Identification and Expression Analysis of Acid Phosphatase Gene (PAP) in Brassica napus: Effects of cis-Acting Elements on Two BnaPAP10 Genes in Response to Phosphorus Stress

Purple acid phosphatases (PAPs) play a key role in phosphorus (P) assimilation and redistribution in plants, catalyzing the hydrolysis of phosphate esters to produce inorganic phosphate (Pi). In this study, a total of 77 PAP genes were identified in B. napus. The candidate genes were divided into three groups and ten subgroups based on the phylogenetic analyses and exon-intron organization. Among these 77 BnaPAP proteins, 35 exhibit typical metal-ligating residues characteristic of known PAPs, whereas certain unaltered amino acid residues were absent or displaced in other BnaPAPs. A computational prediction was conducted, revealing that the majority of PAPs contain signal peptide motifs and display a range of N-glycosylation levels, as well as transmembrane helix motifs. An analysis of previously obtained RNA-seq data revealed that 55.84% (43 of 77) of the BnaPAPs responded to Pi deficiency. Moreover, we conducted a preliminary examination of the expression profiles of BnaPAP genes in response to salt stress, and discovered that 42.86% (33 of 77) of these genes were induced under salt stress, either in the shoots or in the roots. Further qRT-PCR and GUS analyses revealed that BnaC9.PAP10 and BnaA7.PAP10, two paralogs of BnaPAP10s, were induced by Pi deficiency. Notably, BnaC9.PAP10 exhibits robust induction, compared to the relatively mild induction observed in BnaA7.PAP10. Our research shows that BnaA7.PAP10 uniquely responds to Pi stress via the W-box, while BnaA7.PAP10 predominantly responds via the P1BS element, and the differences in cis-regulatory elements (CREs) within their promoter regions specifically contribute to their distinct expression levels under Pi stress. Our findings provide valuable insights and establish a foundation for future functional studies of BnaPAPs.