Plant Molecular Biology 33: 737–743, 1997.
1997 Kluwer Academic Publishers. Printed in Belgium.
Isolation of pathogen/stress-inducible cDNAs from alfalfa by mRNA
Gina M. Truesdell
and Martin B. Dickman
School of Biological Sciences and
Department of Plant Pathology, University of Nebraska-Lincoln, Lincoln, NE
author for correspondence)
Received 9 May 1996; accepted in revised form 5 November 1996
Key words: alfalfa, differential display, gene family, plant defense, plant stress, wound-inducedgene expression
Differential display of mRNA was used to isolate a full-length (SRG1) and a partial (SRG2) alfalfa cDNA induced
during infection with the fungal pathogen Colletotrichum trifolii. The deduced amino acid sequences are similar
to each other and resemble plant defense-related proteins and tree pollen allergens. SRG1 is a member of a gene
family in alfalfa, which may also include the putative defense-related gene PR10. Unlike many defense-related
genes described in similar systems, expression of SRG1-like genes does not correlate with resistance to C. trifolii.
We speculate SRG1 is induced in response to plant stress.
Stress responses in plants are important for envir-
onmental adaptation and are associated with rapid
changes in gene expression. The pathogenesis-related
(PR) proteins are one class of proteins that accumulate
following the stress of pathogen attack. These proteins
were ﬁrst identiﬁed in tobacco leaves , where they
rapidly accumulate in the extracellular spaces follow-
ing infection with tobacco mosaic virus (TMV) .
They also accumulate upon treatment with bacterial
and fungal pathogens and chemical elicitors (such as
salicylic acid) . Activities have been assigned to
some members of the tobacco PR group of proteins,
such as the
-1,3-glucanases  and the chitinases
. The function of the PR1 class is still not known.
teins  has recently been identiﬁed. These proteins
include the disease resistance response [7, 13] and
abscisicacid-responsive proteinsfrompea, stress-
induced proteins from soybean , a wound-induced
protein from asparagus , and pathogenesis-related
proteins from parsley , kidney bean  potato
, and alfalfa . These proteins are small (17–
Thenucleotidesequencedatareportedwill appear intheEMBL,
GenBank and DDBJ Nucleotide Sequence Databases under the
accession numbers U42752 (SRG1) and U42753 (SRG2).
19 kDa), like tobacco PR1 proteins, and their cor-
responding mRNAs accumulate in plant tissues or in
cultured cells in response to various stresses. These
proteins are distinct, however, because they do not dis-
play signiﬁcant sequence similarity to the ‘classical’
PR proteins [13, 28] and their respective cDNAs do
not encode signal sequences . Because database
searches do not point to any known biochemical activ-
ity, a speciﬁc function has not been assigned to any
member of this protein class. Another characteristic
common to these proteinsis that many of their respect-
ive genes comprise gene families, which likely arose
from duplication events .
In addition to being similar to each other, this lat-
ter group of defense-related proteins is also similar to
major pollen allergens from trees of the order Fagales
(birch , alder , hazel  and hornbeam ),
which cause Type I human allergies. The pollen aller-
gens are all about 17 kDa and consist of a heterogen-
eous mixtureof different isoforms,whichare probably
encoded by gene families [3, 29]. The allergens are
proposed to function in pollen defense responses .
Due to the similarities in the primary structures of the
defense-related proteins and the pollen allergens, it
has been suggested that these proteins display a sim-