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.