4.4 Is P metabolism an evolutionary and ecological feature in Tibetan wild barley for its adaptation to acid soil?
The major center of origin for barley is the Middle East which is mainly characterized by non-acid soils (Figure 10a,b). Most acid soils are often deficient in P (Kochian, 1995), thus it seems unlikely that plants would acquire an Al-tolerance mechanism that relies on releasing an essential and scarce macro-nutrient like P from their roots. However, Tibetan wild barley and domesticated Qingke barley are widely distributed and cultivated in the southeast regions of Tibet which has acidic soils with high total soil P (Figure 10b; Wang et al., 2008). It is worth noting that P concentration in root was positively correlated with Al tolerance, while Al concentration was negatively correlated with Al tolerance in 12 Tibetan wild barley accessions (Figure 10c,d). It is not unexpected that Tibetan wild barley has evolved unique mechanisms to adapt to these acidic and high P soil conditions. A plentiful supply of soil P facilitates the Al-induced Pi transport and metabolism in the Tibetan wild barley accession XZ29. We propose that Al tolerance in XZ29 relies on a new mechanism that high cytosolic Pi availability and Al-induced Pi efflux from root apexes (Figure 9). The phosphate release from roots was largely restricted to the elongation zone of root (Figure 7c), so its drain on plant nutrition could be minimal while protecting the root apices. Furthermore, Pi efflux from plant roots is a natural behavior in P homeostasis (Elliott, Lynch, & Lauchli, 1984; Cogliatti & Santa Maria, 1990), and other plant species well-supplied with P exhibit larger Pi efflux than plants deficient in P (Pettersson & Strid, 1989; Cogliatti & Santa Maria, 1990; Chen et al., 2012). Therefore, we conclude that Tibetan wild barley have developed an Al tolerance mechanism based on phosphate efflux from vacuole and phosphate release from the root apices which suited the low pH and high P soils on the Tibetan plateau.