Seagrasses are a group of monocotyledonous angiosperms that diverged from the terrestrial monocots about 130 MYA and subsequently adapted to completely submerged conditions of the marine environment (Janssen and Bremer, 2004). Eelgrass (Zostera marina L.) is an important aquatic weed found in the Atlantic and Pacific oceans as far as the Arctic circle. It provides habitat for several species of fish and invertebrates. Eelgrass improves water quality by absorbing pollutants, and prevents erosion by binding sediments (Moore, 2004). Considering its structural and functional role and importance in many coastal ecosystems, it was recently fully sequenced (Olsen et al., 2016). Genome analysis revealed loss and gain of multiple genes in Z. marina compared to terrestrial or floating aquatic plants, changes assumed to facilitate its adaptation to marine life (Olsen et al., 2016). These adaptations include morphological, physiological and breeding pattern modifications along with the ability to tolerate high salt levels of marine environments.

Notwithstanding the benefits it provides to marine ecosystems, worldwide estimation of eelgrass population suggests a 30% reduction over the past 30 years (Waycott et al., 2009). This is mostly associated with human disturbances (overfishing, eutrophication) and climate-change factors such as increased temperature, changes in mean sea level and biochemical composition of the sea water (Jarvis et al., 2014; Thom et al., 2014). Lower levels of silicon (Si) in marine water is also considered as one of the main reasons for reduced eelgrass population (Kamermans et al., 1999; Disney et al., 2014). In this context, a better understanding of the molecular mechanisms involved in the transport/acquisition of Si and other solutes in eelgrass could find relevance in explaining the alarming reduction in eelgrass populations.

The adverse effect of lower concentrations of dissolved Si on the growth of diatoms has been frequently reported. The diatoms are an important component of ecosystems, and changes in diatom populations substantially affect the marine food web (Ragueneau et al., 2006). Significant changes in the biogeochemistry and aquatic food webs of coastal marine environments have been observed with reduction in dissolved Si content in the Black and Baltic Seas (Humborg et al., 2000). A decreasing proportion of diatoms associated with an increase of flagellates was one of the interesting observations thought to explain the reduction in dissolved Si. The role of Si in aquatic ecosystems has been mostly studied with observational methods, so molecular experiments would contribute to a better definition of this role (Schoelynck and Struyf, 2016). More specifically, a better understanding of Si-transporting aquaporins in aquatic species would adequately support ecological experiments.

Aquaporins are small (21–34 kD) integral proteins, which form transmembrane channels to facilitate movement of water and many other solutes across the cell membrane. The topology of AQPs resembles that of an hourglass structure formed by six transmembranes (TM) α helices (H1 to H6) connected with five inter-helical loops (A to E). At the center of the pore formed by the six TM domains, two distinct constricts are formed, one with highly conserved NPA (Asn-Pro-Ala) motifs and another with four amino acid aromatic arginine (ar/R) region in the channel. These two constrictions determine solute permeability of the AQPs (Lee et al., 2005; Törnroth-Horsefield et al., 2006; Deshmukh et al., 2015).

Aquaporins are found in most living organisms including microbes, animals, and plants. However, AQP’s are comparatively more abundant and diverse in plants than in any other organisms. Based on sequence similarity, plant AQPs were grouped into five subfamilies: plasma membrane intrinsic proteins (PIPs), tonoplast intrinsic proteins (TIPs), nodulin 26-like intrinsic proteins (NIPs), small basic intrinsic proteins (SIPs) and uncategorized intrinsic protein (XIPs) (Chaumont et al., 2001; Johanson et al., 2001; Quigley et al., 2002; Kaldenhoff and Fischer, 2006). Among these, the PIP, TIP, and NIP subfamilies are well-characterized with regards to their localization and function. In addition to water, members of these subfamilies are known to transport urea (Li and Wang, 2014), lactic acid (Bienert et al., 2013), glycerol (Biela et al., 1999), metalloids like boron and silicon (Ampah-Korsah et al., 2016; Song et al., 2016; Ouellette et al., 2017), and gasses like ammonia (NH₃) (Jahn et al., 2004), carbon dioxide (CO₂) (Kaldenhoff et al., 2014) and hydrogen peroxide (H₂O₂) (Tian et al., 2016). All AQP subfamilies are widely distributed in different plant species including primitive land plants, with the exception of XIPs that are absent in some higher plants such as Brassicaceae and monocots (Gupta and Sankararamakrishnan, 2009; Deshmukh et al., 2015; Sonah et al., 2017). With the availability of complete genome sequences, the genes encoding AQPs have been characterized in many land plants such as Arabidopsis, rice, Populus, soybean, canola and tomato (Deshmukh et al., 2016a). Surprisingly enough, no systematic study of AQPs has been carried out in aquatic plant species. The recent availability of the annotated genome sequences of eelgrass provides an opportunity to investigate and compare the role of AQPs in plants adapted to aquatic environments.

