PI Efflux and Influx by P-Adequate and P-Deficient Spirodela and Lemna

Abstract

Net Pi influx by P-adequate ( + P ) and P-deficient (-P) Spirodela and Lemna plants was determined by gross Pi influx and Pi efflux. Rates of Pi influx (gross) and Pi efflux were sensitive to both internal [PI and external [Pi]. The different responses of these two fluxes to external [Pi] resulted in their rates converging at low external [Pi], from Pi efflux being 9-10% of gross Pi influx at 1000 p~ Pi to being over 70% at low (1 p~) external Pi in + P plants. At very low external Pi (0.34 p~, 0.22 p~ Pi for + P Spirodela and Lemna, respectively), rates of gross Pi influx and Pi efflux were assumed to be either equal or zero (referred to as the equilibrium [Pilext), such that net Pi influx was zero. There was assumed to be insufficient Pi diffusing to the carrier site on the root surface at very low external [Pi] (i.e. equilibrium [P,],,,) for the phosphate pump to operate and Pi influx was zero. P deficiency resulted in a simultaneous reduction in rates of Pi efflux and an increase in gross Pi influx such that the equilibrium [Pilext was reduced 3-4 times (0.8, 0.07 p~ Pi for -P Spirodela and Lemna respectively) compared with + P plants. Some of the limitations of Pi diffusion to the carrier sites in P-adequate plants are overcome as internal [PI decreases during P deficiency. As a consequence the ability of Spirodela and Lemna to extract Pi from solution during P deficiency was considerably greater than for plants with adequate P status.Inhibitors of metabolism (low temperatures, arsenate, azide, cyanide and CCCP) reduced rates of gross Pi influx but increased rates of Pi efflux such that net Pi influx was reduced. These findings are discussed in relation to previous work on the kinetics of Pi influx in -P versus + P plants and the role of changes in membrane potentials in the enhancement of Pi influx and reduction of Pi efflux capacity in P-deficient Spirodela and Lemna.

Uptake and translocation of phosphate by pho2 mutant and wild-type seedlings of Arabidopsis thaliana

“…These studies suggest that the shoots can act as a sink to regulate Pi uptake by roots through enhancement of Pi¯ux to the xylem (Drew and Saker 1984). The capacity of plant roots to take up Pi increases when plants are deprived of P and is repressed when the internal concentration recovers (Clarkson and Scattergood 1982;Cogliatti and Clarkson 1983;McPharlin and Bieleski 1989). Under P-sucient conditions, pho2 has a greater uptake rate than wild-type plants and accumulates Pi to shoots beyond requirements.…”

“…The uptake rate is enhanced severalfold by P de®ciency, and there is evidence that a high-anity Pi transporter is induced in root and leaf cells (Drew and Saker 1984;McPharlin and Bieleski 1989;Mimura et al 1990). Similar responses have been observed in Escherichia coli and fungi (Burns and Beever 1977;Tamai et al 1985;Torriani-Gorini 1987) where P-de®ciency stress induces activation of a P-regulation system known as the pho regulon.…”

Moderating mycorrhizas: arbuscular mycorrhizas modify rhizosphere chemistry and maintain plant phosphorus status within narrow boundaries

“…Klein Atalaysa) was grown in a solution with a high P concentration (5 m m ) as compared with that in plants grown at 0.05 m m P. Other studies have also shown efflux to be higher at high P supply and in plants with a high P status (Elliott et al . ; Schjørring & Jensen ; McPharlin & Bieleski ). Alternatively, the elevated rhizosphere Colwell P concentration might be due to efflux of P from the mycorrhizal hyphae in close proximity to the root surface.…”

Molecular characterisation of an S-like RNase of Nicotiana alata that is induced by phosphate starvation

“…This system involves an enhanced capacity to absorb Pi from the environment [16,36] as well as the synthesis of enzymes such as acid phosphatases [13,18,48] that could release Pi from organic sources in the environment or reallocate Pi resources in internal pools. RNA is the most abundant form of organic phosphate in plant cells but has a comparatively low rate of turnover [8].…”