High humidity suppresses ssi4-mediated cell death and disease resistance upstream of MAP kinase activation, H2O2 production and defense gene expression.
The Arabidopsis ssi4 mutant, which exhibits spontaneous lesion formation, constitutive expression of pathogenesis-related (PR) genes and enhanced resistance to virulent bacterial and oomycete pathogens, contains a gain-of-function mutation in a TIR-NBS-LRR type R gene. Epistatic analyses revealed that both PR gene expression and disease resistance are activated via a salicylic acid (SA)- and EDS1-dependent, but NPR1- and NDR1-independent signaling pathway. In this study, we demonstrate that in moderate relative humidity (RH; 60%), the ssi4 mutant accumulates H(2)O(2) and SA prior to lesion formation and displays constitutive activation of the MAP kinases AtMPK6 and AtMPK3. It also constitutively expresses a variety of defense-associated genes, including those encoding the WRKY transcription factors AtWRKY29 and AtWRKY6, the MAP kinases AtMPK6 and AtMPK3, the powdery mildew R proteins RPW8.1 and RPW8.2, EDS1 and PR proteins. All of these ssi4-induced responses, as well as the chlorotic, stunted morphology and enhanced disease resistance phenotype, are suppressed by high RH (95%) growth conditions. Thus, a humidity sensitive factor (HSF) appears to function at an early point in the ssi4 signaling pathway. All ssi4 phenotypes, except for MAP kinase activation, also were suppressed by the eds1-1 mutation. Thus, ssi4-induced MAP kinase activation occurs downstream of the HSF but either upstream of EDS1 or on a separate branch of the ssi4 signaling pathway. SA is a critical signaling component in ssi4-mediated defense responses. However, exogenously supplied SA failed to restore lesion formation in high RH-grown ssi4 plants, although it induced defense gene expression. Thus, additional signals also are involved.
Zhou
Fasong
F
Boyce Thompson Institute for Plant Research, Tower Road, Ithaca, NY 14853, USA.
Menke
Frank L H
FL
Yoshioka
Keiko
K
Moder
Wolfgang
W
Shirano
Yumiko
Y
Klessig
Daniel F
DF
D017360:Arabidopsis / Q000235:genetics / Q000254:growth & development / Q000502:physiology*; D029681:Arabidopsis Proteins / Q000235:genetics / Q000502:physiology*; D001483:Base Sequence; D016923:Cell Death; D004789:Enzyme Activation; D018506:Gene Expression Regulation, Plant; D006813:Humidity; D006861:Hydrogen Peroxide / Q000378:metabolism*; D007113:Immunity, Innate; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism*; D018749:RNA, Plant / Q000235:genetics / Q000302:isolation & purification
D029681:Arabidopsis Proteins; D018749:RNA, Plant; C472141:SSI4 protein, Arabidopsis; D006861:Hydrogen Peroxide; D020928:Mitogen-Activated Protein Kinases