In the present study, we have performed a genome-wide identification of AQPs in Z. marina. Subsequently, characterization of AQPs was conducted based on phylogenetic analysis, gene structure organization, conserved motifs, ar/R selectivity filters, and homology-based 3D protein structure. Finally, AQP expression profiling in different tissues was studied using available transcriptomic data.

Aquaporins have gained increased attention recently because of their reported pivotal role in biotic and abiotic stress tolerance (Hu et al., 2012; Liu et al., 2013; Shivaraj et al., 2017; Sonah et al., 2017). Considering the importance of eelgrass in marine environments, and its declining population associated with different stressful conditions, we were therefore interested in identifying all potential aquaporins in the species with the objective that it would become an invaluable resource to better understand its ecology and predicament.

Our genome-wide analyses led to the identification of 25 putative AQPs in Z. marina genome, a number inferior to reference land plants such as rice (34) (Sakurai et al., 2005; Nguyen et al., 2013) and Arabidopsis (35) (Johanson et al., 2001). Lineage of Z. marina diverged from the terrestrial monocots about 130 MYA. Its adaption to submerged conditions occasioned a loss and gain of multiple genes compared to terrestrial or floating aquatic plants (Janssen and Bremer, 2004; Olsen et al., 2016). The reduced number of AQPs in Z. marina compared to terrestrial plants represents likely an adaptive strategy during its evolution to a new environment. More specifically, the aquatic environment may call for a lesser need in proteins involved in water and solute acquisition through the roots.

Phylogenetic analysis of the Z. marina AQPs identified only four different sub-families, PIPs, TIPs, NIPs, and SIP, as observed in monocots and Brassicaceae, with the notable absence of XIP subfamily members. Interestingly, most of the dicots harbor five AQP subfamilies including XIPs (See Supplementary Table 5), which suggests a clear conservation of the absence of XIPs in monocots even following the divergence between terrestrial and aquatic plants. On the other hand, members of NIP2 (NIP-III), NIP3, TIP2, and TIP4 were not observed in Z. marina genome, while they are systematically found in monocots. The loss of the TIP subfamily is rather surprising since TIPs are relatively more conserved across diverse species compared to the rest of AQP subfamilies (Deshmukh et al., 2015). This likely means that TIPs have specific functions in terrestrial plants that are obsolete or redundant in eelgrass. Notwithstanding the differences in the members and number of AQPs in eelgrass, the gene structure and exon–intron organization were found to be quite similar compared to other terrestrial monocots (Danielson and Johanson, 2008). The conserved exon-intron organization is suggestive of functional redundancy across species. Since the exon–intron organization is known to be affected by gene duplication, diversification and changes in exon–intron organization may lead to changes in gene expression profile (Roy and Penny, 2007; Deshmukh et al., 2016b).

The substrate specificity of AQPs is determined by the size and hydrophobic nature of the amino acids forming the pores (Lee et al., 2005; Törnroth-Horsefield et al., 2006). All PIP family members from Z. marina contained a very hydrophilic ar/R selectivity filter, FHTR, a hallmark of water-transporting aquaporins, in contrast to other families. A similar ar/R selectivity filter is also observed in PIP family of aquaporins from other plant species such as O. sativa, A. thaliana, Brassica rapa, Glycine Max, and Ricinus communis (Johanson et al., 2001; Deshmukh et al., 2013; Zhang et al., 2013; Diehn et al., 2015; Zou et al., 2015). Among Z. marina TIP sub family members, TIP1s were found to have residues HIAV, YIGA, and HIAM forming a more hydrophobic ar/R filter compared to the one found in ZmTIP3 and ZmTIP5, which contained SIAR and QVGR residues, respectively. The residues present in ar/R selectivity filter of ZmTIPs were found to be similar to the TIPs from other plant species. TIPs act as functional water transporters and facilitate transport of small solutes such as NH₄⁺, H₂O₂, and urea (Liu et al., 2003; Holm et al., 2005; Bienert et al., 2007). Conserved NPA motifs and ar/R filter in TIPs suggests their involvement in transport of water as well as solutes in Z. marina.

