Bacterial Resistance

BACTERIAL RESISTANCE

Every year bacterial diseases cause loss of yield on both tropical and temperate fruit trees. They have effects varying from death of the entire plant to loss of quality of fruits. Important bacterial diseases of fruit trees are fire blight (apple, pear, quince and other ornamental species of Rosaceae caused by Erwiniaamylovora), bacterial blight and canker of stone fruits (by Pseudomonas syringae), blight of persian walnut(by Xanthomonas campestris pv. Juglandis) and canker of citruses (by Xanthomonas citri). Research on resistance to bacterial diseases has focused on genes producing anti-microbial proteins like lytic peptids (cercopins, attacins and synthetic analogs: shiva-1, SB-37), and lysozymes (egg white, T4 bacteriophage and human lysozyme). Transformation of apple Malling 26 by attacin E (Norelli et al., 1994; Borejsza-Wysocka etal., 1999), and pear cv. Passe Crassane by attacin E and SB-37 (Reynoird et al., 1999a, b; Mourgues et al., 1998) for resistance to E. amylovora,are examples of this approach. Recently, relationships betweenattacin expressed in transgenic apple and disease resistance were detected using immunoblot assays with the fusion attacin polyclonal antibody (Ko et al., 1999).

Recent advances in our understanding of harpin gene clusters of P. syringae and E. amylovora, the apoplast conditions for the expressionof these genes, their products and secretion systems, and their effectson host plants, have contributed to clarify the interaction between bacteria and host cells. Other strategies against bacteria effective also in fruit crops are represented by the induction of overproduction of H₂O₂ in the plant cells.

Hydrogen peroxide triggers local hypersensitive cell death, exerts direct antimicrobial activity (Peng and Ku‘c, 1992) and is involved in the reinforcement of plant cell wall (Bolwell et al., 1995). Glucose oxidase (Gox) gene from Aspergillus niger, which induces the production of H₂O₂, increased the level of resistance to Erwinia carotovora and Phytophthora infestans in potato (Wu et al., 1995) and to Pseudomonas syringae and Xanthomonas campestris in tomato (Caccia et al., 1999).The resistance orsusceptibility to pathogens can be modified by over expressing hormone genes (Fladung and Gieffers, 1993; Storti et al., 1994). These authors found an increase of resistance to fungi in tomato transgenic plants overexpressing auxin- and cytokinin-synthesising genes (iaaH or iaaM, ipt) from Agrobacterium tumefaciens, when the equilibrium of phytormone of transgenic plants was modified in favour of auxins. Transgenic kiwifruit plants and their transgenic offspring (resulting from crossing rolABC staminate cv GTH X normal pistillate Hayward) artificially infected with Pseudomonas siryngae and P. viridiflava, became more sensitive to these bacteria than untransformedplants (both cv. ‘GTH’ and ‘T1 offspring’ noncarrying rol genes) (Rugini etal., 1999; Balestra et al., 2001).