Resistance mechanisms of Solanum species to Myzus persicae

Alvarez, A.E. (2007), Resistance mechanisms of Solanum species to Myzus persicae, Ph.D. thesis, Wageningen University, Wageningen, the Netherlands. The aphid Myzus persicae (Sulzer) constitute a threat to potato crops because of their efficiency to transmit viruses. Many wild Solanum species have been reported to have resistance to M. persicae. These species represent an important potential source of resistance, which can be used to enhance resistance in crops. The aim of this thesis was to find and study in tuber bearing Solanum species different mechanisms of resistance to M. persicae that also would reduced the spread of viruses. Resistance and susceptibility of wild Solanum genotypes and cultivated potatoes were characterized and linked to plant-aphid interactions with respect to aphid behaviour and performance and plant responses to aphid attack. A combination of aphid colony-development assay with the study of probing activities was used to localize resistance factors in tissues of Solanum genotypes. Genotypes with different degrees of resistance were found. S. stoloniferum showed pre-phloem resistance to M. persicae, and hence it was used as a model plant to unravel and characterise phenotypically and at molecular level the interactions of a single plant with different aphid species. Aphid performance, settling behaviour, and probing studies of M. persicae and M. euphorbiae on S. stoloniferum showed that while S. stoloniferum could be a host plant for M. euphorbiae, it is a poor host for M. persicae. The resistance found in S. stoloniferum against M. persicae seems to rely on constitutively expressed physical traits with some age effects. The transcriptional response of S. stoloniferum to the attack of M. euphorbiae (compatible interactions) was stronger than in response to M. persicae (incompatible). This stronger response to M. euphorbiae involves up-regulation of genes related to pathogenesis (PR), regulatory, and protein metabolism and down-regulation of regulatory genes, general metabolism and photosynthesis related genes. Infestation of S. stoloniferum leaves with M. persicae aphids leads to the development of pustules. Microscopic analysis of the pustules showed a burst of tissue on the surrounding of the vascular bundle. In contrast, the infestation with M. euphorbiae did not induce any visible cellular changes. The formation of pustules and the induction of some genes in S. stoloniferum suggest that a similar situation might exist between plants-pathogenic bacteria interactions, and plants-aphids interactions. Response of S. tuberosum cv. Kardal to the attack of M. persicae was evaluated by studying gene expression. The plant responses depend on foliage maturity. Young leaves of cv. Kardal are resistant to M. persicae whereas mature to senescent leaves are susceptible. In old leaves M. persicae attack elicits higher number of differentially regulated genes than in young leaves. The transcriptional results obtained with the two systems: (1) S. stoloniferum after the attack of M. persicae and M. euphorbiae, and (2) S. tuberosum cv. Kardal at different maturity leaf stages after the attack of M. persicae were compared. The gene-expression studies provide evidence that S. tuberosum and S. stoloniferum respond by activating the salicylic acid (SA) and ethylene (ET) pathways. Genes responsive to jasmonic acid were differentially regulated in low number in the S. stoloniferum-M. euphorbiae but not in the cv. KardalM. persicae interaction. At the local level a compatible plant-aphid interaction resulted in a broader gene expression response than that a systemic level. Genes were identified related to changes in sink-source relationship at the feeding site which may indicate a plant manipulation by the aphids related to the process of changing the physiological status of the tissue towards a local metabolic sink; also genes related to signal-transduction pathways, regulation and signalling, protein metabolism, maintenance of cell homeostasis, transport, secondary metabolism, and structural features were found to be differentially regulated. On the contrary, in incompatible interactions the transcriptional response of the plant seems to be more restricted. PLRV-infection in potato plants of cv. Kardal was found to affect aphid behaviour. M. persicae responses to volatiles emitted from PLRV-infected and non-infected plants depend on the age of the leaf. PLRV infection affects also the probing behaviour of M. persicae. On virus-infected plants stylet penetration into the plant tissue is enhanced. The transmission efficiency of PLRV is also expected to be affected in PLRV-infected plants because these plants attract more aphids than noninfected plants. The results obtained on this research (e.g., aphid performance, probing behaviour, colony development, settling behaviour, and gene expression analysis of Solanum spp. attacked by aphids) contribute to the understanding of plant responses towards aphid attack as a basis for further unravelling the resistance mechanisms at the metabolic, molecular, and genetic levels.