Plant Molecular Biology 38: 623–632, 1998.
© 1998 Kluwer Academic Publishers. Printed in the Netherlands.
Induction of mRNA accumulation corresponding to a gene encoding a cell
wall hydroxyproline-rich glycoprotein by fungal elicitors
e Antonio Mart
ınez-Izquierdo and Pere Puigdom
Departament de Gen`etica Molecular, CID-CSIC, Jordi Girona, 18, 08034 Barcelona, Spain (
Received 28 July 1997; accepted in revised form 24 April 1998
Key words: elicitor, hydroxyproline-rich glycoproteins, Zea mays
The Hrgp (hydroxyproline-rich glycoprotein) gene codes in maize for one of the most abundant proteins of the
cell wall. HRGPs may contribute to the structural support of the wall and they have also been involved in plant
defense mechanisms. This second aspect has been tested for the Hrgp gene in maize where, in contrast with the
situation in dicot species, the gene is encoded by a single-copy sequence. Hrgp mRNA accumulation is induced in
maize suspension-cultured cells by elicitors, isolated either from maize pathogenic or non-pathogenic fungi. The
inductionof Hrgp mRNA accumulation by elicitor extracted fromFusarium moniliforme has been studied in detail.
The level of induction depends on elicitor concentration and remains high until at least 24 h. Ethylene and protein
phosphorylation appear to be involved in the transduction pathway of Hrgp gene activation by the F. moniliforme
elicitor but not by 5 µM methyl jasmonate or 1 mM salycilic acid. Different compounds known to participate
in plant stress responses such as ascorbic acid or reduced glutathione have also a positive effect on Hrgp mRNA
Abbreviations: α-AB, α-aminobutyric acid; ACC, 1-aminocyclopropane-1-carboxylic acid; AOA, amino oxyac-
etate; BAP, 6-benzylaminopurine; BMS, Black Mexican Sweet; HRGP, hydroxyproline-rich glycoprotein; MeJa,
methyl jasmonate; STS, silver thiosulfate; St, staurosporine; 2,4-D, 2,4-dichlorophenoxyaceticacid.
The plant cell wall is a dynamic structure, changing
with physiologicalﬂuctuationscaused by environmen-
tal stimuli, tissue differentiation and maturation [7,
55, 59]. HRGPs (hydroxyproline-rich glycoproteins)
constitute one of the most abundant structural pro-
teins in the plant cell wall  and one of the most
abundant classes of proteins rich in proline in plants.
HRGPs have been described both in mono- and di-
cotyledonse. In the dicot species HRGPs are often
called extensins and they have been studied in more
detail than in monocots,albeit the presence of hydrox-
yproline in cell walls have been detected in different
cereal species [33, 63].
Studies in different species have shown that, al-
though extensins are synthesized as soluble precur-
sors, once secreted to the cell wall they become in-
soluble, presumably due to the formation of covalent
cross-linkages . The expression of the extensin
genesin dicotyledonousplantsis developmentallyreg-
ulated [31, 69] and mRNA accumulation has been
foundin phloem and cambiumtissues in healthy plants
. Expression of extensin is induced in response to
mechanical wounding [15, 40, 45, 53, 68], microbial
elicitors [15, 54, 68], pathogen attack [3, 15, 37, 54]
or ethylene treatment [9, 19, 22, 40].
At least two main functions have been proposed
for extensins in plants . First, they may contribute
to the mechanical properties of the cell wall by creat-
ing a network formed by glycoprotein elements, even
though direct functional evidence is absent. Second,
they contribute to plant defense, by strengthening the
wall upon pathogen attack or mechanical wounding