Phosphorus deficiency reduces forage yield and stand persistence of alfalfa (Medicago sativa L.). Our objectives were to isolate and characterize a high-affinity phosphate-transporter (P-transporter) from alfalfa roots (Medicago sativa L.); determine how phosphorus (P) nutrition impacts P-uptake, growth, and carbohydrate and protein metabolism of alfalfa cells; and learn how expression of the P-transporter is influenced by P nutrition. An 1087-base pair (bp) sequence was isolated using RT-PCR that possessed high nucleotide and amino acid sequence similarity to high-affinity P-transporters. Cultured cells were sampled at 3-day intervals for 9 days while growing in media containing P concentrations ranging from 0 to 10 mM. Media P concentrations declined rapidly in all P treatments by day 6. Low media P concentrations (0, 0.1 and 0.5 mM) reduced cell growth rates compared to higher media P levels (2.5, 5 and 10 mM). Suspension cell cultures supplied 0.5, 2.5, 5, and 10 mM P had lower starch concentrations by day 3 compared to cells cultured in media containing 0 and 0.1 mM P. Steady-state transcript levels for the high-affinity P-transporter were high in P-deprived cells, but declined within 1 day when cells were provided 10 mM P.Abbreviations: 2,4-D D -2,4-dichlorophenoxyacetic acid; MS -Murashige and Skoog; P -phosphorus; P-transporter -phosphate-transporter
Phosphorus (P) deficiency reduces forage yield and stand persistence of alfalfa (Medicago sativa L.). The objective of this study was to determine the influence of P nutrition and defoliation on alfalfa shoot growth, root carbohydrate and protein metabolism, and steady-state mRNA levels for high-affinity P transporters. In a greenhouse study, P deprived plants were provided with 0, 0.25, 2, and 6 mM P beginning 7 d before shoot removal. Plants were sampled immediately (day-7) on days-5,-2, 0 (day of shoot removal), and on days 1, 2, 6, and 9 post-shoot removal. Addition of P to P-deficient plants stimulated growth of shoots but not roots. Taproot bark sugar concentrations were reduced significantly in cut plants at any rate of P, whereas only the 6 mM P treatment reduced taproot wood sugar concentrations in uncut plants. There was a significant defoliation-induced decline in both wood and bark sugar and amino-acid concentration that was enhanced at high P rates. Low P reduced utilization of starch and protein reserves in taproots. Transcripts for a high-affinity P transporter were not detected in any root or shoot tissue assayed, irrespective of defoliation or P treatment. The uncertain relationship between P availability and P-transporter transcript abundance in our greenhouse-grown plants requires additional investigation.
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