A cDNA, encoding a cysteine protease inhibitor (AhCPI), was isolated from an immature
seed cDNA library of grain amaranth (Amaranthus hypochondriacus L.) and characterized.
It encoded a polypeptide of 247 amino acids (aa), including a putative N-terminal
signal peptide. Other relevant regions found in its sequence included the G and PW
conserved aa motifs, the consensus LARFAV sequence for phytocystatins and the reactive
site QVVAG. The predicted aa sequence for AhCPI showed a significant homology to other
plant cystatins. Gene expression analyses indicated that AhCPI was constitutively
expressed in mature seeds, and gradually decreased during germination. In vegetative
tissues, AhCPI was expressed in the radicle and hypocotyls of seedlings and in the
stems and roots of young plantlets. Its expression in roots and stems increased substantially
in response to water deficit, salinity-, cold- and heat-stress, whereas heat-stress
induced a rapid and transient accumulation of AhCPI transcripts in leaves. The results
obtained were suggestive of multiple roles for AhCPI in grain amaranth, acting as
a regulator of seed germination and as a protective agent against diverse types of
abiotic stress, which induced this gene in a tissue- and stress-specific manner. The
work herewith described reports a novel, and apparently, single cystatin protein in
which, in agreement with other plant model systems, could have a regulatory role in
germination, and further expands previous findings linking the accumulation of protease
inhibitors, mostly of the serine proteinase type, with protection against (a)biotic
stress in A. hypochondriacus.