Abstract
The aim of this research was to test a hypothesis that the biosynthetic and degradative pathways of chlorophyll are altered in insect-induced galls using two cecidomyiid galls as a model to test. These galls are oval-pointed and obovate galls respectively induced by the cecidomyiids Daphnephila taiwanensis, and D. sueyenae on Machilus thunbergii (Lauraceae) leaves. The contents of chlorophyll (Chl), biosynthetic intermediates such as protoporphyrin IX (PPIX), magnesium protoporphyrin IX (MGPP), and protochlorophyllide (Pchlide), breakdown intermediates including chlorophyllide (Chlide), pheophytin (Phe), pheophorbide (Pho), and carotenoid (Car) in two different galls and their host leaves were determined. Compared to infected leaves, all chlorophyll-related compounds pertaining to photosynthesis in insect-induced galls were drastically 9-31 fold lower, while Car declined by 9-32 fold. The molar percentages of PPIX, MGIX, and Pchlide in M.
thunbergii leaves were 62.6, 31.7, and 5.7%, respectively, while those in the two galls were 73.6- 81.2%, 18.8- 26.1%, and 0- 0.3%, respectively.
Ratios of Chlide/Pho in the galls were greater than those in the
herbivorous insects altered both chlorophyll biosynthetic and degradation pathways of galls on host plants, and insect-induced galls may utilize Chl→Chlide→Pchlide and Chlide→Phe→Pchlide as the respective major and minor degradative routes.
Introduction
Chlorophylls (Chls) are the most abundant and widely distributed pigments in nature, and play extremely important roles in plant photosynthesis, linking solar magnetoelectric irradiance to millions of organism and constructing complex ecosystems at different levels on earth. All Chls are covalently associated with polypeptides to form pigment-protein complexes on thylakoid membranes of chloroplasts (Markwell et al. 1979). Traditionally, the biosynthetic and degradative pathways of Chls were explored using normal plant leaves and their Chl-deficient mutants as samples (Yang et al. 1995). However, Chls with lower contents and a/b ratios were also discovered in non-leaf green tissues, such as the exocarp, mesocarp, and endocarp of fruits, seed coats, cotyledons, flower sepals, petals, stamens, and carpels, the stem skin, light-illuminated roots, and so forth (Yang et al. 2004). Chl synthetic and degradative pathways differ among normal leaves, mutant leaves, and non-leaf green tissues (Yang and Chen 1996). In the Chl synthetic pathway, mole percentages of individual porphyrins, such as
protoporphyrin IX (PPIX), magnesium protoporphyrin IX (MGIX), and protochlorophyllide (Pchlide) of normal leaves, greatly differ from those of Chl-deficient mutant leaves, such as of Sansevieria trifasciata and sweet potatoes (Chen et al. 2003; Hsu et al. 2003). Normal leaves use Chl→pheophytin (Phe)→pheophorbide (Pho) and Chl→Chlide→Phe as the major and minor chlorophyll degradative routes, respectively.
However, those of mutant leaves and non-leaf green tissues are totally reversed, suggesting that Chl-deficient mutant leaves and non-leaf green tissues, dramatically but non-lethally, alter the biosynthetic and degradative pathways of Chls (Hsu et al. 2003).
Chl-deficient mutants were found in higher plants with the following common characteristics: (1) lower Chl contents and higher Chl a/b values than normal plants; (2) decreasing contents of the pigment-protein complex in the plant's lifetime; and (3) sensitive to temperature, light, and photoperiod (Yang et al. 1995; Lu et al. 1995;
Yang and Chen 1996). In previous studies found that the contents of Chl in leaves were higher than in gall tissues of Daphnephila taiwanensis (Yang et al. 2003, 2007). In addition, two kinds of cecidomyiid galls induced by Daphnephila taiwanensis and D. sueyenae on Machilus thunbergii leaves were totally deficient in the pigment-protein complexes, CP1, A1, AB1, and AB2 throughout the entire period of gall
mutant leaves and non-leaf green tissues found in an oval-pointed cecidomyiid gall of Daphnephila taiwanensis on M. thunbergii leaves (Yang et al. 2003, 2007).
Impacts of herbivores on their host plants extend beyond the simple removal of tissue or phloem sap, which include a triggering of inducible defenses, increasing food qualities, and changing architecture, all of which may impact other herbivores by changing the resource quality of the remaining tissues (Larson 1998). Multiple physiological effects in response to gall inducers were observed in host plant tissues, including changes in pH, polarity, and nuclear and nucleolar hypertrophy, excesses of amino acids and sugars, and the presence of hydrolytic enzymes such as amylase and protease (Mani 1992; Rohfritsch 1992). More than 65%
of insect galls were found on plant leaves after plant organs had been subjected to insect-infected conditions (Dreger-Jauffret and Shorthouse 1992; Yang and Tung 1998). Both the morphology and anatomy of insects and insect-induced galls were studied (Meyer 1987); however, no work has been done on chloroplasts of galls and their photosynthesis, or on the biosynthesis and degradation of photosynthetic pigments such as Chl in gall chloroplasts. Although a number of categories of host-plant traits (i.e., morphology, physiology, phenology, and plant vigor) are known to influence host choice by gall-inducing insects, it is expected that host plant chemistry should be among the most important
factors, given the specificity of interactions between gall-inducers and their hosts (Abrahamson et al. 2003). Hartley (1998) demonstrated that plant galls are closely associated between arthropods and plants, in which the plant produces an unnatural growth of tissue in response to a specific stimulus from a gall-inducer, which has the ability to affect the growth and development of plant tissues. Moreover, levels of secondary compounds in gall tissue usually markedly differ from those of surrounding plant tissues, and gall-inducers produce species-specific and temporally variable changes in the chemical composition of gall tissues.
Fay et al. (1993), Larson (1998), and Dorchin et al. (2006) studied gall-inducers and their impacts on photosynthesis of the host; their results suggested no general trends, because a range of effects was seen from negative to positive. By comparing the Chl biosynthetic and degradation pathways of a mature oval-pointed cecidomyiid gall and an infected leaf of the host plant Machilus thunbergii Sieb and Zucc (Lauraceae), Yang et al. (2003) reported that the infected leaf may use the degradation pathway of Chl→ Phe→ Pho as the major route, but the cicedomyiid gall may use the degradation pathway of Chl→ Chlide→
Pho as the major route. The current study used two gall-inducers and host plants to test a hypothesis that the galls derived from infected