, D.A., Lapchyk, L., Kachroo, A. Plant Physiol. (2005) [Pubmed]
  4. A recessive mutation in the Arabidopsis SSI2 gene confers SA- and NPR1-independent expression of PR genes and resistance against bacterial and oomycete pathogens. Shah, J., Kachroo, P., Nandi, A., Klessig, D.F. Plant J. (2001) [Pubmed]
  5. The Arabidopsis stearoyl-acyl carrier protein-desaturase family and the contribution of leaf isoforms to oleic acid synthesis. Kachroo, A., Shanklin, J., Whittle, E., Lapchyk, L., Hildebrand, D., Kachroo, P. Plant Mol. Biol. (2007) [Pubmed]
  6. Restoration of defective cross talk in ssi2 mutants: role of salicylic acid, jasmonic acid, and fatty acids in SSI2-mediated signaling. Kachroo, P., Kachroo, A., Lapchyk, L., Hildebrand, D., Klessig, D.F. Mol. Plant Microbe Interact. (2003) [Pubmed]
  7. Arabidopsis ssi2-conferred susceptibility to Botrytis cinerea is dependent on EDS5 and PAD4. Nandi, A., Moeder, W., Kachroo, P., Klessig, D.F., Shah, J. Mol. Plant Microbe Interact. (2005) [Pubmed]
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