The function of hydrogen sulphide in iron availability: Sulfur nutrient or signaling molecule?

ABSTRACT

Hydrogen sulphide (H₂S) has traditionally been considered as a phytotoxin, having deleterious effects on the plant growth and survival. Recently, it was recongnized as a potential signaling molecule involving in physiological regulation similar to nitric oxide (NO) and carbon monoxide (CO) in plants. In a recent study, we mainly focused on the signaling function of H₂S in improving adaptation of Zea mays seedlings to iron deficiency. We reported that H₂S was closely related to iron uptake, transport, and accumulation, and consequently increased chlorophyll biosynthesis, chloroplast development, and photosynthesis in Z. mays seedlings. Here, we provide more commentary on the signaling roles of H₂S in coping with Fe deficiency in plants through increasing sulfur containing metabolites and regulating the expression level of iron homeostasis and sulfur metabolism-related genes in maize seedlings.

Iron (Fe) is an essential microelement for plants and all other living organisms, which is a component of a number of proteins and enzymes with functions in key metabolic process and Fe.¹ Despite being the fourth most abundant element in the earth's crust, Fe deficiency is one of the major limiting factors for crop production in calcareous soils all over the world.² Higher plants have two strategies for the uptake of Fe(III) from the rhizosphere. Strategy I plant species respond to lack of Fe by three steps including acidification of rhizosphere by an H⁺-ATPase, reduction of Fe (III) to Fe (II) by ferric-chelate reductase and uptake of Fe(II) by iron transporters in the roots.^3-5 In contrast, in Strategy II plants, iron acquisition includes biosynthesis of phytosiderophores (mugineic acids, MAs) inside the roots; secretion of phytosiderophores to the rhizosphere; solubilization of insoluble iron in soils by chelation of phytosiderophores; and uptake of the ferric-phytosiderophore complex by the roots.^6,7 However, strategies I and II are not sufficient to support the iron requirement for plant development when iron availability is under a threshold level, thus stress symptoms become evident in iron-deficient plants.

In the last few years, there has been a renewed interest in the effect of hydrogen sulphide (H₂S) on plant physiology.⁸ Literatures published from the last 30 y showed that this gas can affect the growth of plants, but more recent works suggested H₂S can act as a signaling molecule similar to nitric oxide (NO) and carbon monoxide (CO) in plants at low concentrations by participating in various biological process.^9,10 For instance, previous studies showed that H₂S promoted seed germination, alleviated oxidative damage, inhibited boron toxicity, salt toxicity, and aluminum toxicity and so on in plants.^11-13

H₂S is endogenously generated during the metabolism of L-cysteine by the catalysis of cystathionine β-synthase and cystathionine γ-lyase in plants.¹⁴ Besides, H₂S is thought to be released from cysteine via a reversible O-acetyl-L-serine(thiol)lyase (OAS-TL) reaction in plants.¹⁵ Moreover, the uptake of H₂S is largely dependent on its rate of metabolism into cysteine by OAS-TL and subsequent assimilation into other organic sulfur compounds.¹⁶ Therefore, H₂S as an important compound involved in plant sulfur metabolism. It is noteworthy that S supply could help plants cope with the Fe shortage.^17-20 For instance, Astolfi et al.,¹⁸ reported that barley exhibited a positive correlation between the S nutritional status and its capability of coping with Fe deficiency emerged. Moreover, one of the responses to Fe deficiency in strategy II plant is the extrusion of phytosiderophores in the root rhizosphere in order to chelate and solubilize Fe³⁺.^18,19 Phytosiderophores are derived from nicotianamine that is synthesized from three molcules of S-adenosyl-methionine, thus representing another possible junction between Fe and S metabolism. Under S deficiency condition the release of phytosiderophores was reduced.^19,21 However, it is not clear whether H₂S as sulfur compound or signaling molecule play a key role in response to Fe deficiency in plants?

