1. Introduction
In recent decades, heavy-metal contamination in the environment has increased due to the rapid development of industry and the use of pesticides [
1,
2]. In some parts of the world, such as Ziyang County, China [
3], soil conditions are detrimental to the growth of many or most plant species due to the increasing level of certain inorganic ions [
4]. Sometimes, even low concentrations of heavy metals such as Cd, Pb, Al, and Hg can cause great damage to plants [
5,
6,
7,
8]. Heavy metals generally produce toxic effects in plants, including chlorosis, inhibition of growth and photosynthesis, low biomass accumulation, imbalance of nutrient assimilation and water, and senescence, which eventually lead to plant death [
9]. Cd is a nonessential heavy metal and is ranked among the top 20 toxins [
10], and it can affect many processes of plant growth and development. Contamination of soil with 5 μM Cd can significantly affect the seed germination and seedling growth of barley [
11]. Different concentrations of Cd can also affect the root growth of wheat and rice [
12,
13]. Photosynthetic indices of tomato seedlings, such as the photosynthetic rate (Pn) and the intracellular CO
2 concentration (Ci), can be severely affected under Cd stress [
14]. In order to survive in environments containing heavy metals, plants have to develop a range of strategies to cope with these heavy metals [
9]. Research reports that several protein families are involved in the detoxification and sequestration of heavy metals, such as ATP-binding cassette (ABC) transporters [
15], zinc/iron-regulated transporter proteins (ZIPs) [
16], heavy-metal ATPases (HMAs) [
17,
18,
19], cation diffusion facilitators (CDFs) [
20], natural resistance-associated macrophage proteins (Nramps) [
21], and heavy-metal-associated isoprenylated plant proteins (HIPPs) [
22].
HIPPs are a group of metal-binding metallochaperones characterized by a heavy-metal-associated (HMA) domain and an isoprenylation motif [
23]. Although the HMA domain and the isoprenylation motif commonly occur in many organisms, from bacteria to humans, the presence of both interacting in the same protein has been observed only in vascular plants [
24]. Analysis of
HIPP-family genes has mainly been done for
Oryza and
Arabidopsis, with a few studies in other species [
23,
24]. Thus far, 45
HIPP genes have been identified in
Arabidopsis, 59 in
Oryza, 74 in
Populus trichocarpa, 52 in
Setaria italic, and 5 in
Selaginella moellendorffii, and these genes have been divided into five distinct clusters [
23]. All HIPP proteins have a conserved structure, including an HMA domain and a C-terminal isoprenylation CaaX motif (where “C” is cysteine, “a” is an aliphatic amino acid, and “X” is any amino acid); some of them also contain other domains, such as glycine-rich repetitions and a proline-rich motif [
23].
The functions of
HIPPs have been extensively studied in
Arabidopsis [
25],
Oryza [
26], tomato [
27], barley [
28], wheat [
29], etc., and
HIPPs have been revealed to play an important role in the maintenance of heavy-metal homeostasis and detoxification. In
Arabidopsis, the expression of
AtCdI19 can be induced by Cd, Hg, Fe, and Cu, and overexpression of
CdI19 confers Cd tolerance in transgenic
Arabidopsis [
30]. Expression of
Arabidopsis HIPP20,
HIPP22,
HIPP26, and
HIPP27 in yeast confers increased Cd resistance to the Cd-sensitive yeast strain
ycf1. The
hipp20/21/22 triple mutant is more sensitive to Cd and shows significantly decreased shoot fresh weight compared to the wild-type [
24].
AtFP6 expression can be induced by Cd and Zn and the protein can bind Pb, Cd, and Cu; overexpression of
AtFP6 can enhance tolerance to Cd compared to the wild-type [
22]. AtHIPP44 can interact with the transcription factor MYB49, thus leading to its upregulated expression and the subsequent reduction of Cd accumulation [
31]. In rice, the expression of
OsHIPP16,
OsHIPP28,
OsHIPP34,
OsATX1, and
OsHIPP60 in roots and shoots can be induced by Mn, Cd, and Cu [
26]. The expression of
OsHIPP34,
OsHIPP60, and
OsHIPP16 in a yeast mutant showed that
OsHIPP34 can increase resistance to Cu,
OsHIPP60 can increase resistance to Zn, and
OsHIPP16 can increase resistance to Cd and Zn [
26]. Additionally, the
oshipp42 mutant grows more weakly than the wild-type under Cu, Zn, Cd, and Mn stresses [
26].
OsHIPP29 is upregulated by high Cd and Zn concentrations in the shoots and roots; the mutants and RNAi lines of
OsHIPP29 show decreased plant heights and dry biomass compared to the wild-type under Cd exposure [
32]. Some
HIPP genes may also be involved in other abiotic-stress responses.
HvFP1 from barley shows a complex expression pattern with induction under different abiotic stress conditions (e.g., cold, drought, and heavy-metal exposure) during leaf senescence and in response to abscisic acid [
28]. Like
HvFP1,
HIPP26 from
Arabidopsis can also be induced by cold, salt, and drought stresses [
33]. Evidence has accumulated for the critical role of HIPP in response to biotic stresses. In
Arabidopsis, AtHIPP3 acts as an upstream regulator of the salicylate-dependent pathway of pathogen response [
25]. HIPP27 is a host-susceptibility factor required for beet cyst nematode infection and development [
34]. In tobacco, NbHIPP26 interacts with TGB1 (the potato mop-top virus movement protein) to activate the drought-stress response and to facilitate the long-distance movement of the virus [
35]. In wheat, transient silencing of
TaHIPP1 enhances stripe-rust resistance, indicating that
TaHIPP1 acts as a negative regulator [
36].
Common wheat (
Triticum aestivum L.) is one of the most important food crops around the world, and the growth and yield of wheat are affected by various environmental factors, such as heavy metals [
37]. To date, some genes related to heavy metal tolerance have been reported in wheat, including
TaVP1 [
38],
TaGolS3 [
37],
TaEXPA2 [
39],
TaHMA2 [
40],
TaPCS1 [
41],
TaCNR2 [
42], and
TaPCS1 [
43]. In this study, genome-wide identification of the
HIPP gene family in
Triticeae species was performed by searching the published sequences. A phylogenetic tree of the
HIPP genes was constructed and the evolutionary relationships were analyzed. Chromosome distribution and protein structure were further studied to gain a better understanding of
HIPP genes in wheat. The potential functions of the common wheat
HIPP genes were predicted through their expression profiles based on in silico analysis.
Haynaldia villosa L. (2
n = 2
x = 14, VV) is a diploid wild relative of wheat. Previous studies have shown that
H. villosa is resistant to various wheat diseases such as stripe rust and powdery mildew, and also possesses the characteristics of resistance to drought, cold, and salt; therefore, it provides excellent material for wheat genetic improvement [
44,
45,
46]. As
H. villosa has not been sequenced, we cloned
H. villosa’s
HIPPs through homology-based cloning. One of the
HIPP genes,
HIPP1-V, was transformed into wheat to study its role in Cd tolerance. Our results will support the understanding of the evolution and diversification of
HIPPs in
Triticeae species at a genome-wide scale.