PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 45

( PROPEP2 , PROPEP3 , PROPEP4 , PROPEP5 , PROPEP6 and PROPEP7 ).
PROPEP1 orthologs have been found in rice, maize, wheat, barley, canola, potato,
soybean, Medicago , and poplar plants (Huffaker et al. 2006 ). Transcripts of
PROPEP1 , PROPEP2 and PROPEP3 genes were induced by pathogen infection
and also by PAMPs (Huffaker et al. 2006 ; Huffaker and Ryan 2007 ).
At Pep1 activates the expression of its own precursor gene PROPEP1 (Huffaker
et al. 2006 ). Because Pep peptides induced the transcription of their own precursor
genes, it is likely that Pep peptides, which are initially induced by PAMPs, feed
back into the signaling pathways to generate additional processed peptides to
further upregulate downstream defense responses (Ryan et al. 2007 ). Thus, PROPEP
genes are components of a feedback signaling system that is mediated by the PEPR1
receptor to amplify the innate immune response of Arabidopsis . At Pep1 and its
homologs ( At Pep2 to At Pep7) are endogenous amplifi ers of innate immunity of
Arabidopsis thaliana .

2.10.4 Soybean GmSubPep Peptide

In soybean, a 12-aa peptide was found to activate transcription of defense genes. The
HAMP peptide was named G lycine m ax Subtilase Peptide (GmSubPep) (Pearce et al.
2010 ). The amino acid sequence of the peptide was determined and was found to be
derived from a member of the subtilisin-like protease (subtilase) family. The sequence
of the peptide was located within a region of the protein that is unique to subtilases in
legume plants and not found within any other plant subtilases. GmSubPep peptide is
processed from a unique region of an extracellular subtilisin-like protease (subtilase)
(Pearce et al. 2010 ). It was capable of producing a pH change within 10 min and a
maximal alkalinizing response in 15 min (Pearce et al. 2010 ). Among the suspension-
cultured cells tested from a wide array of species, only the suspension cells produced
from Glycine max (soybean) were capable of producing an alkalinizing response to
GmSubPep (Pearce et al. 2010 ). The gene encoding this peptide, Glyma18g48580 ,
has been cloned. The peptide was active at extremely low concentrations. The recep-
tor for this peptide has not yet been isolated (Pearce et al. 2010 ).

2.10.5 Maize Zm Pep1 Peptide

A pathogen-inducible gene orthologous to the Arabidopsis AtPROPEP1 gene, which
encodes the precursor protein of elicitor peptide 1 ( At Pep1), has been identifi ed in
maize ( Zea mays ) and called ZmPROPEP1 (Huffaker et al. 2011 ). Both At PROPEP1
and Zm PROPEP1 do not have a conventional signal sequence for export through the
secretory pathway and both are predicted to localize to the cytosol (Huffaker et al.
2011 ). ZmPROPEP1 encodes a peptide, Zm Pep1, which is an endogenous elicitor.
The native length of Zm Pep1 is predicted to be 23 amino acids similar to the At Pep
peptides of Arabidopsis thaliana .