Members of NIP-III subfamily are known to be involved in transport of metalloids like boron (Takano et al., 2006) and silicon (Ma et al., 2006). Several studies have reported beneficial effects of Si on monocotyledonous plants. Different plant species accumulate a wide range of Si, from 0.2% or less to 10% Si on a dry weight basis (Lux et al., 2002; Hodson et al., 2005; Kuliková and Lux, 2010; Grégoire et al., 2012; Deshmukh et al., 2015). Very few studies have reported Si levels in Z. marina leaves; Herman et al. (1996) found a range between 0.02 and 0.66%, with higher concentrations in plant associated with higher dissolved Si in water. Interestingly, a tight positive correlation was also observed between dissolved Si levels and Z. marina population (Herman et al., 1996; Disney et al., 2014). On the other hand, Kamermans et al. (1999) could not establish if increase in dissolved Si levels resulted in increased biomass of Z. marina. This is an important feature to clarify in light of the association of declining populations of eelgrass with Si concentrations. If lower Si is indeed responsible for this situation, one must be able to establish that eelgrass benefits from Si through its uptake. Indeed, in terrestrial plants, the benefits of Si are directly correlated with a plant’s ability to absorb silicic acid. Recent studies have clearly shown that uptake of Si is mediated by aquaporins, and more specifically by NIP-IIIs containing a GSGR selectivity filter (Deshmukh and Bélanger, 2015). In the present study, we found that NIP-IIIs were completely absent from the Z. marina genome, which would indicate that the species is unable to uptake Si, a conclusion supported by the low Si levels found in the tissue of eelgrass. Under these conditions, it is difficult to rationalize how dissolved Si levels in water would influence eelgrass populations. Better controlled studies are thus needed to determine the role of Si, if any, on eelgrass. In any event, results from this study, namely the absence of NIP-IIIs, offer new avenues to better understand the ecology of eelgrass in the context of Si fluxes in marine environments.

Eelgrass has been extensively studied to understand the plant physiology particularly under aquatic and high saline condition (Yu, 2015; Yang et al., 2017). However, very limited attention was paid to study the involvement of AQPs in physiological processes. In Posidonia oceanica (Seagrass), significant role of PIPs have been observed in salinity tolerance and maintaining water balance in the leaves (Serra et al., 2013). Similarly, expression profiling performed by Cozza and Pangaro (2009) also suggested the role of AQPs in salinity and water balance in seagrass. Apart from such few reports, no significant study evaluating physiological role of AQPs has been performed in aquatic plants.

Analysis of RNA-seq data revealed higher or similar expression of different aquaporins in vegetative compared to root tissues. In terrestrial plants, in addition to providing anchorage, roots play an important role in the absorption of water and nutrients from the soil. However, in submerged aquatic plants, vegetative parts are also actively involved in uptake of water and nutrient from the surrounding environment (Barko and Smart, 1986). PIPs are known to play a central role in transport of water; additionally PIPs are known to facilitate CO₂ diffusion in mesophyll tissue of A. thaliana and Nicotiana tabacum affecting photosynthesis (Flexas et al., 2006; Heckwolf et al., 2011). Our expression analysis showed higher expression of PIPs in different plant parts analyzed suggesting a possible role of PIPs in water transport and CO₂ diffusion in Z. marina. The members of TIP1 and TIP5 family showed higher expression specifically in male flowers. Different studies in A. thaliana have shown pollen specific accumulation of TIP family members (TIP1;3 and TIP5;1), which are expected to be involved in pollen maturation and germination (Soto et al., 2008; Wudick et al., 2014). Similar expression pattern of homologs of these TIP members in Z. marina indicates their conserved role in pollen development affecting reproduction.

Genome-wide analysis of AQPs performed in the first fully sequenced marine angiosperm, Z. marina revealed several salient features. It has shown loss of AQPs in Z. marina compared to reference land plants such as rice and Arabidopsis. The reduced number of AQPs in Z. marina compared to terrestrial plants represents likely an adaptive strategy during its evolution to a new environment. The Z. marina AQPs formed only four different sub-families as observed in monocots and Brassicaceae, with the notable absence of XIP subfamily members. We also found absence of NIP-III members from the Z. marina genome, which would indicate that the species is unable to uptake Si, a conclusion supported by the low Si levels reported in the tissue of eelgrass. The absence of NIP-IIIs in Z. marina, offer new avenues to understand the ecology of eelgrass in the context of the role of Si in marine environments. The higher expression for most of the AQPs in shoots compared to roots observed with RNA-seq data suggests, as one could expect, a predominant role of vegetative tissues in uptake of water and nutrient from the surrounding environment. The members of TIP1 and TIP5 family showed higher expression specifically in male flowers as observed in A. thaliana indicating their probable role in pollen development affecting reproduction. The identification, classification, and expression of AQPs performed in the present study will be helpful for enhancing our knowledge of distribution and evolution of AQPs in aquatic plant species.

SS, RD, and HS compiled the data, performed analysis, and wrote first draft of the MS. HS and JB performed expression analysis. RB planned the study, drew the conclusions and contributed to the writing of the manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.