Characterization of genes coding for small hypervariable peptides in Globodera rostochiensis

N.E.M. van Bers

Research output: Thesisinternal PhD, WU


Plant parasitic nematodes secrete a cocktail of effector molecules, which are involved
in several aspects of the interaction with the host, eg. in host defense suppression, in
migration and in feeding cell formation. In this thesis, we performed the first study on
10 novel peptide genes, believed to be important for parasitism of the potato cyst
nematode, Globodera rostochiensis. Nine of the peptide genes described here belong
to the SECPEP gene family. The SECPEP genes are all expressed in the dorsal
esophageal gland, which is one of the main sites for the production of effector
molecules. This, together with the predominant expression in preparasitic and early
parasitic juvenile nematodes, makes it very likely that the SECPEPs code for effector
peptides essential for succesful infection and feeding site formation.
In chapter 2, we show that diversifying selection is a likely driver of the molecular
evolution of the SECPEPs. The sequences of the mature peptides appear to be highly
diverse, while the non)coding 3’UTR and intronic regions as well as the region coding
for the signal peptide for secretion are relatively conserved. In fact, a pairwise
comparison of the SECPEPs reveals no significant sequence similarity between family
members at all. In chapter 5 we further speculate on a possible role for RNA)editing as
a mechanism to yield hypervariability in the SECPEPs, because the sequence diversity
at the transcript level significantly exceeds that of the genomic locus. Chapter 5 further
elaborates on the analysis of trans)splicing in SECPEP1 transcripts. We show that
SECPEP1 transcripts are trans)spliced to a surprising diversity of novel spliced)leader
sequences. The first approach to unravel the role of the members of the SECPEP family
in plant parasitism, is described in chapter 4. We generated transgenic potato and
Arabidopsis plants expressing SECPEP3 while using the CaMV 35S promotor. The
phenotype associated with SECPEP3 in both potato and Arabidopsis plants includes a
reduction of root growth and an alteration of the leaf morphology. The SECPEP3
peptide harbors several sequence motifs first found in the cyclin)dependent kinase
inhibitors ICK1/KRP1, SIM and Smr1. We, therefore, suggest a role for SECPEP3 in cell
cycle alteration in nematode feeding site formation. Although the SECPEP genes show
only a low level of primary sequence similarity, all code for positively charged,
hydrophilic peptides with a C)x)G γ)core motif (chapter 2). These are characteristics
typical for host defense peptides, and in chapter 6 we investigate whether these
characteristics are also found for other peptides involved in plant)parasite interactions.
We show that a considerable number of these effector peptides share a positive
charge, hydrophilicity and C)x)G γ)core motif with the SECPEPs, and we speculate on a
role for the positive charge in peptide)ligand interaction.
In chapter 3 we describe the NEMPEP peptide, secreted by G. rostochiensis. NEMPEP
is also a positively charged, hydrophilic peptide with a C)x)G γ)core motif, although it is
genetically unrelated to the SECPEP gene family. During the life cycle of G.
rostochiensis, the expression pattern of NEMPEP reveals a striking regulation. NEMPEP
is highly expressed in preparasitic juveniles and in the parasitic life stages after initial
feeding cell formation. However, NEMPEP expression was hardly detectable in the
juveniles just after entering the plant root. Several disease resistance genes condition
nematode resistance at the onset of parasitism. The downregulation of NEMPEP at
exactly this timepoint could be a strategy to avoid recognition by the host’s immune
system. In planta expression of NEMPEP, as a fusion to GFP, shows that NEMPEP
accumulates in the nucleolus of tobacco cells. Potato plants transformed with
35S::NEMPEP were slow at forming roots and the internodes between the leaflets were
shortened. This, together with a reduced transformation efficiency, led us to
hypothesize a role for NEMPEP in cytokinin signaling (Chapter 3).
Currently, there are two models regarding the functional role of the SECPEPs and
NEMPEP. The first one concerns a role as an antimicrobial peptide, which could protect
the host plant against secondary infections by opportunistic microbes. As a competing
hypothesis, the high hydrophilicity of the peptides may point to a role as peptide
hormone. As such, they may be involved in redirecting cell cycle or hormonal regulation
upon feeding cell formation.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Wageningen University
  • Bakker, Jaap, Promotor
  • Smant, Geert, Co-promotor
  • Goverse, Aska, Co-promotor
Award date17 Jun 2008
Place of Publication[S.l.]
Print ISBNs9789085049579
Publication statusPublished - 2008


  • globodera rostochiensis
  • plant parasitic nematodes
  • peptides
  • genes
  • gene expression
  • solanum tuberosum
  • arabidopsis
  • nicotiana
  • gene splicing
  • host pathogen interactions
  • molecular interactions

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