Transcriptomic and metabolomic responses to varying nutrient conditions in the growing medium reveal new insights into pitcher formation in Nepenthes khasiana

Pitchers enable Nepenthes to colonize habitats poor in soil nutrients. So far, pitcher formation in Nepenthes is still a poorly understood biological process. To shed light on the formation of pitchers in Nepenthes, we grew dissected shoot apices of 3-month-old N. khasiana seedlings in Murashige and Skoog (MS) medium of varying strengths, including those lacking N, P and K and those containing 5μM N-1-naphthylphthalamic acid (NPA). Shoots grown in 1 MS or NPA-containing 1/4 MS and 1/8 MS failed to develop pitchers, while pitcher formation is restored when N is removed from 1 MS. Transcriptomic responses to sufficient and deficient nutrient conditions are associated with the enrichment of several defence-related genes, including two JA-mediated defence response genes, WRKY51 and WRKY11, respectively. Our findings suggest that failure to form pitchers is associated with the suppression of JA-signalling pathway whereas the induction of JA-mediated defence responses is linked to pitcher formation in Nepenthes. Further, metabolomic responses to the varying nutrient conditions identify glutamic acid as a key metabolite, accumulating at lower and higher levels in shoots with and without pitchers, respectively. We hypothesize that Nepenthes activates a defence response mechanism, seen in herbivore-damaged Arabidopsis leaves, during pitcher formation under low nutrient conditions.