Gloria Nombela

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.

Differential Variation in Development of the B- and Q-Biotypes of Bemisia tabaci (Homoptera: Aleyrodidae) on Sweet Pepper at Constant Temperatures

Abstract Developmental rates of the B- and Q-biotypes of Bemisia tabaci (Gennadius, 1889) were studied at seven constant temperatures (17, 20, 23, 26, 30, 33, and 35°C) on sweet pepper, Capsicum annuum L. (‘Morrón’). The egg incubation period and the times required to complete development at all immature stages decreased with increasing temperature up to 33°C, but at 35°C were found to be greater than at 33°C. The relationships between developmental rate of B. tabaci and temperature were influenced by the insect biotype. The lower and upper developmental thresholds as well as the optimal temperatures and thermal constant for the preoviposition period and 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 reciprocal of development times against temperature could be described by the modification 2 of the Logan’s model. The simple linear model tT = K+ct suffices for predicting B- and Q-biotype phenologies on sweet pepper for the temperature range of 17–33°C. The shortest developmental times as well as the lowest developmental thresholds and thermal constant were mostly obtained with the Q-biotype. Overall, the most favorable temperature range appeared to be 31–33°C. Mean generation times (adult-adult) ranged from 17 d (Q-biotype) and 18 d (B-biotype) at 33°C to 49 d (B-biotype) and 46 d (Q-biotype) at 17°C.

A differential interaction study of Bemisia tabaci Q-biotype on commercial tomato varieties with or without the Mi resistance gene, and comparative host responses with the B-biotype

Three tomato varieties (Motelle, Ronita, and VFN8) bearing the Mi‐1.2 gene providing resistance to nematodes Meloidogyne spp. and to the potato aphid Macrosiphum euphorbiae Thomas, and three varieties not bearing this gene (Moneymaker, Roma, and Río Fuego), were compared by choice assay for host preference using the Q‐biotype of Bemisia tabaci (Gennadius). The most preferred hosts, determined by infestation levels and numbers of feeding adults were Moneymaker, Río Fuego and Roma, all of which were not carrying the Mi gene. Ronita and Motelle, both of which bore the Mi gene, were the least preferred hosts. In a no‐choice assay, B. tabaci females laid a significantly lower number of eggs on the varieties that carried the Mi gene than on those lacking the gene. Differences were more dramatic when plants carrying the Mi gene were pooled together and compared with pooled plants without this gene. Significantly greater values were obtained for the Mi‐lacking group for all parameters tested. Comparing these results with those from a previous study on the B‐biotype of B. tabaci, Q‐biotypes were found to produce higher daily infestation rates on most of the tomato varieties. When results from plants carrying Mi were pooled, they showed lower infestation levels of Q‐biotypes than B‐biotypes. The Q‐biotype infested less Mi‐plants and more non‐Mi plants than B‐biotype. Q‐biotype females produced significantly less pupae than the B‐biotype females on both groups of plants. These results suggest the existence of an antixenosis and antibiosis‐based resistance to the Q‐biotype of B. tabaci in Mi‐bearing commercial tomato varieties, which is greater than that previously reported for the B‐biotype.

Analysis by DC-EPG of the resistance to Bemisia tabaci on an Mi-tomato line

The tomato Mi gene confers resistance to nematodes, Meloidogyne spp., and to the potato aphid, Macrosiphum euphorbiae (Thomas). Previous greenhouse choice assays with Bemisia tabaci (Gennadius) showed that tomato commercial varieties carrying this gene had significantly lower values of host suitability and whitefly reproduction than varieties lacking Mi. This indicated that Mi, or another gene in its region, could regulate partial resistance. In order to characterise this resistance, probing and feeding behaviour of Bemisia tabaci B‐biotype was studied with DC Electrical Penetration Graph (EPG) technique on the near‐isogenic tomato lines Moneymaker (without Mi) and Motelle (carrying Mi). Significant differences (P < 0.05) between tomato lines were found in EPG parameters related to epidermis and/or mesophyll tissues. On Motelle, a lower percentage of whiteflies achieved phloem phase and they made more probes before attaining first phloem phase, had a higher ratio (number of probes before first phloem phase)/(total number of probes), had a longer total duration of non‐probing time, and a longer time before making the first intracellular puncture and before making the first phloem phase. In contrast, most of the parameters related to phloem phase were found not to differ significantly between these near‐isogenic lines. The behavioural data strongly suggest that the partial resistance in the variety Motelle is due to factors in the epidermis and/or mesophyll that inhibit the whiteflies from reaching phloem sieve elements. However, once the stylets reach a sieve element, whitefly behaviour did not differ between the two varieties. Thus, phloem sap of the two varieties appears to be equally acceptable to the whiteflies. Further studies are necessary to provide a better understanding of these mechanisms of resistance to whiteflies in tomatoes.

