The cloning and analysis of two different cDNA clones encoding putativ
e maize (Zea mays L.) chitinases obtained by polymerase chain reaction
(PCR) and cDNA library screening is described. The cDNA library was m
ade from poly(A)(+) RNA from leaves challenged with mercuric chloride
for 2 d. The two clones, pCh2 and pCh11, appear to encode class I chit
inase isoforms with cysteine-rich domains (not found in pCh11 due to t
he incomplete sequence) and proline-/glycine-rich or proline-rich hing
e domains, respectively. The pCh11 clone resembles a previously report
ed maize seed chitinase; however, the deduced proteins were found to h
ave acidic isoelectric points. Analysis of all monocot chitinase seque
nces available to date shows that not all class I chitinases possess t
he basic isoelectric points usually found in dicotyledonous plants and
that monocot class II chitinases do not necessarily exhibit acidic is
oelectric points. Based on sequence analysis, the pCh2 protein is appa
rently synthesized as a precursor polypeptide with a signal peptide. A
lthough these two clones belong to class I chitinases, they share only
about 70% amino acid homology in the catalytic domain region. Souther
n blot analysis showed that pCh2 may be encoded by a small gene family
, whereas pCh11 was single copy. Northern blot analysis demonstrated t
hat these genes are differentially regulated by mercuric chloride trea
tment. Mercuric chloride treatment caused rapid induction of pCh2 from
6 to 48 h, whereas pCh11 responded only slightly to the same treatmen
t. During seed germination, embryos constitutively expressed both chit
inase genes and the phytohormone abscisic acid had no effect on the ex
pression. The fungus Aspergillus flavus was able to induce both genes
to comparable levels in aleurone layers and embryos but not in endospe
rm tissue. Maize callus grown on the same plate with A. flavus for 1 w
eek 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.