Peptides as signals - Intercellular Communication in Plants - page 33

cell division cycle, leading to formation of the nodule primordium. The nodule
primordium differentiates to form a nodule after bacteria are released from the
infection thread.
ENOD40 is one of the early nodulin genes ( ENOD ). The first ENOD40 gene,
GmENOD40 ,was identified from soybean (Yang et al. , 1993). GmENOD40 is
strongly induced during nodule development prior to the start of N 2 fixation. This
suggests that the gene is involved in the nodulation process but not N 2 -fixation pro-
cess. During early stages of nodule development, the GmENOD40 gene is expressed
first in the pericycle of root vascular bundle and then in dividing root cortical cells
and the nodule primordium.
ENOD40 produces a 0.7-kb transcript that was initially thought to function as
a nontranslatable RNA because it lacked a significant open reading frame (ORF)
(Crespi et al. , 1994). Further insights into the possible function of ENOD40 come
from the study on the ENOD40 gene from Medicago truncatula ( MtENOD40 )
(Sousa et al. , 2001). When MtENOD40 was delivered into epidermal cells and
the outermost cortical layer by ballistic microtargeting, it induced division of inner
cortical cells to initiate nodule formation. This phenomenon was used as an assay
for the MtENOD40 activity. The alfalfa ENOD40 contains two small ORFs (sORF
I and sORF II) that encode 13 and 27 amino acids respectively. The sORF I region
is highly conserved among the ENOD40 genes from different species. It was found
that sORF I or sORF II alone is sufficient for ENOD40 activity. In addition, their
activity requires translation of the sORFs. The other regions of the transcript may
play a role in controlling stability or regulating the translation process. Therefore,
the study indicates that the short peptides encoded by the sORFs are responsible for
ENOD40 activity.
A study using in vitro translation of soybean ENOD40 in wheat germ extracts re-
vealed that two peptides (Peptides A and B) of 12 and 24 residues, respectively, were
translated from the single mRNA (Rohrig et al. , 2002). Both peptides are translated
from the region conserved among the ENOD40 genes. The studies on MtENOD40
and Soybean ENOD40 demonstrate that multiple peptides are produced from the
polycistronic transcripts by translation of multiple small ORFs. Interestingly, both
Peptides A and B bind to nodulin 100, which is a subunit of sucrose synthase (Rohrig
et al. , 2002). The discovery suggests that ENOD40 peptides may control photosyn-
thate use, but it does no provide an obvious answer as to how the regulation of
sucrose synthase activity by ENOD40 leads to cortical cell dedifferentiation and
proliferation.
The ENOD40 gene is expressed not only during nodulation but also in phloem
cells of stems. In addition, ENOD40 genes have been isolated from non-legume
dicot species and monocot species such as tomato, rice, and maize that do not form
nodules (van de Sande et al. , 1996; Kouchi et al. , 1999). Therefore, ENOD40 may
have a general role as a regulator of cell proliferation in plants.
The peptides derived from ENOD40 are apparently localized in the cytosol. How-
ever, expression of ENOD40 gene in outer cell layers of alfalfa roots induces cell