Carnivorous plants capture and digest insects for nutrients, allowing them to survive in soil deprived of nitrogenous nutrients. Plants from the genus Nepenthes produce unique pitchers containing secretory glands, which secrete enzymes into the digestive fluid. We performed RNA-seq analysis on the pitcher tissues and liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis on the pitcher fluids of Nepenthes × ventrata to study protein expression in this carnivory organ during early days of pitcher opening. This transcriptome provides a sequence database for pitcher fluid protein identification. A total of 32 proteins of diverse functions were successfully identified in which 19 proteins can be quantified based on label-free quantitative proteomics (SWATH-MS) analysis while 16 proteins were not reported previously. Our findings show that certain proteins in the pitcher fluid were continuously secreted or replenished after pitcher opening, even without any prey or chitin induction. We also discovered a new aspartic proteinase, Nep6, secreted into pitcher fluid. This is the first SWATH-MS analysis of protein expression in Nepenthes pitcher fluid using a species-specific reference transcriptome. Taken together, our study using a gel-free shotgun proteomics informed by transcriptomics (PIT) approach showed the dynamics of endogenous protein secretion in the digestive organ of N. × ventrata and provides insights on protein regulation during early pitcher opening prior to prey capture.
Carnivorous pitcher plants produce specialised pitcher organs containing secretory glands, which secrete acidic fluids with hydrolytic enzymes for prey digestion and nutrient absorption. The content of pitcher fluids has been the focus of many fluid protein profiling studies. These studies suggest an evolutionary convergence of a conserved group of similar enzymes in diverse families of pitcher plants. A recent study showed that endogenous proteins were replenished in the pitcher fluid, which indicates a feedback mechanism in protein secretion. This poses an interesting question on the physiological effect of plant protein loss. However, there is no study to date that describes the pitcher response to endogenous protein depletion. To address this gap of knowledge, we previously performed a comparative RNA-sequencing experiment of newly opened pitchers (D0) against pitchers after 3 days of opening (D3C) and pitchers with filtered endogenous proteins (>10 kDa) upon pitcher opening (D3L). Nepenthes ampullaria was chosen as a model study species due to their abundance and unique feeding behaviour on leaf litters. The analysis of unigenes with top 1% abundance found protein translation and stress response to be overrepresented in D0, compared to cell wall related, transport, and signalling for D3L. Differentially expressed gene (DEG) analysis identified DEGs with functional enrichment in protein regulation, secondary metabolism, intracellular trafficking, secretion, and vesicular transport. The transcriptomic landscape of the pitcher dramatically shifted towards intracellular transport and defence response at the expense of energy metabolism and photosynthesis upon endogenous protein depletion. This is supported by secretome, transportome, and transcription factor analysis with RT-qPCR validation based on independent samples. This study provides the first glimpse into the molecular responses of pitchers to protein loss with implications to future cost/benefit analysis of carnivorous pitcher plant energetics and resource allocation for adaptation in stochastic environments.
The carnivorous plants of genus Nepenthes produce unique pitchers containing secretory glands, which secrete proteins into the digestive fluid. We investigated protein profile in the pitcher fluid during the first three days of opening to understand carnivory trait of Nepenthes × ventrata. The proteome analysis of pitcher fluid from N. × ventrata was performed by label-free quantitative liquid chromatography mass spectrometry (nLC-MS/MSALL). Raw MS data have been deposited to the ProteomeXchange with identifier PXD007251. This dataset allows the identification and quantification of proteins from pitcher fluids to elucidate proteins involved in carnivory physiology of Nepenthes species.