Abstract
Aluminum (Al) toxicity in acid soils significantly affects plant growth,
agricultural productivity and ecosystem health. Here we investigated
plant Al tolerance from evolutionary physiological, molecular, and
ecological perspectives. Genetic similarity and phylogenetic analysis of
Al tolerance-associated gene families showed that many of these were
conserved from streptophyte algae to angiosperms, indicating land plants
have evolved gradually in
adaptation to Al-rich acid soil during plant terrestrialization. In
particular, vacuolar phosphate transporter SPX-major facility
superfamily (SPX-MFS) and inorganic phosphate (Pi) transporter 1
subfamily (PHT1s) of streptophyte algae showed higher genetic similarity
to land plants than chlorophyte algae. PHT1 subfamily exhibited a
significant expand during the evolution from streptophyte algae to
liverworts and then to eudicots. Moreover, we identified an Al-tolerant
Tibetan wild barley accession XZ29, showing high levels of
Phosphorus(P)-containing glycolytic intermediates under Al stress. We
found a new Al-tolerance mechanism that Al-induced Pi efflux from root
elongation zone to chelate rhizosphere Al3+ and
immobilization of Al with P reduce Al accumulation in barley root cells.
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.
Key words: acid soil, gene family evolution, Hordeum
spontaneum , phylogenetic analysis, phosphate metabolism, phosphate
transporters 1. Introduction
Aluminum (Al) is the most abundant metal in Earth’s crust and Al
toxicity is a major factor limiting crop production especially in acid
soil, which cover almost 40% of the world’s arable land (Kochian,
Pineros, Liu, & Magalhaes, 2015). Phosphorus (P) is an important
macro-nutrition for all living organisms, and functions as a substrate
in many metabolic processes such as glycolysis. P deficiency in soil
causes significant stress to plants and limiting global food production
(Runge-Metzger, 1995). Inorganic phosphate (Pi) can bind with Al to
format Al-P complex (Zheng et al., 2005). P availability is dramatically
reduced in acid soils because it forms sparingly soluble complexes with
Al, thus P deficiency and Al toxicity usually coexist in acid soils
(Kochian, Hoekenga, & Pineros, 2004). Soil P and Al have significant
ecological and evolutionary implication for the evolution of plant
nutrient uptake and crop domestication for the adaptation to acid soils
(Liang et al., 2013).
Despite the importance of crop Al tolerance for sustainable food
production and human health, there is limited investigation on the
evolution of plant Al tolerance. Streptophyte algae, the sister lineage
of land plants, have gradually evolved and equipped with new innovations
in gene families for the adaptation of green plants to terrestrial
habitats, including Al-rich acidic soils (Taylor et al., 2000; Nishiyama
et al. 2018; Leebens-Mack et al. 2019; Zhao et al. 2019; Wang et al.,
2020). Herburger, Remias, & Holzinger (2016) reported that a
streptophyte alga Zygogonium ericetorum (Zygnematophyceae,
Charophyta) has evolved the capability of Al tolerance by binding Al
with pectin-rich parental cell wall matrix, thereby reducing Al
accumulation in unpigmented filaments. It is thus suggested that
acquisition of Al tolerance by modification of cell wall components
(Yang et al., 2011; Zhu et al., 2012) may be originated from
streptophyte algae. On the contrary, Al-induced organic acids secretion
was mostly reported in seed plants, but not in lower plants (Yang, Fan,
& Zheng, 2019) while the vital roles of P in plant nutrition and stress
tolerance have been conserved across green plants (Lambers et al., 2015;
Prodhan, Finnegan, & Lambers, 2019). Moreover, key P metabolism, Pi
transporters SPX-MFSs (Syg1/Pho81/XPR1 with Major Facilitator
Superfamily) and PHT1s (phosphate transporter 1 family) have been
implicated to function in plants Al tolerance (Cardoso, Pinto, & Paiva,
2019; Rae, Cybinski, Jarmey, & Smith, 2003; Wang et al., 2012; Wang et
al., 2015). However, the evolutionary origin of P metabolism and Pi
transporters and their roles in Al tolerance is still elusive.
Micro-molar concentrations of Al3+ can rapidly inhibit
root elongation, leading to reduction of water and nutrient uptake, and
biomass production and yield reduction (Jones, Blancaflor, Kochian, &
Gilroy, 2006). Some of the key mechanisms conferring this tolerance
include the Al-induced release of organic anions from roots (Ma, Ryan,
& Delhaize, 2001; Sasaki et al., 2004; Wang et al., 2007), the
modification of cell wall chemistry to reduce accumulation in the
apoplast (Yang et al., 2011; Zhu et al., 2012), and phytohormone
mediated inhibition of root growth under Al stress (Yang et al., 2014,
Yang et al., 2017). Application of phosphate alleviated Al toxicity by
reducing Al3+ availability and stimulating Al
tolerance genes (Jiang, Tang, Zheng, Lie, & Chen, 2009; Liang et al.,
2013; Sun, Shen, Zhao, Chen, & Dong, 2008). Immobilization of Al by P
in the cell wall conferred higher Al tolerance in buckwheat (Zheng et
al., 2005). Although the interactions between P and Al in plants have
been found (Chen et al., 2012; Du et al., 2009; Liang et al., 2013), the
underlying mechanisms are still not fully understood.
The center of barley origin and domestication is in the Fertile Crescent
(Nevo, Baum, Beiles, & Johnson, 1998; Dai et al., 2012), where soils
are largely saline with higher pH (IGBP-DIS, 1998). Wild barley in Tibet
(Hordeum spontaneum ) is well adapted to the harsh conditions of
the Tibetan plateau, where large areas of the soil is acidic and
contains high P content (IGBP-DIS, 1998; Wang, Yang, & Ma, 2008).
Tibetan wild barley is one of the progenitors of the modern cultivated
barley, possessing a wider genetic diversity and a larger variation to
abiotic stress tolerance than cultivated barley (Dai et al., 2012; Dai
et al., 2014; Wu et al., 2013; Feng et al., 2020). By screening more
than one hundred wild barley genotypes, we identified accessions with
similar tolerance to Al toxicity as Dayton, a well-known Al-tolerant
cultivar (Cai et al., 2013; Dai et al., 2011). Two novel loci bpb-9458
(Chromosome 2H) and bpb-8524 (Chromosome 7H) were linked with Al
tolerance in Tibetan wild barley (Cai et al., 2013). These results
suggested that the mechanisms controlling Al tolerance are likely to be
different between Tibetan wild barley and modern barley cultivars.
It was thus hypothesized that unique P metabolism and transport
facilitate Tibetan wild barley’s adaptation to low pH and high P soil
during the evolution. In this study, we have combined evolutionary
bioinformatics (Zhao et al., 2019), metabolomics (Urano, Kurihara, Seki,
& Shinozaki, 2010), confocal microscopy (Klug, Specht, & Horst, 2011),
secondary ion mass spectrometry (SIMS) (Kopittke et al., 2015), and
electrophysiology (Newman, 2001) to investigate the Al tolerance
mechanisms in Tibetan wild barley. We found that most of Al
tolerance-related genes, including genes encoding Pi transporters and
glycolysis, were conserved from streptophyte algae to angiosperms.
Tibetan wild barley has developed an Al tolerance mechanism based on
P-related metabolites and gene expression and phosphate efflux from the
root apices to chelate toxic Al3+.