Calcium Ion Signaling System: Calcium Signatures and Sensors - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 237

4.14.15 Calmodulin-Binding Homeodomain

Transcription Factors

Homeodomain proteins in the homoeobox gene family play important roles as tran-
scription factors in plants (Williams 1998 ). Some of the homeodomain transcription
factors have been shown to be involved in modulating plant immune responses
(Park et al. 2007 ). An Arabidopsis homeodomain transcription factor,
OVEREXPRESSOR OF CATIONIC PEROXIDASE 3, mediates resistance to
infection by necrotrophic pathogens (Coego et al. 2005 ). The homeodomain tran-
scription factors have also been reported as calmodulin-binding proteins. In soy-
bean, transcription of calmodulin isoform 4 ( GmCaM4 ) is rapidly induced within
30 min after pathogen stimulation. The GmCaM4 promoter contains two repeats of
conserved homeodomain binding site, ATTA (Park et al. 2007 ). Two proteins,
GmZF-HD1 and GmZF-HD2, belonging to the zinc fi nger homeodomain (ZF-HD)
transcription factor family have been detected in soybean. These transcription fac-
tors bind to the two repeats of ATTA homeodomain binding site in the calmodulin
GmCaM4. This binding was induced in response to the pathogen. Regulation of
GmCaM4 gene by the GMZF-HD transcription factors may be a signifi cant compo-
nent of the plant defense-signaling pathway (Park et al. 2007 ).

4.14.16 CaM-Binding Protein Involved in Glucosinolate

Metabolism

A nuclear protein, IQ-DOMAIN1 (IQD1), is a CaM-binding protein and integrates
intracellular Ca 2+ signals towards stimulation of glucosinolate accumulation and
plant defense (Levy et al. 2005 ). IQD1 encodes a novel protein that contains puta-
tive nuclear localization signals and several motifs known to mediate calmodulin
binding, which are arranged in a plant-specifi c segment of 67 amino acids, called
the IQ67 domain. IQD1 protein is targeted to the cell nucleus and binds to calmodu-
lin in a Ca 2+ -dependent fashion. IQD1 affects expression of multiple genes with
roles in glucosinolate metabolism. It is suggested that IQD1 is a nuclear factor that
integrates intracellular Ca 2+ signals to fi ne-tune glucosinolate accumulation in
response to pathogens (Levy et al. 2005 ).

4.14.17 CaM Binds with MLO Protein to Regulate

Defense Response

CaM binds with MLO protein, which exhibits a dual role as docking molecule and
defense modulator for the powdery mildew pathogen (Panstruga and Schulze-Lefert
2005 ). MLO has been detected in Arabidopsis , barley, and rice (Kim et al. 2002a , b ;