Hari S. Gaur

Chemotaxis can take plant-parasitic nematodes to the source of a chemo-attractant via the shortest possible routes

It has long been recognized that chemotaxis is the primary means by which nematodes locate host plants. Nonetheless, chemotaxis has received scant attention. We show that chemotaxis is predicted to take nematodes to a source of a chemo-attractant via the shortest possible routes through the labyrinth of air-filled or water-filled channels within a soil through which the attractant diffuses. There are just two provisos: (i) all of the channels through which the attractant diffuses are accessible to the nematodes and (ii) nematodes can resolve all chemical gradients no matter how small. Previously, this remarkable consequence of chemotaxis had gone unnoticed. The predictions are supported by experimental studies of the movement patterns of the root-knot nematodes Meloidogyne incognita and Meloidogyne graminicola in modified Y-chamber olfactometers filled with Pluronic gel. By providing two routes to a source of the attractant, one long and one short, our experiments, the first to demonstrate the routes taken by nematodes to plant roots, serve to test our predictions. Our data show that nematodes take the most direct route to their preferred hosts (as predicted) but often take the longest route towards poor hosts. We hypothesize that a complex of repellent and attractant chemicals influences the interaction between nematodes and their hosts.

Comparison of host recognition, invasion, development and reproduction of Meloidogyne graminicola and M. incognita on rice and tomato

The rice root-knot nematode Meloidogyne graminicola normally infects rice, wheat and several other graminaceous plants. Meloidogyne incognita is a serious pest of dicotyledonous crops, although it can infect and reproduce on some cereals. This paper demonstrates and compares host recognition, development and reproduction of these two species of root-knot nematodes on rice and tomato plants. Attraction bioassays in pluronic gel clearly showed that M. incognita preferred tomato roots to rice or mustard roots, whilst M. graminicola was more attracted towards rice compared with tomato or mustard roots. Based on the attraction data from this study, it can be hypothesised that either: i ) the blend of attractants and repellents are different in good and poor hosts; or ii ) relatively long-range attractants, together with shorter-range repellents, might affect nematode movement patterns. Some host specific attractants might also be involved. Meloidogyne incognita was able to invade and develop to adult female but did not produce eggs in rice roots. By contrast, M. graminicola developed and reproduced faster on both rice and tomato plants compared with M. incognita. Nevertheless, second-stage juveniles of both these root-knot nematodes showed a similar pattern of distribution inside the roots, preferring to accumulate at the root tips of rice or in the vascular cylinder and cortical region of tomato.

The influence of root diffusate, host age and water regimes on hatching of the root-knot nematode, Meloidogyne triticoryzae

The root-knot nematode, Meloidogyne triticoryzae, produces three kinds of unhatched second stage juveniles (J2): i) those that hatch freely in water, ii) those that require stimulus from rice root diffusate (RRD), and iii) those that do not hatch even in the presence of RRD. The proportion of these three types varies with generation, with the final generation produced on senescing plants having a large proportion of unhatched J2 of the third type, which is likely to equate with diapause. Dilution of RRD reduced its hatching activity and there is no evidence of the presence of inhibitors in the diffusate. The hatching response of M. triticoryzae may be modified additionally by the growing conditions: submergence of infected plants may have delayed hatch, possibly by causing anoxybiosis in unhatched J2, and thereby delayed the appearance of the second and third generations of the nematodes. Le nematode Meloidogyne triticoryzae produit trois types de juveniles de 2eme stade (J2) non encore eclos: i) ceux qui eclosent d’eux-meme dans l’eau; ii) ceux qui requierent le stimulus produit par le diffusat radiculaire de riz (RRD); enfin iii) ceux qui n’eclosent pas meme en presence de RRD. La proportion de ces trois types varie avec la generation du nematode, la derniere generation, produite sur un hote senescent, contenant une grande proportion de J2 non eclos de type 3, ce fait representant l’equivalent d’une diapause. La dilution du RRD diminue l’eclosion et aucune evidence d’inhibiteurs n’a ete relevee dans le diffusat. Chez M. triticoryzae l’eclosion peut de plus etre modifiee par les conditions de croissance de l’hote; ainsi la submersion des plants infestes peut retarder l’eclosion, l’anoxybiose ainsi induite affectant vraisemblablement les J2 non eclos et retardant de ce fait l’apparition des deuxieme et troisieme generations du nematode.

