Proceedings of the XLV Italian Society of Agricultural Genetics - SIGA Annual Congress

Salsomaggiore Terme, Italy - 26/29 September, 2001

ISBN 88-900622-1-5

Poster Abstract

HORIZONTAL DEFENSE RESPONSE IN TOMATO PLANTS WITH ALTERED PHYTOHORMONE EQUILIBRIA

BETTINI P.*, GUERRIERO I.*, MICHELOTTI S.*, PELLEGRINI M.G.*, MELANI L.*, BINDI D.*, GIANNINI R.**, CAPUANA M.**, BUIATTI M.*

* Dipartimento di Biologia animale e Genetica "Leo Pardi", Università degli Studi di Firenze

** Istituto per il Miglioramento Genetico delle Piante Forestali, C.N.R., Firenze

tomato, rol genes, horizontal resistance, physiological background

Horizontal resistance, particularly Systemic Acquired Resistance (SAR), is characterized by a long lasting, broad-spectrum resistance. These features make it attractive as an alternative to specific, gene-for-gene-based systems in order to confer to plants an increased resistance or tolerance to several pathogens. In this frame the physiological state of the plant, and especially the endogenous hormone balance, seem to play a crucial role in the establishment of the defense response. We have therefore obtained tomato plants from a cv. (Tondino) susceptible to the phytopathogenic fungus Fusarium oxysporum f.sp. lycopersici, whose hormone equilibrium has been altered through the introduction of Agrobacterium rhizogenes rolA, B, C genes and are currently testing them for the acquirement of "competence" for active defense through parameters known to be correlated with resistance such as tolerance to fungal toxins, inhibition of fungal growth in dual culture, PR-protein gene expression (Storti et al. 1992 Theor. Appl. Genet. 84: 123-128; Bettini et al. 1998 Theor. Appl. Genet. 97: 575-583).

Pleiotropic alterations, that may be indicative of phytohormone imbalance, were observed both for the rolA and rolC-transgenic plants, while the presence of rolB did not induce significant differences when compared to the controls. The introgression of the rolA gene had a major effect, with plants showing wrinkled, intensely green leaves, thick stems, less branching, reduced yield and small fruits often lacking seeds. This effect is in part consistent with the observations made on rolA tobacco plants, where the phenotypic alterations were suggested to reflect a functional alteration in the auxin/cytokinin balance in favour of cytokinin (Schmulling et al. 1993 The Plant Journal 3: 371-382). Interestingly, the phenotypic effect of the rolA gene in our system was related to the number of inserted copies, being more severe in the clones where more than one copy was present than in the ones harboring only one copy. rolC plants on the other hand were characterized mainly by a reduced height with respect to controls and higher branching, and also in this case the results are in agreement with the phenotype shown by transgenic tobacco, potato and petunia plants (Schmulling et al. 1993 The Plant Journal 3: 371-382; Winefield et al. 1999 Molecular Breeding 5: 543-551).

Plant tolerance to the toxin produced by F. oxysporum was assessed in ion leakage experiments in the presence of 5 mM fusaric acid. In all cases a higher resistance to the cellular damage caused by the toxin was observed in the plants harboring each of the three rol genes, with respect both to the regenerated, untransformed controls and the seed-grown original cv. Tondino. This behaviour suggests that the physiological modifications induced by the rol genes can indeed positively affect the plant defense response in our system.

Finally, basal PR-protein gene expression was analysed by means of Reverse Transcriptase-PCR for chitinase, b-1,3-glucanase and PR-1. Due to the mostly qualitative nature of the RT-PCR data, differences in amplification between the transgenic plants and the respective controls are not to be considered significant, unless confirmed by a quantitative technique. For this reason, a quantitative analysis based on Real Time Quantitative RNA PCR is currenltly underway using as a probe the PR-1 gene, known to be a marker of SAR.