Evaluating the Influence of Water Scarcity on the Host Response of Garlic to the Stem and Bulb Nematode Ditylenchus dipsaci.

Ditylenchus dipsaci is a plant-parasitic nematode with a great economic impact on bulbous crops, including garlic (Allium sativum L.), and is distributed worldwide, particularly in the Mediterranean region. Traditionally, garlic was a rainfed crop in Spain, but irrigated areas have increased during the last few decades. However, the expected climatic conditions, with longer and more intense droughts, will make it necessary to reduce the water supply to garlic crops. This poses the urgent need to select garlic cultivars more tolerant to water scarcity and that are also more resistant to plant pathogenic organisms. The aim of this work was to analyze the influence of water stress on the host response of garlic plants to D. dipsaci. The specific objectives were to evaluate the level of nematode infestation in plants from four garlic genotypes treated with a reduced irrigation regime and compare them with those of control plants not subjected to water stress. The observed results were correlated with changes in the bulb and root development, as well as in the physiological parameters (total chlorophyll concentration and proline accumulation). The effects were different depending on whether the plants were subjected to water stress before or after nematode inoculation, as well as whether the water stress was continuous or discontinuous. Garlic inter-cultivar variability also affected the obtained results.

Hsp90 Gene Is Required for Mi-1-Mediated Resistance of Tomato to the Whitefly Bemisia tabaci.

The Mi-1 gene of tomato (Solanum lycopersicum) confers resistance against some nematodes and insects, but the resistance mechanisms differ depending on the harmful organism, as a hypersensitive reaction (HR) occurs only in the case of nematodes. The gene Rme1 is required for Mi-1-mediated resistance to nematodes, aphids, and whiteflies, and several additional proteins also play a role in this resistance. Among them, the involvement of the chaperone HSP90 has been demonstrated in Mi-1-mediated resistance for aphids and nematodes, but not for whiteflies. In this work, we studied the implication of the Hsp90 gene in the Mi-1 resistance against the whitefly Bemisia tabaci by means of Tobacco rattle virus (TRV)-based virus-induced gene silencing (VIGS). The silencing of the Hsp90 gene in tomato Motelle plants carrying the Mi-1 gene resulted in a decrease in resistance to whiteflies, as oviposition values were significantly higher than those on non-silenced plants. This decrease in resistance was equivalent to that caused by the silencing of the Mi-1 gene itself. Infiltration with the control TRV vector did not alter Mi-1 mediated resistance to B. tabaci. Similar to the Mi-1 gene, silencing of Hsp90-1 occurs partially, as silenced plants showed a significant but not complete suppression of gene expression. Thus, our results demonstrate the requirement of Hsp90 in the Mi-1-mediated resistance to B. tabaci and reinforce the hypothesis of a common model for this resistance to nematodes and insects.

Basal differences in the transcriptional profiles of tomato leaves associated with the presence/absence of the resistance gene Mi-1 and changes in these differences after infestation by the whitefly Bemisia tabaci.

The tomato Mi-1 gene mediates plant resistance to whitefly Bemisia tabaci, nematodes, and aphids. Other genes are also required for this resistance, and a model of interaction between the proteins encoded by these genes was proposed. Microarray analyses were used previously to identify genes involved in plant resistance to pests or pathogens, but scarcely in resistance to insects. In the present work, the GeneChip™ Tomato Genome Array (Affymetrix®) was used to compare the transcriptional profiles of Motelle (bearing Mi-1) and Moneymaker (lacking Mi-1) cultivars, both before and after B. tabaci infestation. Ten transcripts were expressed at least twofold in uninfested Motelle than in Moneymaker, while other eight were expressed half or less. After whitefly infestation, differences between cultivars increased to 14 transcripts expressed more in Motelle than in Moneymaker and 14 transcripts less expressed. Half of these transcripts showed no differential expression before infestation. These results show the baseline differences in the tomato transcriptomic profile associated with the presence or absence of the Mi-1 gene and provide us with valuable information on candidate genes to intervene in either compatible or incompatible tomato-whitefly interactions.

Temperature-dependent development of Aleyrodes proletella (Homoptera: Aleyrodidae) on two cultivars of broccoli under constant temperatures.

