Altered nutrient homeostasis under P-stress
Pi is the main source of P absorbed by and distributed within plants.
Internal Pi concentration is a good indicator of P nutrition status of
plants (Kanno et al., 2016). As expected, Pi concentrations decreased
progressively in both roots and shoots with decreasing Pi availability
in the soil (Figure 1 ). Interestingly, and unlikeArabidopsis thaliana and other annual plant species such asOryza sativa , Medicago truncatula , Brachypodium
distachyon, Setaria viridis , and soybean where long-term (3 week)
P-stress reduced Pi concentrations in shoots by about 90% (Morcuende et
al., 2007; Secco et al., 2013; Mo et al., 2019; Pandey-Pant & Scheible,
personal communication), Pi levels in shoots of severely P-deprived
switchgrass remained relatively high (~ 30%) after 3
weeks of P-stress, compared to P-replete plants. In contrast, Pi levels
in roots of severely P-deprived switchgrass were very low, at around
10% of P-replete levels (Figure 1 ). Apparently, severely
P-stressed switchgrass invests almost all the Pi retained in roots to
root growth and maintenance, presumably to maximize P-acquisition from
the soil, while holding significant reserves of Pi in shoots for other
reasons. Although it seems odd that switchgrass shoots should retain
substantial pools of Pi when P availability is severely growth-limiting,
such reserves may increase the fitness of plants by providing a source
of readily-mobilizable P for seed production should P availability not
increase prior to reproductive development. Alternatively, maintenance
of a significant Pi reservoir during P-stress may facilitate P
translocation to the root crown, during shoot senescence, for storage
until regrowth of this perennial in the following year. Interestingly,
in maize, the ability to maintain a relatively high shoot Pi
concentration during a long-term P-stress has been associated with a
higher rate of photosynthesis and better growth (Zhang et al., 2016).
P-stress affected levels of other nutrients in switchgrass in different
ways. Nitrate and sulfate levels in shoots and/or roots increased during
P-stress, presumably reflecting decreased assimilation of N and S
concomitant with slower plant growth. In contrast, chloride and malate
levels tended to decrease under P-stress, possibly in response to the
increasing levels of nitrate in order to maintain charge balance.
Likewise, the observed decline in K and Na levels in shoots and/or roots
during Pi deprivation may have reflected the need to preserve charge
balance within the plant (Amtmann et al., 2005). Ultimately, changes in
the content of inorganic ions like K+ and
Cl- that are not metabolized, must reflect changes in
the activities of myriad transporters at the root surface under
P-limitation. The utilization and uptake of other macro- and
micronutrients was also altered upon Pi stress in Arabidopsis (Misson et
al., 2005; Rouached et al., 2011). Such interaction of Pi with other
nutrients indicate the importance of Pi in numerous energy requiring
metabolic and transport processes (Plaxton & Carswell, 1999).