A carnivorous sundew plant prefers protein over chitin as a source of nitrogen from its traps

Highlights

• Carnivorous sundew plant is able to derive nitrogen from its insect prey.

• Digestion and nitrogen uptake from protein is more effective in comparison to chitin.

• This nitrogen is incorporated into chlorophyll and chlorophyll-binding proteins.

• Carnivorous plants use nitrogen from the prey to building their photosynthetic apparatus.

Abstract

Carnivorous plants have evolved in nutrient-poor wetland habitats. They capture arthropod prey, which is an additional source of plant growth limiting nutrients. One of them is nitrogen, which occurs in the form of chitin and proteins in prey carcasses. In this study, the nutritional value of chitin and protein and their digestion traits in the carnivorous sundew Drosera capensis L. were estimated using stable nitrogen isotope abundance. Plants fed on chitin derived 49% of the leaf nitrogen from chitin, while those fed on the protein bovine serum albumin (BSA) derived 70% of its leaf nitrogen from this. Moreover, leaf nitrogen content doubled in protein-fed in comparison to chitin-fed plants indicating that the proteins were digested more effectively in comparison to chitin and resulted in significantly higher chlorophyll contents. The surplus chlorophyll and absorbed nitrogen from the protein digestion were incorporated into photosynthetic proteins – the light harvesting antennae of photosystem II. The incorporation of insect nitrogen into the plant photosynthetic apparatus may explain the increased rate of photosynthesis and plant growth after feeding. This general response in many genera of carnivorous plants has been reported in many previous studies.

Introduction

Carnivorous plants have independently evolved several times by the process of convergent evolution (Ellison and Gotelli, 2009). They usually grow in sunny, wet and nutrient-poor habitats where the nutritional benefit gained from captured prey exceeds the costs of modifying leaves into traps (Givnish et al., 1984, Pavlovič et al., 2009). The costs of carnivory represents extra energy costs with prey attraction (production of lures), capture (production of traps) and digestion (production of digestive enzymes) being required. There is also a decreased rate of photosynthesis (A_N) and in some species an increased rate of respiration (R_D) as a result of leaf adaptation for carnivory (Givnish et al., 1984, Ellison, 2006, Pavlovič and Saganová, 2015). On the other hand, a potential benefit from carnivory is an increase in A_N through improved nutrient supply; particularly in foliar/shoot nitrogen and phosphorus contents (Farnsworth and Ellison, 2008, Pavlovič et al., 2009, Pavlovič et al., 2014, He and Zain, 2012, Kruse et al., 2014, Gao et al., 2015). The greatest enhancement in photosynthetic gains from prey capture occurs under conditions of soil nutrient shortage, together with sufficient humidity and light. Such conditions have favoured the evolution of botanical carnivory. It has been suggested that the mechanism accounting for the increased A_N in response to nitrogen uptake from prey is an increased concentration in photosynthetic proteins (mainly Rubisco) (Givnish et al., 1984); however, this has never been tested. This suggestion seems reasonable because Rubisco is present at very high levels in the photosynthesizing cells of C₃ plants and may contribute up to 50% of soluble leaf protein and 20–30% of total leaf N (Evans, 1989, Feller et al., 2008). The second major fraction of nitrogen directly related to photosynthesis consists of the pigment–protein complexes in thylakoid membranes (Evans, 1989).

Carnivorous plants obtain a substantial amount of nutrients from prey capture and have evolved five basic trapping mechanisms (Juniper et al., 1989, Król et al., 2012, Pavlovič and Saganová, 2015). The most important nutrients, which restrict the carnivorous plant growth in nutrient-poor soils are nitrogen (N), phosphorus (P) and potassium (K) (Ellison, 2006). Insect prey is a rich source of N and P, the contents of which exceed that in leaf tissue by 5–10 times, and these elements are markedly absorbed (Adamec, 2002, Pavlovič et al., 2014). This accounts for the relatively high contribution of insect-derived N to total leaf N content in carnivorous plants which successfully capture insect prey (10–90%, Schulze et al., 1991, Schulze et al., 1997, Chapin and Pastor, 1995, Schulze et al., 2001, Moran et al., 2001, Millett et al., 2003), and is allocated mainly in the new foliage (Schulze et al., 1997, Gao et al., 2015). Adamec (2002) and Pavlovič et al. (2014) analysed prey carcasses after their digestion and found that a high amount of N (40–60%) in insect carcasses was unavailable for absorption by sundew traps. On the other hand, P and K were absorbed much more effectively. They hypothesized that the less effective N uptake was due to the large proportion of N in insect chitin exoskeletons (as poly-N-acetylglucosamine) which is not available for absorption. Indeed, the exoskeleton of the digested prey does not seem to be significantly affected by the digestive processes (Juniper et al., 1989). However, this seems to be in contrast to the recent molecular findings that several classes of chitinases in the digestive fluid in different species of carnivorous plants have been identified, and are even up-regulated by the presence of prey and/or chitin (Matušíková et al., 2005, Eilenberg et al., 2006, Rottloff et al., 2011, Hatano and Hamada, 2012, Renner and Specht, 2012, Paszota et al., 2014).

In our previous work, we have shown that feeding the sundew plant Drosera capensis on fruit flies significantly increased its leaf nitrogen and phosphorus content as well as the photosynthetic rate (Pavlovič et al., 2014). In this work, we focused on the importance of two nitrogen-rich compounds (chitin and protein) in the nutrition of the carnivorous sundew plant D. capensis. The sundew plants were fed either on chitin or protein (bovine serum albumin, BSA) to estimate the contribution of both N sources to the total N budget in plants. We measured the biomass, elemental composition, stable N isotopes and chlorophyll content to reveal the uptake and nutritional value of both insect nitrogen-rich compounds. In addition, we did Western blot analyses for important photosynthetic proteins to find the role of N nutrition in the photosynthesis of carnivorous plants.