INTRODUCTION
Phosphorus (P) is essential for plant growth and development. Low
P-availability limits plant growth in most soils because soluble,
plant-available inorganic phosphate (Pi) readily forms insoluble
complexes with Fe3+ and Al3+ in
acidic soils and with Ca2+ in alkaline soils
(Hinsinger, 2001; Bertrand et al., 2003). Plants have evolved a variety
of developmental, physiological, molecular, and symbiotic strategies to
optimize acquisition and utilization of P for growth (Lambers et al.,
2011; O’Rourke et al., 2013; Lambers et al., 2015a). Traits associated
with adaptation and acclimation to P-limitation include: remodeling of
root architecture and development of more and longer root hairs and
lateral roots (Lynch, 2011; Lambers et al., 2015b); increased root to
shoot ratio (Lynch, 1995); induction of phosphate scavenging and
recycling enzymes (Ding et al., 2016); release of carboxylates (Tomasi
et al., 2009); P homeostasis by adjustment of major P-pools (Veneklaas
et al., 2012); and engagement of specific signal transduction pathways
and transcriptional regulatory networks (Misson et al., 2005; Morcuende
et al., 2007; Secco et al., 2013). Qualitative and quantitative
differences in P-strategies exist between and within species (Lambers et
al., 2015b; Pang et al., 2018). Therefore, in order to optimize P
efficiency (uptake and utilization) in specific crop species, it is
important to investigate the responses and adaptations to P-limitation
and the underlying mechanisms in that species and the natural variation
that is available for breeding.
Switchgrass, native to the North American tallgrass prairies, is a
perennial plant with water-efficient C4 photosynthesis that was targeted
for development as a bioenergy crop (Casler et al., 2011; Meyer et al.,
2014). Switchgrass exhibits high biomass production potential,
relatively low input requirements, and is adapted to much of the eastern
half of the USA, including areas considered marginal for food-crop
production (Casler et al., 2011; Gopalakrishnan et al., 2011; King et
al., 2013; Meyer et al., 2014). Some research suggests that
biomass-to-energy schemes using marginal lands would provide substantial
ecosystem services, particularly in terms of carbon sequestration and
other environmental benefits (Bhardwaj et al., 2011; Gelfand et al.,
2013). Data on switchgrass production on marginal sites are limited.
Previous research has shown that switchgrass biomass yields respond to
nitrogen fertilizer rates of up to 168 kg ha-1,
depending on ecotype and location (Sanderson et al., 1999; Muir et al.,
2001; Guretzky et al., 2011). In soils with low plant-available P,
application of 45 kg P ha-1 increased biomass yield by
up to 17% (Kering et al., 2012).
Switchgrass has been subjected to a genome sequencing effort (Casler et
al., 2011) as well as transcriptome analyses, using Expressed Sequence
Tags (ESTs), Affymetrix oligonucleotides arrays, and RNA-seq (Sharma et
al., 2012; Zhang et al., 2013; Meyer et al., 2014; Yang et al., 2016).
Transcriptome analyses have identified thousands of genes associated
with drought stress (Meyer et al., 2014) and leaf senescence in
switchgrass (Yang et al., 2016), but transcriptional responses to P
limitation have not been reported. Likewise, metabolic responses of
switchgrass to P deficiency remain unknown, although advanced
technologies are available (Sanchez et al., 2008; Luo et al., 2017). We
characterized the physiological and developmental responses of
switchgrass to P-limitation and explored underlying transcriptional and
metabolic responses in shoots and roots. Results and insights are
presented here.