Vacuoles are reservoirs of many metabolites, such as inorganic substances, organic acids, amino acids, and sugar in seeds and nutrient tissues [
61,
62]. Special cells in these organs accumulate proteins primarily as amino acid stores. The most common storage proteins are globulin, found in embryos, and glutenin, specific to cereal endosperm [
63,
64]. The results show that prolamins accumulation in the ER is an important step for the subsequent accumulation in storage vacuoles [
63].
Many reports indicate that toxic pollutants in the environment have a serious negative impact on a variety of organisms [
65]. Metals and metalloid pollutants come from natural or anthropogenic factors [
66]. One study focused on the tolerance and accumulation mechanism of the heavy metals cadmium (Cd) and arsenic (As) and discussed how to use the knowledge collected on this subject to develop pollution-free crops and utilize phytoremediation [
67]. Iron (Fe) is an essential micronutrient for both plant growth and human health. It can be used as a necessary cofactor for the electron transport chain of cell redox reactions involved in DNA biosynthesis, respiration, photosynthesis, and other reactions [
68,
69]. Iron deficiency leads to yellowing and growth retardation of young leaves, which leads to a decrease in photosynthetic efficiency and crop productivity [
70]. Micronutrient malnutrition undermines the health and well-being of women and pre-school children in particular [
71]. Conversely, excessive iron can cause severe dysfunction and cell damage, which is harmful to cells and organisms [
72]. Metal contamination and toxicity in soil limits food production. Scientific research shows that biofortification is a way to solve hidden hunger by increasing the dietary iron content in staple food crops [
73].
The storage of iron in seeds is a good example of plant vacuole storage of different heavy metals. At least 95% of the iron in
Arabidopsis seeds is stored in vacuoles. However, in other seeds (such as
P. sativum), the iron content of the vacuole is very low [
74,
75,
76]. Iron is a rich element in most soils, but its solubility is low in aerobic environments, especially in alkaline calcareous soils [
77]. Plants use two different strategies to absorb iron. In strategy I, plants, including dicots (such as cassava and
Arabidopsis) and non-grass monocots, rely on the following processes: (1) plants secrete protons into the rhizosphere to reduce the pH of the soil, thereby increasing the solubility of ferric iron complexes (Fe
3+); (2) the root protein ferric chelate reductase (FRO2) reduces Fe
3+ into the more soluble Fe
2+ (ferrous ion) on the root surface [
78]; (3) the iron-regulated transporter 1 (IRT1)-type ferric transporter, of the zinc-iron transporter (ZIP) family, moves Fe
2+ onto the cortical membrane of the root epidermal plasma membrane; (4) flavins are secreted to further facilitate the solubilization of ferric iron [
79]. Under the condition of iron deficiency, the four mechanisms are upregulated in the root system. Incredibly, graminaceous plants have another unique strategy for absorbing iron, i.e. strategy II. This strategy has been described as a “chelation” strategy, similar to that used by many bacteria and fungi [
80], and may result from adaptation to alkaline soils [
81]. The plant secretes phytosiderophore (PS) into the rhizosphere to form the chelating complex Fe
3+–PS, which is then absorbed into root cells by the yellow stripe 1 (YS1) transporter [
82,
83]. Iron is then transported from the roots through the xylem to the shoots, such as branches, leaves, and seeds, for use [
69]. To reach its final destination, iron must also be transported to the appropriate cell compartments for use (
Figure 2).