Laboratory experiments were conducted to estimate developmental rates and nymphal survival of Aleyrodes proletella Linnaeus (Homoptera: Aleyrodidae) on two broccoli Brassica oleracea L. variety italica Plenck cultivars (Marathon and Agripa) at eight constant temperatures (16, 18, 20, 22, 24, 26, 28, and 30 degrees C). The times required to complete development of egg and first instar decreased with increasing temperature, but the developmental times of second, third, fourth instars, all instars, and egg-adult period were greater at 30 degrees C than at 28degrees C. The relationships between developmental rate of A. proletella and temperature were slightly influenced by broccoli cultivar. The optimal temperatures and thermal constant as well as the lower and upper thresholds of development for all immature stages were estimated by fitting the observed developmental rates versus temperature with a nonlinear model and two linear models. For all stages, graphs obtained by plotting the developmental rates against temperature could be described by the modification two of the Logan's model. Overall, developmental times for immature stages and egg-adult periods were similar on both Agripa and Marathon cultivars. The most favorable temperature range for nymphal development seemed to be 28-29 (second and third instars) and 31-33 degrees C (fourth instar). Mean generation times (egg-adult) ranged from 19 d ('Marathon' and 'Agripa') at 28 degrees C to 47 ('Marathon') and 46 d ('Agripa') at 16 degrees C.

Benzothiadiazole induces local resistance to Bemisia tabaci (Hemiptera: Aleyrodidae) in tomato plants.

Plant resistance to the B and Q biotypes of sweetpotato whitefly, Bemisa tabaci (Gennadius), induced by benzo [1,2,3] thiadiazole-7-carbothioic acid-S-methyl ester (BTH or acibenzolar-S-methyl) in tomato 'Marmande' plants was evaluated in free-choice and no-choice assays under different conditions. BTH is the active ingredient of the Syngenta plant activator Bion. BTH treatment affected host preference of B. tabaci (B and Q biotypes) adults on plants sprayed with Bion at 0.2 and 0.4 g/liter during the earlier days of free-choice assays. As a consequence, a decrease in the total number of eggs (although female fecundity was not affected) and in the final number of pupae and empty pupal cases was observed. The effect produced by BTH applied at 0.1 g/liter Bion was not significant. In no-choice assays, a reduction of the numbers of first-stage larvae and total individuals and a delay in insect development were observed when local treatment was restricted to one leaflet per plant, 5 d before B. tabaci (biotype B) infestation. This acquired resistance induced by BTH seemed to be locally expressed because of the differences between treated and nontreated leaflets in the same plants, whereas no differences in nontreated leaflets were observed between BTH-treated and control plants.

Rme1 is necessary for Mi-1-mediated resistance and acts early in the resistance pathway.

The tomato gene Mi-1 confers resistance to root-knot nematodes (Meloidogyne spp.), potato aphid, and whitefly. Using genetic screens, we have isolated a mutant, rme1 (resistance to Meloidogyne spp.), compromised in resistance to M. javanica and potato aphid. Here, we show that the rme1 mutant is also compromised in resistance to M. incognita, M. arenaria, and whitefly. In addition, using an Agrobacterium-mediated transient assay in leaves to express constitutive gain-of-function mutant Pto(L205D), we demonstrated that the rme1 mutation is not compromised in Pto-mediated hypersensitive response. Moreover, the mutation in rme1 does not result in increased virulence of pathogenic Pseudomonas syringae or Mi-1-virulent M. incognita. Using a chimeric Mi-1 construct, Mi-DS4, which confers constitutive cell death phenotype and A. rhizogenes root transformation, we showed that the Mi-1-mediated cell death pathway is intact in this mutant. Our results indicate that Rme1 is required for Mi-1-mediated resistance and acts either at the same step in the signal transduction pathway as Mi-1 or upstream of Mi-1.

The root-knot nematode resistance gene Mi-1.2 of tomato is responsible for resistance against the whitefly Bemisia tabaci.

The tomato gene Mi-1.2 confers resistance against root-knot nematodes and some isolates of potato aphid. Resistance to the whitefly Bemisia tabaci previously has been observed in Mi-bearing commercial tomato cultivars, suggesting that Mi, or a closely linked gene, is responsible for the resistance. The response of two biotypes of B. tabaci to tomato carrying the cloned Mi was compared with that of the isogenic untransformed tomato line Moneymaker. Our results indicate that Mi-1.2 is responsible for the resistance in tomato plants to both B- and Q- biotypes. Mi-1.2 is unique among characterized resistance genes in its activity against three very different organisms (root-knot nematodes, aphids, and whiteflies). These pests are among the most important on tomato crops worldwide, making Mi a valuable resource in integrated pest management programs.