Effect of nitrogen supply form on the invasion of rice roots by the root-knot nematode, Meloidogyne graminicola

Rice is capable of taking up both nitrate (NO3 −) and ammonical (NH4 +) forms of nitrogen (N). It is largely grown under flooded cultivation when NH4 + is the main form of available soil N. The root knot nematode, Meloidogyne graminicola, can cause serious damage to rice crops and disturb N uptake and translocation. We have investigated if the N supply form can influence the susceptibility of rice to this important pest. Roots supplied with a 100-fold lower supply of calcium nitrate (0.1 mM Ca(NO3)2) showed a higher level of nematode infection as measured by the gall index. Plants supplied with 2.85 mM of Ca(NO3)2 were more infected compared with the same dose of ammonium nitrate (NH4NO3) or ammonium chloride (NH4Cl). 15NO3 − influx studies showed significantly smaller uptake of nitrate in nematode-infected roots when compared with non-infected control plants. The electrophysiological studies showed that resting cell membrane potentials of nematode-infected plants were significantly smaller (less negative) than those of non-infected control rice. Furthermore, when the cellular responses to NO3 − were compared, these also showed significantly smaller nitrate transport activity in nematode-infected roots (4.7 ± 1.2 mV) when compared with non-infected control plants (11.9 ± 3.4 mV). Taken together, the 15NO3 − influx and electrophysiological measurements clearly showed that the root NO3 − transport activity was severely decreased in nematode-infected roots. The results also show that regulation of NO3 − concentration at critical periods of nematode infection of rice root can provide a non-nematicidal method of nematode management.

Effect of small lipophilic molecules in tomato and rice root exudates on the behaviour of Meloidogyne incognita and M. graminicola

Plant chemicals in the rhizosphere originating from root exudates or sites of previous nematode penetration can influence nematode behaviour, and a number of plant compounds, some present in root exudates, have been shown either to attract nematodes to the roots, or to result in repellence, motility inhibition, or even death. The present work was conducted to isolate small lipophilic molecules (SLMs) emitted by root exudates of Solanum lycopersicum and Oryza sativa to investigate their effect on root-knot nematodes. SLMs extracted, through solid phase extraction, from hydroponically collected root exudates of 40 tomato and rice plants had an inhibitory impact on the motility of second-stage juveniles of Meloidogyne incognita and M. graminicola and showed a nematotoxic or nematostatic (upon dilution) effect on both species. The semiochemicals present in the SLMs induced a very small, albeit statistically significant, effect on stylet thrusting. A small quantity of salivary secretion around the stylet tip and a significant decrease in nematode head movement were observed. The semiochemicals negatively influenced behaviour of M. incognita and M. graminicola by strongly affecting their mobility. Therefore, it is proposed that SLMs present in both tomato and rice root exudates play important roles during the interaction of Meloidogyne spp. with their host plant, and that they might exert a repellent, or allellopathic, effect on these nematodes.

Hatching Behaviour of Six Successive Generations of the Pigeon-Pea Cyst Nematode, Heterodera Cajani, in Relation To Growth and Senescence of Cowpea, Vigna Ung Ui c uLa Ta

Hatching from cysts and eggsacs of six successive generations of Heterodera cajani produced on cowpea during a single growing season in the glasshouse was compared in distilled water, soil leachate and host root diffusate from four week old plants. The majority of eggs in eggsacs hatched within 7 days in all treatments and there were no differences between generations. Hatch from cysts in all treatments was similar over the first four generations but the fifth and sixth generations, produced on senescing plants, showed a marked dependency on host root diffusate for hatch. The ratio of eggs in eggsacs to eggs in cysts decreased with each succeeding generation and a comparison between third and sixth generations indicated that, in the older generation, more lipid reserves are partitioned into the encysted J2 than into the J2 in eggsacs. The results are discussed in the context of the survival requirements of H. cajani in the absence of a host.

