- Phloem (1) (remove)
- Phosphate nutrition in the Ricinus communis L. seedling (2007)
- Phosphate (Pi) is one of the essential macronutrients required for growth and development of plants. Pi plays an important role in various metabolic processes, such as photosynthesis, respiration, energy conservation, carbohydrate metabolism and signal transduction. Although various Pi starvation induced genes have been isolated from different plant species grown under conditions of Pi starvation, information about their functions during germination and growth of seedlings is still lacking. During germination Pi stored in the endosperm is mobilized and transported to growing organs of seedlings, thus a phosphate transporters and acid phosphatases are expected to be involved in these processes. The aim of this study was to clarify the translocation of Pi within the seedlings and to identify the involvement of phosphate transporters and acid phosphatases in the growth of seedlings. Uptake into the phloem was analyzed by incubating the cotyledons in Pi. The movement of 32P-labeled applied as an inorganic phosphate (Pi) was detected from the cotyledons to the hypocotyl, in particular to its apical hook near the cotyledons, suggesting that Pi moves from the Ricinus communis L. cotyledons through the hypocotyl via phloem and partially re-circulates in the xylem or leaks out through the roots. Therefore reducing the efflux could be as important for the plant as increasing the efficiency of the uptake mechanism. Following the Pi uptake into the roots, the translocation of 32P-labeled Pi to the cotyledons through the hypocotyl via the xylem showed that a high amount of radiotracer accumulated in the cotyledons. The accumulated Pi in the cotyledons can be retranslocated to the roots via phloem. This work describes the cloning of the phosphate transporter RcPT1 and the acid phosphatase RcPS1 genes by RT-PCR from Ricinus seedlings grown under Pi starvation conditions. Phosphate transporter RcPT1 contains an open reading frame encoding a 530 amino acid polypeptide with a calculated molecular mass of 59 kD. The expression of RcPT1 in the yeast high-affinity phosphate transporter mutant strain complemented the mutant and enhanced the cell growth significantly. Southern blot analysis showed that the RcPT1 gene is present as a single or low-copy gene in the Ricinus genome. The transcripts of RcPT1 were expressed in the endosperm, cotyledons, hypocotyl and roots during germination. In detail in situ hybridization studies revealed RcPT1 expression in the adjacent area of endosperm to cotyledon, in the phloem and in the lower epidermis of cotyledons; Immunolocalization analysis showed RcPT1 accumulation at the same sites as its mRNA. In addition, RcPT1 transcripts were also found in the phloem of hypocotyl, and the epidermis and the steles of roots. These results implicated that RcPT1 is involved in the movement of Pi from endosperms to cotyledons and the redistribution of Pi within seedlings via phloem during germination. Acid phosphatase RcPS1 shows a 747 bp open reading frame encoding a 248 amino acids polypeptide with a calculated molecular mass of 27,5 kD. The amino acid sequence of RcPS1 shares significant similarity with the acid phosphatase LePS2 from tomato and highly conserved motifs, which are typical for a member of haloacid dehalogenase and DDDD superfamilies of enzymes catalyzing a diverse number of hydrolytic and phosphotransferase reactions. The functional analysis after expression of RcPS1 in E.coli showed significant acid phosphatase activity. The high transcript level of RcPS1 in endosperms, cotyledons and roots at the first few days of germination suggested that this acid phosphatase gene might be expressed during mobilization of storage products. RcPT1 and RcPS1 mRNA are detectable in the seedlings grown under Pi starvation and Pi sufficient conditions, indicating that both genes were expressed independently from exogenous Pi supply during germination. Moreover, RcPT1 and RcPS1 were expressed in leaves, stems and roots of plants grown under Pi starvation; furthermore, in situ hybridization studies localized RcPT1 and RcPS1 mRNA in the epidermis and the stele of roots of Pi-starved plants, suggesting that these genes also play a role in response to Pi starvation. Thus, it is concluded that there are different signals regulating RcPT1 and RcPS1 expression in response to Pi starvation and during germination.