Profili di espressione di geni di difesa in genotipi di pomodoro

The development of an appropriate defence requires that plants recognize pests as early as possible. Consequently, plant responses are correlated to the mode of insect feeding and the degree of tissue damage at the feeding site. Although phloem-feeding pests, including aphids, cause extensive crop damage, little is known about how plants perceive these arthropods and activate endogenous defences. Aphids are highly specialized in their mode of feeding and impose a complex stress on plants. Differently from herbivorous pests, the prolonged interaction of aphid stylets and plant tissue causes minimal mechanical damage and consequently, plant respond to aphids activating various processes. They include a gene-for-gene recognition of aphid-derived elicitors by plant resistance genes and an activation of pathogen- and wound-induced pathways (Smith C.M. and Boyko E.V., 2007).
We analyzed the transcriptional changes of some defensive genes in tomato genotypes that are susceptible or resistant to aphids. The material analyzed included two landraces, AN5 and AN7, that were collected in Campania. These landraces have high levels of direct and indirect resistance to Macrosiphum euphorbiae in comparison to the control tomato cultivar M82.
A CAPS analysis of four resistance genes to biotic stress (including REX1, associated with Mi that confers resistance against the potato aphid, M. euphorbiae , and root-knot nematodes) did not show differences, being all the genotypes susceptible. The expression analysis of genes involved in direct and indirect defense against insects was carried out by real-time PCR in absence and after controlled infestations with M. euphorbiae. The genes analyzed were selected considering their known role in different biotic stress-response such as those involved in the synthesis of PR proteins (P4 and Pti4), direct and indirect defense against herbivores (TomLoxC, TomLoxD, HPL and GCS) and jasmonate biosynthesis (prosystemin).
The data indicated that the plant response to aphids involves different metabolic pathways, as expected. More interestingly, the results indicated that the constructive over-expression of genes that respond to aphid associates with higher level of direct and indirect defense in tomato. Differences after infestations between resistant and susceptible genotypes were limited. The results suggest that the resistance observed in the landraces AN5 and AN7, although probably complex and multifactorial, should be related to constitutive higher level of expression of defensive genes, implying that the modulation and the exploitation of endogenous plant defense should be a practicable strategy to improve tomato resistance against aphids.