Recent progress has shown that vacuolar Pi transporters (VPTs) are important for cellular Pi homeostasis against external Pi variations in Arabidopsis and rice, while it is poorly understood for the identity and regulatory mechanism of VPTs in Brassica napus ( B. napus). Here, we identified two vacuolar Pi influx transporters BnA09PHT5;1b and BnCnPHT5;1b in B. napus and uncovered their necessity for cellular Pi homeostasis through functional analysis. BnPHT5;1bs are the homologs of Arabidopsis AtPHT5;1 with the similar sequence, structure, tonoplast localization, and VPT activity. BnPHT5;1b double mutants had smaller shoot growth and higher shoot cellular Pi than the wild-type B. napus, which are largely different from the report in At PHT5;1 mutant, suggesting PHT5;1-VPTs play a distinct mechanism of cellular Pi homeostasis in seedlings of B. napus and Arabidopsis. By contrast, disruption of BnPHT5;1b genes slowed vegetative growth accompanied by Pi toxicity in floral organs, reduced seed yield and impacted seed traits, agreeing with the role of AtPHT5;1 in floral Pi homeostasis. Taken together, our studies identified two vacuolar Pi influx transporters in B. napus and revealed the distinct and conserved regulatory mechanisms of BnPHT5;1bs in cellular Pi homeostasis in this plant species.

The concentration and homeostasis of intracellular phosphate (Pi) are crucial for sustaining cell metabolism and growth. During short-term Pi starvation, intracellular Pi is maintained relatively constant at the expense of vacuolar Pi. After the vacuolar stored Pi is exhausted, the plant cells induce the synthesis of intracellular acid phosphatase (APase) to recycle Pi from expendable organic phosphate (Po). In this study, the expression, enzymatic activity and subcellular localization of ACID PHOSPHATASE 1 (OsACP1) were determined. OsACP1 expression is specifically induced in almost all cell types of leaves and roots under Pi stress conditions. OsACP1 encodes an acid phosphatase with broad Po substrates and localizes in the endoplasmic reticulum (ER) and Golgi apparatus (GA). Phylogenic analysis demonstrates that OsACP1 has a similar structure with human acid phosphatase PHOSPHO1. Overexpression or mutation of OsACP1 affected Po degradation and utilization, which further influenced plant growth and productivity under both Pi-sufficient and Pi-deficient conditions. Moreover, overexpression of OsACP1 significantly affected intracellular Pi homeostasis and Pi starvation signalling. We concluded that OsACP1 is an active acid phosphatase that regulates rice growth under Pi stress conditions by recycling Pi from Po in the ER and GA.

Phytate is the storage form of phosphorus in angiosperm seeds and plays vitally important roles during seed development. However, in crop plants phytate decreases bioavailability of seed-sourced mineral elements for humans, livestock and poultry, and contributes to phosphate-related water pollution. However, there is little knowledge about this trait in oilseed rape B. napus (oilseed rape). Here, a panel of 505 diverse B. napus accessions was screened in a genome-wide association study (GWAS) using 3.28 x 106 single nucleotide polymorphisms (SNPs). This identified 119 SNPs significantly associated with phytate concentration (PA_Conc) and phytate content (PA_Cont) and six candidate genes were identified. Of these, BnaA9.MRP5 represented the candidate gene for the significant SNP chrA09_5198034 (27kb) for both PA_Cont and PA_Conc. Transcription of BnaA9.MRP5 in a low -phytate variety (LPA20) was significantly elevated compared with a high -phytate variety (HPA972). Association and haplotype analysis indicated that inbred lines carrying specific SNP haplotypes within BnaA9.MRP5 were associated with high- and low-phytate phenotypes. No significant differences in seed germination and seed yield were detected between low and high phytate cultivars examined. Candidate genes, favorable haplotypes and the low phytate varieties identified in this study will be useful for low-phytate breeding of B. napus.