75 Integrating Membrane Transport with Male Gametophyte ... - TAIR

271 Scarecrow-like Protein SCL14 Interacts with TGA Factors and Functions as an Essential Coactivator
of as-1-dependent Transcription
Tanja Siemsen, Corinna Thurow, Benjamin Fode, Ralf Weigel, Christiane Gatz
University of Goettingen, Untere Karspuele 2, 37073 Goettingen, Germany
Plant defense signalling molecule salicylic acid (SA) as well as xenobiotic stress cues like 2.4D (dichlorphenoxy
acetic acid) and the herbicide paraquat induce a number of defense genes containing the cis regulatory element activation
sequence-1 (as-1). As-1 type cis elements are bound by basic/leucine zipper (bZIP) transcription factors of the TGA
family. In order to identify factors regulating TGA activity, a yeast protein interaction screen with Arabidopsis thaliana
TGA2 as bait and an Arabidopsis thaliana cDNA library was performed and led to the identification of SCARECROW-
LIKE protein 14. Functional analysis in scl14 knock out plants revealed that SCL14 is required for the expression of
the “truncated” CaMV 35S promoter, which contains the as-1 element as the only regulatory sequence. Chromatin
immunoprecipitation (ChIP) experiments revealed 2.4D-enhanced association of SCL14 and TGA2 to the as-1 element
in planta. As the activity of known as-1-containing promoters like the GST6 and the PR-1 promoter was not altered, a
whole genome microarray analysis was carried out to identify endogenous promoters that required SCL14 for expression.
Two genes that were down-regulated in the scl14 mutant were further analysed. ChIP analysis showed recruitment of
SCL14 and TGA2 to these promoters. In the tga2tga5tga6 background, SCL14 showed no in vivo binding to its target
promoters, supporting the model that SCL14 is tethered to the promoter by its interaction with as-1-bound TGA factors,
where it functions as a transcriptional co-activator.
272 Signalling Pathways Involved in the “defense, no death” Phenotypes of Arabidopsis
Mutants dnd1 and dnd2
Ruth Genger 2 , Grace Jurkowski 1 , John McDowell 3 , Andrew Bent 2
1
Department of Oncology, University of Wisconsin-Madison, Madison WI 53706, 2 Department of Plant
Pathology, University of Wisconsin-Madison, Madison WI 53706, 3 Fralin Center for Biotechnology, Virginia
Tech, Blacksburg VA 24061
Plant disease resistance pathways have as key mediators the signaling molecule salicylic acid (SA) and the two plant hormones
ethylene and jasmonic acid. Numerous studies have demonstrated various levels of antagonism between these defense signaling
pathways, indicative of crosstalk. The Arabidopsis “defense, no death” (dnd) mutants dnd1 and dnd2(hlm1) exhibit constitutive
activation of plant defenses (including expression of defense marker genes), elevated SA levels, and enhanced resistance to
biotrophic and hemibiotrophic pathogens. In addition, they exhibit no, or a greatly reduced, hypersensitive response to avirulent
pathogens. These phenotypes result from mutations in two separate cyclic nucleotide-gated ion channels, AtCNGC2 or AtCNGC4
respectively. We generated and characterized double and triple mutants in the dnd1 and dnd2 genetic backgrounds using wellcharacterized
mutant alleles of NPR1, SID2 (EDS16), NDR1, and EIN2 to evaluate in greater detail the contributions of the
wild-type genes DND1 and DND2/HLM1 with regard to known defense signaling cascades. We found that both SID2 and NPR1
were required for resistance of dnd1 and dnd2 to virulent and avirulent Pseudomonas syringae and virulent Hyaloperonospora
parasitica, indicating a requirement for signaling through SA. Although mutation of NPR1 did not affect SA levels in dnd1, SID2
was required for increased SA levels in dnd1. EIN2 was not required for resistance of dnd1 or dnd2 to P. syringae or H. parasitica,
or for increased SA levels in dnd1. Mutation of ein2 compromised resistance of dnd1 to the necrotroph Botrytis cinerea. NDR1
was required for resistance of dnd1 and dnd2 to avirulent P. syringae expressing avrRpt2, and resistance of dnd1 to virulent
H. parasitica. However, for resistance to virulent P. syringae, NDR1 was required in the dnd2 background, but not in the dnd1
background, indicating a rare difference in defense signaling between dnd1 and dnd2. NDR1 was required for increased SA levels
in the dnd1 background. At the molecular level, we noted that npr1 and sid2 mutations reduced/eliminated constitutive PR-1
gene expression of dnd1 plants, but intriguingly induced PDF1.2 expression. This suggests that the defense signaling activated
by DND1 or DND2 mutations uses SA-mediated NPR1-dependent pathways as the default pathway, but when SA or NPR1 are
not available, defense signaling is diverted toward PDF1.2 expression. None of the double mutants showed restoration of the
loss-of-HR phenotype, indicating at least partly separate control of the HR from the other phenotypes of dnd1 and dnd2.