Three GPI-anchored alkaline phosphatases are involved in the intoxication of Cry1Ca toxin to Spodoptera exigua larvae

Glycosylphosphatidylinositol (GPI)-anchored alkaline phosphatase (ALP) isoforms are crucial for the intoxication of crystal proteins (Cry) to several insect species. We cloned five SeALPs from the larval midgut of Spodoptera exigua, a major pest of many crops. All five SeALPs contain a signal peptide at the N-terminus, a phosphatase domain, and a GPI-anchor site at the C-terminus. Additionally, the sequences encode two or three potential N-glycosylation sites. The five SeALPs were highly expressed at the larval stage, especially in the larval gut or Malpighian tubules. Ingestion over four consecutive days of double-stranded RNAs (dsRNAs) targeting SeALP1, SeALP2, SeALP3, SeALP4, and SeALP5 significantly reduced the corresponding mRNA levels by 60.0%, 40.0%, 65.6%, 48.1%, and 69.1% respectively, compared with the levels in control larvae that fed on non-specific dsRNA (dsEGFP). When larvae that previously ingested phosphate buffered saline (PBS)-, dsEGFP-, or five dsSeALPs-overlaid diets were then exposed to a diet containing Cry1Ca, the larval mortalities after six days were 70.0%, 71.8%, 49.1%, 54.9%, 65.3%, 52.5%, and 77.4%, respectively. ANOVA analysis revealed that the larvae that previously fed on the dsSeALP1-, dsSeALP2-, and dsSeALP4-overlaid diets had significantly lower mortalities than those that previously ingested the PBS-, dsEGFP-, dsSeALP3- and dsSeALP5-overlaid diets. The results suggest that SeALP1, SeALP2 and SeALP4 are involved in the intoxication of Cry1Ca to S. exigua larvae.