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).