PLANT PHYSIOLOGY , Vol 105, Issue 4 1097-1105, Copyright © 1994 by American Society of Plant Biologists
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MOLECULAR BIOLOGY AND GENE REGULATION |
Molecular Analysis of Two cDNA Clones Encoding Acidic Class I Chitinase in Maize
S. Wu, A. L. Kriz and J. M. Widholm
Department of Agronomy, University of Illinois, 1201 West Gregory Avenue, Urbana, Illinois 61801
The cloning and analysis of two different cDNA clones encoding putative
maize (Zea mays L.) chitinases obtained by polymerase chain reaction (PCR)
and cDNA library screening is described. The cDNA library was made from
poly(A)+ RNA from leaves challenged with mercuric chloride for 2 d. The two
clones, pCh2 and pCh11, appear to encode class I chitinase isoforms with
cysteine-rich domains (not found in pCh11 due to the incomplete sequence)
and proline-/glycine-rich or proline-rich hinge domains, respectively. The
pCh11 clone resembles a previously reported maize seed chitinase; however,
the deduced proteins were found to have acidic isoelectric points. Analysis
of all monocot chitinase sequences available to date shows that not all
class I chitinases possess the basic isoelectric points usually found in
dicotyledonous plants and that monocot class II chitinases do not
necessarily exhibit acidic isoelectric points. Based on sequence analysis,
the pCh2 protein is apparently synthesized as a precursor polypeptide with
a signal peptide. Although these two clones belong to class I chitinases,
they share only about 70% amino acid homology in the catalytic domain
region. Southern blot analysis showed that pCh2 may be encoded by a small
gene family, whereas pCh11 was single copy. Northern blot analysis
demonstrated that these genes are differentially regulated by mercuric
chloride treatment. Mercuric chloride treatment caused rapid induction of
pCh2 from 6 to 48 h, whereas pCh11 responded only slightly to the same
treatment. During seed germination, embryos constitutively expressed both
chitinase genes and the phytohormone abscisic acid had no effect on the
expression. The fungus Aspergillus flavus was able to induce both genes to
comparable levels in aleurone layers and embryos but not in endosperm
tissue. Maize callus grown on the same plate with A. flavus for 1 week
showed induction of the transcripts corresponding to pCh2 but not to pCh11.
These studies indicate that the different chitinase isoforms in maize might
have different functions in the plant, since they show differential
expression patterns under different conditions.
