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

Abstract

Nepenthes are carnivorous plants that colonize habitats poor in soil nutrients. To survive, Nepenthes develop pitchers capable of capturing and digesting attracted prey. Prey-derived nutrients are then absorbed to support plant growth and reproduction. So far, pitcher formation in Nepenthes is a poorly understood biological process. To shed light on the formation of Nepenthes pitchers, we grew dissected shoot apices of 3-month-old N. khasiana seedlings in Murashige and Skoog (MS) medium of varying strengths viz. full-strength MS (1 MS), quarter-strength MS (1/4 MS), and one-eighth strength MS (1/8 MS), including those lacking nitrogen (N), phosphorus (P), and potassium (K) and in the presence of N-1-naphthylphthalamic acid (NPA). We sequenced the transcriptome and performed gas chromatography-mass spectrometry to determine changes in gene expression patterns and primary metabolite accumulations in response to the varying nutrient conditions. Shoots grown in 1 MS or NPA-containing 1/4 MS and 1/8 MS failed to develop pitchers. Remarkably, pitcher formation is restored when N was removed from 1 MS. Transcriptomic response to nutrient-sufficient and nutrient-deficient conditions are associated with the enrichment of several defence-related genes, including two JA-mediated defence response genes, WRKY51 and WRKY11, respectively. Further, metabolomic response to the varying nutrient conditions identifies glutamic acid as a key metabolite, accumulating at lower and higher levels in shoots with and without pitchers, respectively. Together, our findings suggest that failure to form pitchers may be associated with the suppression of the JA-signalling pathway, whereas the induction of the JA-mediated defence response is linked to pitcher formation in N. khasiana.