In our recent published study, we presented compelling data that revealed a novel effect of H₂S on iron nutrition.²² In our experiment, the S content by exogenously applied H₂S was much lower than that of nutrition solution itself which was only 1%. However, a profound change in chlorophyll content, iron uptake, and iron homeostasis-related gene expression including S-adenosyl homocysteine nucleosidase (ZmMTN), nicotianamine synthase (ZmNAS1 and ZmNAS3), deoxymugineic acid synthase 1 (ZmDMAS1), transporter of MAs (ZmTOM2 and ZmTOM3), iron-regulated transporter (ZmIRT), iron binding protein (ZmIBP), ferric-chelate reductase (ZmFRO1), and yellow stripe 1 (ZmYS1) happened in iron-deficiency Z. mays seedlings when treated by exogenous H₂S (Fig. 1). Therefore, we concluded that H₂S as a signaling molecule played a vital role in improving adaptation of maize seedlings to iron deficiency rather than sulfur nutrition.

Figure 1.

Heat map of the transcripts of iron homeostasis-related genes and sulfur metabolism-related genes of maize seedling leaves. Maize seedlings were pre-treated with 100 µM NaHS for 8 d and then grown in a nutrient solution containing 1 µM Fe(III)-EDTA or 50 µM Fe(III)-EDTA for 12 d. Red color represents higher relative expression and blue color represents lower relative expression when compared with the control samples (−Fe). Scale is the log₂ of the mean concentration values after normalization (n = 4).

The supply of H₂S would directly feed into cysteine and glutathione biosynthesis. Many studies have reported that H₂S exposure generally results in an increased content of water-soluble non-protein thiol compounds including GSH and cysteine in shoot, particularly, in some species an increase of sulfate content in shoot has been observed.^16,23–25 In our study, a high accumulation of endogenous H₂S in maize seedling leaves and roots caused by exogenously applied NaHS was observed under –Fe (0.1 μM FeIII-EDTA) or +Fe (50 μM FeIII-EDTA) conditions. Meanwhile, NaHS treatment caused GSH and NPTs increase in roots and leaves under –Fe or +Fe conditions. Besides, H₂S also could regulate sulfur metabolism-related genes expression including sulfate transporter (ZmST1), sulfate reduction-related genes (ZmATPS and ZmAPR), O-acetyl-L-serine(thiol)lyase (ZmOASTL1 and ZmOASTL2), and cysteine desulfhydrase (ZmDES) (Fig. 1). These results indicated exogenously applied NaHS was not only directly feed into cysteine and glutathione biosynthesis by regulating sulfur metabolism-related enzymes activities and genes expression, but also increased the content of endogenous H₂S in plants.²²

Therefore, our results suggested that H₂S as a signaling molecule could cope with iron deficiency through increasing sulfur containing metabolites including GSH and NPTs and regulating the expression level of iron homeostasis and sulfur metabolism-related genes in maize seedlings. The detailed signaling pathway of H₂S-regulated iron assimilation need to further study.

Disclosure of potential conflicts of interest

No potential conflicts of interest were disclosed.

Funding

This study was financially supported by the Natural Science Foundation of China (NSFC No 31501822, 31570586, 31300505, 31260057), the Postdoctoral Science Foundation of China (2015M580876), the foundation of doctoral Fig. 1. Heat map of the transcripts of iron homeostasis-related genes and sulfur metabolism-related genes of maize seedling leaves. Maize seedlings were pre-treated with 100 mM NaHS for 8 d and then grown in a nutrient solution containing 1 mM Fe(III)-EDTA or 50 mM Fe(III)-EDTA for 12 d. Red color represents higher relative expression and blue color represents lower relative expression when compared with the control samples (iFe). Scale is the log2 of the mean concentration values after normalization (n D 4). 2 J. CHEN ET AL. scientific research in Northwest A&F University (Z109021409), and West Light PhD Project Foundation of the Chinese Academy of Sciences, Chinese Universities Scientific Fund (K318021510).