The effect of root-knot nematode, Meloidogyne graminicola, on the quality and vigour of rice seed

The root-knot nematode, Meloidogyne graminicola, is a serious pest of rice known to reduce grain yield. The effects of a series of infestation levels of M. graminicola on the nutritional and seed quality parameters of the grain have been studied under pot and field conditions using two cultivars of rice, a basmati, cv. Pusa Sugandh-5 and a non-basmati, cv. Pusa-44. The 1000 grain weight was significantly reduced by as much as 44.5% in cv. Pusa Sugandh-5 and 50.7% in cv. Pusa-44 seed when the parent plants were grown in soil with very high levels of M. graminicola infestation. The protein and amylose contents of the grains were significantly reduced in both the rice cultivars. The seed germination percentages of both the rice cultivars were also reduced as the nematode inoculation rates increased from 1-8 second-stage juveniles (J2) cm−3 in a pot experiment. Similar trends were also observed in field conditions. There was a significant decrease in the seedling vigour indices of both the rice cultivars. These results showed that the rice grains produced on plants infected with the nematode M. graminicola were lighter in weight and had poorer nutrient qualities, such as amylose and protein content. Further, if these grains were used as seed, the germination percentage was lower and, even more importantly, seed vigour was poor compared to the seed obtained from plants grown in M. graminicola-free soil.

Differential defence response due to jasmonate seed treatment in cowpea and tomato against root-knot and potato cyst nematodes

Treatment of seeds with jasmonate provides significant reduction in Meloidogyne incognita and Globodera pallida infection. The number of M. incognita inside roots of cowpea and tomato plants derived from seeds treated with jasmonate was greatly reduced; however, the major effect of jasmonic acid and methyl jasmonate treatments was observed on the number of eggs produced by the nematodes wherein a significant reduction was observed in both treatments. By contrast, pre-treatment of tomato seeds with jasmonate not only reduced G. pallida infection by 63% but also affected nematode development inside the roots. These results indicate that jasmonate treatments affected nematode reproduction and/or development. We show here that germination and plant growth parameters of tomato seeds, but not of cowpea and soybean seeds, were unaffected by jasmonate treatment. The use of elicitors to prime plant immunity is a natural way of protecting plants by boosting their immunity to provide nematode resistance. Seed treatment with natural elicitors of plant immunity offers an environmentally friendly alternative for farmers and growers that can contribute to the protection of their crops from pests above and below ground.

Effect of root nitrogen supply forms on attraction and repulsion of second-stage juveniles of Meloidogyne graminicola

Three experiments were conducted to compare the attraction and repulsion of second-stage juveniles (J2) of the root-knot nematode, Meloidogyne graminicola, to rice plants supplied with different forms of nitrogen. The rice plants were hydroponically grown in a full nutrient solution containing different concentrations (0.1 or 10.0 mM) of nitrate (NO3 −) or forms of nitrogen supply (2.85 mM calcium nitrate (Ca(NO3)2), ammonium nitrate (NH4NO3) or ammonium chloride (NH4Cl)) for 2 weeks. Five rice plants were placed with their roots in one corner of a Perspex X or Y-chamber partly filled with agar and J2 were inoculated onto the chambers. The data show that J2 of M. graminicola were significantly attracted towards the roots of rice plants grown in hydroponics containing 0.1 mM NO3 − and 2.85 mM Ca(NO3)2, but repelled by 10.0 mM NO3 −, 2.85 mM NH4NO3 and NH4Cl. The results suggest that the application of ammonia-based nitrogen fertiliser to the rice nursery bed may interfere with nematode attraction and thus reduce invasion, and the application of chemical nitrification inhibitors to rice nursery beds may decrease nematode invasion.

In vitro nematicidal activity of a terrestrial cyanobacterium, Synechococcus nidulans, towards plant-parasitic nematodes

The nematicidal activity of a terrestrial cyanobacterium, Synechococcus nidulans, was investigated. Extracts of S. nidulans cultures collected at weekly intervals for 5 weeks were sonicated and tested against second-stage juveniles (J2) of Meloidogyne incognita. Extracts of 2-week-old cultures caused the maximum immobility (94.2%) and mortality (29.3%) of J2, compared with controls (medium and water). This extract was tested in vitro against infective stages and hatch of M. graminicola, Heterodera cajani, H. avenae and Rotylenchulus reniformis. Extracts of sonicated S. nidulans caused a mean immobility in the range of 91.3-98.4% in infective stages of the nematodes, with no significant difference with an increase in exposure time from 24 to 72 h. The greatest mean percentage mortality was observed in M. graminicola (31.5%) followed by M. incognita (29.3%), H. avenae (20.9%), and R. reniformis and H. cajani (both 17.4%) with a significant increase with the period of exposure from 24 to 72 h. No significant differences in mortality were observed between M. graminicola and M. incognita and between H. avenae and H. cajani. The percentage hatch inhibition over control (water) was greatest in M. incognita (94.2%), followed by H. avenae (91.6%), H. cajani (72.3%) and M. graminicola (70.6%), and least in R. reniformis (58.6%).