Variation in tomato host response to Bemisia tabaci (Hemiptera: Aleyrodidae) in relation to acyl sugar content and presence of the nematode and potato aphid resistance gene Mi

Two commercial cultivars of tomato, Alta and Peto 95, the accession line number LA716 of Lycopersicon pennellii and lines 94GH-006 and 94GH-033 (backcrosses between Peto 95 and LA716), with different leaf acyl sugar contents were screened for resistance to Bemisia argentifolii Bellows & Perring (corresponding to the Spanish B-biotype of Bemisia tabaci (Gennadius)), in greenhouse- and field-no-choice experiments. There was no oviposition on LA716 (with the highest acyl sugar content) while the greatest fecundity and fertility values were observed on the cultivar Alta (no acyl sugar content). However, no clear relationship was found between the low acyl sugar content in the other tomato cultivars tested and whitefly reproduction. Thus, resistance to B. tabaci did not appear to correlate with acyl sugar content below a threshold level of 37.8 microg cm-2 leaf. In a greenhouse choice-assay, B. tabaci exhibited reduced host preference and reproduction on the commercial tomato cultivars Motelle, VFN8 and Ronita all of which carry the Mi gene resistance to Meloidogyne nematodes and the aphid Macrosiphum euphorbiae (Thomas), than on the Mi-lacking cultivars Moneymaker, Rio Fuego and Roma. When data of Mi-bearing plants were pooled, the mean values for daily infestation and pupal production of B. tabaci were significantly lower than those of Mi-lacking plants. This reflected a level of antixenosis- and antibiosis-based resistance in commercial tomato and indicated that Mi, or another closely linked gene, might be implicated in a partial resistance which was not associated either with the presence of glandular trichomes or their exudates. These findings support the general hypothesis for the existence of similarities among the resistance mechanisms to whiteflies, aphids and nematodes in commercial tomato 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.

Within‐plant distribution and infestation pattern of the B‐ and Q‐biotypes of the whitefly, Bemisia tabaci, on tomato and pepper

Two biotypes (B and Q) of the sweetpotato whitefly, Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae), are present in Spain (Guirao et al., 1997). These whiteflies cause serious damage by feeding on the underside of leaves and by transmitting devastating plant viruses, such as tomato yellow leaf curl viruses (Carnero et al., 1990; Brown & Bird, 1996; Moriones et al., 1993; Bedford et al., 1994; Blua & Toscano, 1994; Markham et al., 1996; Jiang et al., 1999). Although both biotypes are major pests of horticultural crops in many of the tomato and pepper-growing areas, recent studies on some hosts have shown that the Q-biotype infests a higher percentage of plants and develops faster than the B-biotype (Muniz, 2000; Nombela et al., 2001). Although growers use insecticides to control whiteflies, no monitoring technique has been specifically developed for the Q-biotype. Reliable sampling methods are critical to the development of monitoring programs for pest management application (Naranjo & Flint, 1995; Naranjo, 1996; Ellsworth & Martinez-Carrillo, 2001). Location of whiteflies on the different plant strata and accurate estimation of the infestation capacity (percentage of plants infested by a certain number of insects) of B. tabaci are important aspects in the study of its biology and ecology. In the last ten years, several studies have determined the spatial within-plant distribution of immature stages of B. tabaci in many important crops such as cotton (Butler & Vir, 1990; Rao et al., 1991; Naranjo & Flint, 1994, 1995), peanut (Lynch & Simmons, 1993), melon (Tonhasca et al., 1994), tomato (Carnero & Gonzalez-Andujar, 1994; Shuster, 1998), and alfalfa (Yee et al., 1997). The immature stage may be the stage that is most correlated to plant damage, but immatures are difficult and timeconsumming to count. Consequently, it is likely that the adult stage will continue to be the center of focus for pest management application (Naranjo, 1996). To our knowledge, no reports exist on the within-plant distribution of B. tabaci adults in pepper and tomato. Another main aspect of the spatial distribution (whose knowledge could help growers to make correct decissions on whiteflies control) is between-plant or within-field distribution which is typically described with a theoretical distribution such as the Poisson or negative binomial, or with models as Taylor’s power law (Taylor, 1961) or Iwao’s patchiness regression (Iwao, 1968) which relate the mean and variance over a range of population densities (Naranjo, 1996). In addition, there has been limited effort to describe the inter-relationships between different biotypes of B. tabaci and their host plants (Muniz, 2000; Muniz & Nombela, 1997a,b; Nombela et al., 2000, 2001). Our objectives were to obtain, under greenhouse conditions, the within-plant distribution pattern of both B-and Q-biotype adults of B. tabaci for tomato plants, and to investigate the relationship between the proportion of infested plants and the number of adults present on pepper or tomato plants by a descriptive model under specific greenhouse conditions.

