4.1 Evolution of Al tolerance mechanisms in green plants
Al comprises of 5% of mass in earth’s crust, and it is the most common metallic element in the soil (Sposito, 2008). When and how did the green plants become adapted to Al-rich soil during plant terrestrialization? To answer this question, we conducted evolutionary and phylogenetic analysis of Al tolerance-associated predicted protein families to identify the origin of the Al-tolerance pathway. Many of these gene families were originated from streptophyte algae (Figure 1). Consistent with the recent work, it showed that the gene sequence identity and genetic similarity between streptophyte algae and land plants were significantly higher than those between streptophyte algae and chlorophyte algae (Liang et al., 2020; Zhao et al., 2019). It is proposed that streptophyte algae have generated key genetic innovations for the evolution of land plants in adaptation to terrestrial environment (Liang et al., 2020). In this study, the widely reported interactions between P and Al (Jiang et al., 2009; Liang et al.,2013; Sun et al., 2008; Zheng et al., 2005) led to the hypothesis that Pi homeostasis is crucial for plant Al tolerance. The high genetic similarity of many predicted protein families and large number of PHT1s and SPX-MFSs (Figures 2, S3-5) between streptophyte algae and bryophytes (hornwort/moss/liverwort) strongly suggested their potential significance during plant terrestrialization. Hornworts, liverworts and mosses, as three early diverging clades of land plants, occur in nearly all terrestrial habitats on all continents as pioneers in harsh environments such as acid soils (Blankenship, Condon, & Pyke, 2019; Bowman et al., 2017; Zhang et al., 2020). Highly conserved glycolysis in land plants and streptophyte algae may have secured the energy to cope with Al toxicity, but this process was limited by Pi availability (Figures 4, 5). These results provided first evidence for a conserved role of Pi homeostasis in land plants in adaptation to Al-rich acidic soil.
However, it does not mean that these proteins have conserved function between streptophyte algae and land plants or even between different plant species of the same clade in response to Al stress. For example, wheat TaALMT1 mediates the Al induced malate secretion in T. aestivum under Al stress (Sasaki et al., 2004), while the close homolog HvALMT1 in barley has no function in Al tolerance (Gruber et al., 2010). This is partially explained by the relatively low amino acid similarity in ALMTs family (Figures 1, S1; Table S3). Therefore, it was suggested that Al-induced secretion of organic acids (e.g. malate, citrate and oxalate) have gone through independent convergent evolution among different plant species (Ryan & Delhaize, 2010). Moreover, cell wall functions as the first barrier for Al entry into the cytosol (Jones et al., 2006; Wu, Shen, Yokawa, & Baluska, 2014), and PMEs and XTHs are key enzymes of cell wall modification in response to Al stress (Yang et al., 2008, Zhu et al., 2012). Our phylogenetic analysis indicated that these two protein families are originated from streptophyte algae (Figure 1). This is supported by the evidence that the evolvement of pectin-rich parental cell wall in streptophyte algae Zygogonium ericetorum helps its adaptation to acidic and high Al terrestrial habitats (Herburger et al., 2016).