The acid phosphatase-containing digestive fluid secreted by <i>Nepenthes khasiana</i> pitchers inhibits plant growth, possibly due to the presence of heat-stable growth-inhibitory molecules

Carnivorous plants such as <i>Nepenthes</i> secrete acid phosphatase (ACP) into their digestive fluids, probably to facilitate the scavenging of inorganic phosphate (Pi) from prey-derived organic phosphate (Po) sources, thereby promoting plant growth. To investigate whether the ACP-containing digestive fluid promotes Pi acquisition, 7-day-old Arabidopsis seedlings were grown in liquid half-strength Murashige and Skoog (MS) medium supplemented with the digestive fluid obtained from the unopened pitchers of <i>N. khasiana</i>, which had been incubated with DNA, a likely Po source from digested prey. This fluid exhibited significantly higher ACP activity that remained unaffected despite increased phosphate levels in the growing medium than the fluid from opened pitchers and contained an ACP protein similar to those reported in other <i>Nepenthes</i> species. Surprisingly, the ACP-containing digestive fluid significantly inhibited Arabidopsis plant growth. Even after heat deactivation, the digestive fluid failed to stimulate growth and instead severely repressed it. Similar inhibitory effects were observed with the digestive fluid obtained from the opened pitchers of <i>N. khasiana</i>. To further explore this finding, Arabidopsis seedlings were treated with varying concentrations of two heat-stable 1,4-naphthoquinones, droserone (0–250 µM) and plumbagin (0–400 µM), including the non-radical reactive oxygen species H₂O₂ (0–2 µM), all of which have been reported in the digestive fluids of <i>Nepenthes</i> pitchers. While H₂O₂ had no significant effect on plant growth, droserone caused significant growth inhibition at 250 µM concentration, and plumbagin inhibited growth at concentrations as low as 25 µM. These findings suggest that naphthoquinones may be responsible for the observed growth inhibition.