Minisymposium 2: Signaling

Abs # 12005: FA signaling pathway and its role in plant defense

Presenter:	Kachroo, Pradeep , pk62@uky.edu
Authors	Kachroo, Aardra  (A)   Venugopal, Srivathsa C (A)   Lapchyk, Ludmila  (A)   Hildebrand, David  (B)   Kachroo, Pradeep  (A)
Affiliations:	(A): Department of Plant Pathology, University of Kentucky, Lexington, KY 40546 (B): Department of Agronomy, University of Kentucky, Lexington, KY 40546
Web Site:	http://www.ca.uky.edu/agcollege/plantpathology/PPATHTEST/kachroo/

The salicylic acid (SA)-, jasmonic acid (JA)- and ethylene-mediated plant defense pathways act independently, synergistically as well as antagonistically, thus undergoing extensive cross talk. In an effort to decipher the regulatory components common to both SA and JA signaling, we studied the ssi2 mutant of Arabidopsis, which is altered in both of these pathways. A mutation in the ssi2/fab2, encoding stearoyl-ACP desaturase, constitutively activates NPR1-dependent as well as –independent defense responses, while simultaneously compromising the induction of the JA responsive gene PDF1.2 and resistance to necrotrophic pathogens. Our studies with ssi2 and its suppressors, indicates that reduced 18:1 levels in the ssi2 mutant results in altered defense signaling. Furthermore, replenishing 18:1 levels results in restoration of wild-type like signaling in the mutant (1, 2, 4). This suggests that 18:1 or an 18:1 derived molecule(s) acts as the signal for restoring altered defense signaling in ssi2 plants. We have identified three genes, which participate in the prokaryotic FA pathway and loss-of-function of which restores various phenotypes in ssi2 plants. Mutations in genes encoding for the glycerol-3-phosphate (G3P) acyltransferase (ACT1, 2), G3P dehydrogenase (GLY1, 3, 4) and a membrane associated w6 desaturase (FAD6, 2) restore wild-type like phenotypes via restoration of 18:1 levels in the ssi2 plants. Interestingly, the 18:1 content in wild-type plants can be lowered by glycerol application, resulting in ssi2-mimics. Regeneration of G3P pools in glycerol treated ssi2 and ssi2 gly1-3 plants results in a marked reduction in the 18:1 levels, further enhancing the ssi2-like phenotypes. Furthermore, overexpression of the ACT1 gene, which catalyzes the acylation of 18:1 with G3P, also enhances ssi2- related phenotypes upon glycerol application. Consequently, a mutation in act1 rescues this hypersensitivity to glycerol. Our results show that a FA regulated pathway is involved in modulating both SA- and JA- related responses in Arabidopsis. We demonstrate that 18:1 levels in plastids are regulated via acylation with G3P and a balance between G3P and 18:1 is critical for the regulation of SA- and JA-mediated signaling pathways. 1. Kachroo, P., et al., (2001) PNAS 98, 9448-9453. 2. Kachroo, A., et al., (2003) Plant Cell 15, 2952-2965. 3. Kachroo, P. et al., (2003) MPMI 11,1022-1029. 4. Kachroo, A., et al., (2004) PNAS In press (PMID: 15044700).