Ethylene is involved in root phosphorus remobilization in rice (Oryza sativa) by regulating cell-wall pectin and enhancing phosphate translocation to shoots

Abstract

Background and aims Plants are able to grow under phosphorus (P)-deficient conditions by coordinating Pi acquisition, translocation from roots to shoots and remobilization within the plant. Previous reports have demonstrated that cell-wall pectin contributes greatly to rice cell-wall Pi re-utilization under P-deficient conditions, but whether other factors such as ethylene also affect the pectin-remobilizing capacity remains unclear. Methods Two rice cultivars, 'Nipponbare' (Nip) and 'Kasalath' (Kas) were cultured in the +P (complete nutrient solution), -P (withdrawing P from the complete nutrient solution), +P+ACC (1-amino-cyclopropane-1-carboxylic acid, an ethylene precursor, adding 1 μm ACC to the complete nutrient solution) and -P+ACC (adding 1 μm ACC to -P nutrient solution) nutrient solutions for 7 d. Key Results After 7 d -P treatment, there was clearly more soluble P in Nip root and shoot, accompanied by additional production of ethylene in Nip root compared with Kas. Under P-deficient conditions, addition of ACC significantly increased the cell-wall pectin content and decreased cell-wall retained P, and thus more soluble P was released to the root and translocated to the shoot, which was mediated by the expression of the P deficiency-responsive gene OsPT2, which also strongly induced by ACC treatment under both P-sufficient and P-deficient conditions. Conclusions Ethylene positively regulates pectin content and expression of OsPT2, which ultimately makes more P available by facilitating the solubilization of P fixed in the cell wall and its translocation to the shoot.

Keywords: Rice; cell-wall polysaccharides; ethylene; gene expression; pectin; phosphorus; remobilization; transport.

Fig. 1.

Soluble Pi content and root ethylene production in rice cultivars Nip and Kas. Seedlings (after germination) were subjected to P-deficient/sufficient nutrient solution for 1 week, and the root (A) and shoot (B) soluble Pi contents and root ethylene production (C) were measured. Data are means ± s.d. (n = 4). Columns with different letters are significantly different at P < 0·05.

Fig. 2.

Effect of P deficiency on expression of genes for ethylene biosynthesis enzymes. Seedlings were transferred to P-depleted (−P) or P-supplied (+P) nutrient solution for 1 week. After treatment, total root RNA was subjected to qRT-PCR using gene-specific primers. Data are means ± s.d. (n = 4). Asterisks indicate a significant difference from Nip at P < 0·05.

Fig. 3.

Effect of ACC on root ethylene production (A) and the expression of genes for ethylene biosynthesis enzymes (B, C). Seedlings were transferred to P-depleted (−P) nutrient solution in the presence or absence of ACC for 1 week. After treatment, total root RNA was subjected to qRT-PCR analysis using gene-specific primers. Data are means ± s.d. (n = 4). Asterisks indicate a significant difference from the absence of ACC at P < 0·05.

Fig. 4.

Effect of ACC on the root (A, C) and shoot soluble P content (B, D). Seedlings were transferred to P-depleted (−P) nutrient solution in the presence or absence of ACC for 1 week. Data are means ± s.d. (n = 4). Columns with different letters are significantly different at P < 0·05.

Fig. 5.

Effect of ACC on the retention of the P in the cell wall (A) and the pectin content (B). Seedlings were transferred to P-depleted (−P) nutrient solution in the presence or absence of ACC for 1 week. Data are means ± s.d. (n = 4). Columns with different letters are significantly different at P < 0·05.

Fig. 6.

Identification of a correlation among root total P, cell-wall P content and root ethylene production. (A) Cell-wall P content and root total P content. Nip was transferred to P-supplied (+P) nutrient solution for 1 week. After treatment, total roots were subjected to P content analysis and cell-wall extraction. (B) Root cell-wall P content correlates with root total P content under P-sufficient conditions. Eleven different rice cultivars were transferred to P-supplied (+P) nutrient solution for 1 week. After treatment, total roots were subjected to P content analysis and cell-wall extraction. (C) Root ethylene production correlates with root cell-wall P content. Eleven different rice cultivars were transferred to P-depleted (−P) nutrient solution for 1 week. After treatment, total roots were subjected to cell-wall extraction and ethylene production. Data are means ± s.d. (n = 4).

Fig. 7.

Effect of ACC on the expression of Pi translocation-related genes in Nip. Seedlings were transferred to P-depleted (−P) or P-supplied (+P) nutrient solution in the presence or absence of ACC for 1 week. The inset of B amplified the effect of ACC on the expression of OSPT6 under +P nutrient solution. After treatment, total root RNA was subjected to qRT-PCR analysis using gene-specific primers. Data are means ± s.d. (n = 4). Columns with different letters are significantly different at P < 0·05.

Fig. 8.

Occurrence of ethylene transcription factor-responsive elements in the promoter regions of OsPT2, OsPT6 and OsPT8. A stretch of 2500 bp upstream of the transcription initiation site was examined. The predicted ethylene-responsive elements AWTTCAAA and GCCGCC are represented by triangles and oval symbols, respectively. The numbering below each symbol indicates the position of each motif relative to the site of initiation of transcription.