Aluminum (Al) toxicity in acid soil significantly reduces plant growth, agricultural productivity and ecosystem health. The Al-tolerant barley cultivars were reported to mainly rely on the Al-activated efflux of citrate from root apices, but the key mechanisms for Al tolerance may differ for wild relatives of barley adapted to acid soil.
Here, we investigated plant Al tolerance from evolutionary physiological, molecular, and ecological perspectives.
Phylogenetic analysis of Al tolerance-associated gene families showed that most of these genes were conserved from streptophyte algae to angiosperms, indicating land plants have evolved gradually in adaption to Al-rich acid soil during plant terrestrialization. Vacuolar phosphate transporter SPX-major facility superfamily (SPX-MFS) and inorganic phosphate transporter 1 family (PHT1s) of streptophyte algae showed high genetic similarity to land plants. PHT1s exhibited a significant expand during the evolution from streptophyte algae to liverworts and then eudicots. Al-tolerant Tibetan wild barley accession, XZ29 showed high levels of P-containing glycolytic intermediates including Glu-6-P, Fru-6-P, 3-PGA, 2-PGA and PEP under Al stress. Some primary metabolites were evolutionarily conserved in liverwort, gymnosperm and three tested angiosperms. Furthermore, we found that Al-induced Pi efflux from root elongation zone to chelate rhizosphere Al3+, and immobilization of Al with P at the inner epidermal layer of root mature zone to reduce Al accumulation in the cortical layer in barley.
These results indicated that Tibetan wild barley has evolved unique P transport and metabolism for the adaptation to harsh conditions in eastern and southeastern Tibet where acid soils contain high P.