| Root-knot nematodes (Meloidogyne spp.) are economically important pests in the world and Meloidogyn incognita is the main nematode species. The tomato is one of the most widely-cultivated crops in the world and root-knot nematode disease is going to get worse because of protected cultivation. In the 1940s, the nematode resistant gene Mi was identified and shown to be unique source of resistant in all tomato cultivars. Shortly after, many tomato cultivars and rootstocks carrying the Mi gene emerged. This was regarded as the most safe and effective approach for the prevention of root-knot nematode disease. Beyond the gene, Mi, the main target of researchers worldwide has become to understanding the allelopathic effect of root exudates on nematodes. Plant roots can produce and exude into the rhizosphere agreat variety of compounds ranging from amino acids, complex polysaccharides, and proteins, to smaller, more volatile lipophilic molecules, all of which also have been shown to directly or indirectly influence the soil community of organisms, include nematodes. Plant chemicals originating from root exudates attract nematodes to roots or result in repellence, motility inhibition or even death. Until recently, little was known about the signaling mechanisms occurring before nematodes penetrate host roots. Moreover, root-knot nematode is a kind of soil-borne disease, the study on resistance mechanism needs to be integrated the relations between and among plant, nematodes and soil biological activity. Thus, there is a clearly recognized need for systematic research on the relationship of tomato rootstock and resistance to root-knot nematode.In this paper, the disease resistance of twelve tomato rootstocks was investigated. Root exudates from three tomato rootstocks including highly resistant, moderately resistant and highly susceptible were investigated. The composition of the root exudates was analysed by gas chromatography/mass spectrometry (GC/MS) prior to and following M. incognita inoculation. Four compounds in root exudates were selected for further analysis and their allopathic effect on M. incognita, tomato seed and seeding and soil ecological environment were investigated. At last, this paper verified the function of root exudate by gene expression profiling. The results of the study add to a growing body of important literature illustrating the effect of root exudates on M. incognita.The main results were showed below:1. Tomato rootstocks were inoculated artificially by M. incognita to identify resistance index in solar greenhouse. The disease resistance covered five types including high resistance, moderate resistance, resistant, susceptible and high susceptible. The physiological property, rhizosphere soil microorganism quantity and soil enzyme activity were measured. Correlation analysis results showed that tomato rootstocks resistance to M. incognita were positive correlated with the activities of PAL, PPO and POD in blade and the quantity of bacteria and actinomyces, the activities of catalase, polyphenol oxidase, urease and peroxidase in rhizosphere soil.2. After gathered root exudates of three tomato rootstocks, the effects of the root exudates on egg hatch of M. incognita, survival and chemotaxis of the second-stage juvenile (J2) were assessed. The results showed that root exudates from three rootstocks all suppressed eggs hatch of M. incognita. Inhibitory effect of root exudate from resistant rootstock was most obvious. Root exudates from three rootstocks increased the mortality rate of J2. The root exudate from HR strain was repellent to J2 while that from the MR strain was neutral. The root exudates from T strain attracted M. incognita J2. The composition of the root exudates was analysed by gas chromatography/mass spectrometry (GC/MS) prior to and following M. incognita inoculation. Results showed that the contents of ester, phenol compounds include 2,6-Di-tert-butyl-p-cresol, L-ascorbyl 2,6-dipalmitate, Dibutyl phthalate, Dimethyl phthalate increased obviously after inoculation. These compounds might be related to tomato resistance to M. incognita. They were selected for further analysis and their allopathic effects on M. incognita were investigated. Results showed that the egg hatch of M. incognita was suppressed by each of the compound. L-ascorbyl 2,6-dipalmitate showed the most notable effect in a concentration-dependent manner. All four compounds were associated with increased J2 mortality. The greatest effect was observed with dimethyl phthalate at 2 mmol·L-1. Dibutyl phthalate was the only compound observed to repel M. incognita J2. Each of the four compounds were correlated with a reduction in disease index in the susceptible cultivar, T, and tomato seedlings irrigated with L-ascorbyl 2,6-dipalmitate at 2 mmol·L-1 showed the best resistance to M. incognita.3. The allelopathic effects of four compounds on seed germination and plant growth were determined. For seed germination:L-ascorbyl 2,6-dipalmitate promoted the germination of tomato seeds. Dimethyl phthalate presented inhibition obviously. At low concentration, 2,6-Di-tert-butyl-p-cresol and Dibutyl phthalate promoted the germination of tomato seeds and turned to inhibit at high concentration. For plant growth:allelopathic effects of four simulated components on plant growth expressed promotion at low concentration and inhibition at high concentration. They promoted plant height, stem diameter, overground and underground fresh weight, branch root and made root robust at 0.5 mmol·L-1. Moreover, this treatment promoted tomato root vigor, chlorophyll content and PAL activity in blade.4. The effects of simulated components on soil ecological environment through soil microorganism quantity, soil physicochemical properties, soil enzyme activity were verified. After applying four compounds, the factors including soil pH, rapid available phosphorus content, microorganism gross, bacteria and fungus quantity, urease, catalase and polyphenol oxidase activity were found increased, and the factors including soil available potassium content, actinomycetes quantity, peroxidase activity were founded reduced. At concentration from 0.5 mmol·L-1 to 1.0mmol·L-1,2,6-Di-tert-butyl-p-cresol, dibutyl phthalate and dimethyl phthalate brought the highest soil enzyme activity and optimum soil microorganism structure and soil physicochemical properties, this was a optimum condition for plant growth.5. The differential expressed genes in tomato root irrigated by L-ascorbyl 2,6-dipalmitate were analysed through digital gene expression profile. The results detected 1,896 differential expressed genes,442 up-regulated genes,1454 down-regulated genes. The function of these differential expressed genes were analysed, and the molecular mechanism of allelopathic effect of L-ascorbyl 2,6-dipalmitate at high concentration on tomato resistance to M. incognita and growth. |