Preinfestations of tomato plants by whiteflies (Bemisia tabaci) or aphids (Macrosiphum euphorbiae) induce variable resistance or susceptibility responses

In addition to constitutive plant resistance against pests or pathogens, plants can activate protective mechanisms upon contact with an invader or a chemical elicitor. Studies on induced plant resistance to herbivores, especially piercing-sucking insects, are less abundant than those devoted to pathogens. Several experiments under controlled conditions have been conducted to demonstrate that infestations by Macrosiphum euphorbiae induce plant resistance to Bemisia tabaci in susceptible tomato plants. After three days of exposure to 20 apterous adult aphids, the plants acquired a transiently induced resistance to B. tabaci when aphid removal occurred one or 18 hours prior to B. tabaci infestation; the effect disappeared when four days passed between aphid and whitefly infestations. The resistance observed was both locally and systemically induced. Other assays were performed to evaluate the effect of preinfestation with ten adults of B. tabaci during 48 h on the tomato responses to two different clones (Sp and Nt) of M. euphorbiae. The numbers of nymph and adult aphids were counted after the same time interval as the pre-reproductive period and 20 (Sp clone) or 22 (Nt clone) days after adult aphid removal. The tomato responses induced by whitefly feeding depend on the aphid clone. For the Sp clone, the number of aphid nymphs ten days after adult removal was significantly higher on whitefly preinfested plants than on uninfested plants. However, no significant differences were observed when the aphid clone Nt was tested. The duration of plant response to a previous infestation by B. tabaci is apparently limited.

Comparative Host Suitability of Some Brassica Cultivars for the Whitefly, Aleyrodes proletella (Homoptera: Aleyrodidae)

Abstract Four cultivars of broccoli (Brassica oleracea L. variety ‘italica’), two cultivars of early cauliflower (Brassica oleracea L. variety ‘botrytis’), four cultivars of late cauliflower, and one cultivar of red cabbage (Brassica oleracea L. variety ‘capitata’) were screened to determine some reproductive parameters of Aleyrodes proletella L. in a no-choice assay. The highest and lowest oviposition rates and production of pupae and adults were obtained with late cauliflower (cultivar Picasso) and red cabbage (cultivar Cabeza negra), respectively. The highest percentages of adult emergence (indicating survival from egg to adult) were obtained on broccoli (cultivar Chevalier) and late cauliflower (cultivars Mayfair and Picasso), whereas the lowest was obtained on late cauliflower (cultivar Arbon). In a choice experiment, A. proletella preferred late cauliflower (cultivar Picasso) and broccoli (cultivar Agripa) to red cabbage (cultivar Cabeza negra). Significantly more adults per day, and more pupae and empty pupal cases per plant, were found on broccoli and cauliflower cultivars than on red cabbage. In another no-choice assay at 22 ± 1.5°C, A. proletella required significantly more days for development on red cabbage than on broccoli and cauliflower cultivars. A. proletella developed significantly faster on broccoli cultivars Agripa and Chevalier and late cauliflower cultivars Mayfair and Picasso. These results suggest that it is important to minimize the use of broccoli (cultivars Agripa and Chevalier) and late cauliflower (cultivars Mayfair and Picasso) to avoid the risk of further expansion of whitefly populations where these Brassica crops and A. proletella are present.

Nematodes as bioindicators of dry pasture recovery after temporary rye cultivation

Abstract Soil and plant parasitic nematodes were sampled from the top 10 cm of a luvisol in dry grassland north of Madrid (Central Spain). A natural pasture and two grazed pastures which had been cropped with rye 1 and 6 yr before, respectively, were compared by multivariate statistical analysis of the nematode fauna. Results from stepwise discriminant analysis demonstrated the influence of agricultural management on the abundance of the nematode community of these pasture soils. Specifically, it was found that Paratylenchus and Rhabditidae can have a discriminative value of the recovery of these soil ecosystems. Among the indices used to analyze the abundance, diversity and maturity of the nematode community, only plant parasite index (PPI) detected significant differences among pastures, decreasing its values as recovery time after human intervention increased, with the significantly highest value in the recently cropped (C) pasture. It is concluded that length of time after cropping influences the abundance more than the diversity of the nematodes present in pastures of these areas, and that multivariate statistical analyses are the most useful tests to detect differences. The role of nematodes as biological indicators of the degree of recovery of these soils after human disturbance is noted.