Effects of pear tree physiology on fire blight progression in perennial branches and on expression of pathogenicity genes in Erwinia amylovora

Abstract

The interaction between Erwinia amylovora (the causal agent of fire blight) and the physiological status of pear trees was examined under orchard conditions. The physiological status of the trees was defined qualitatively, using host phenology and vigour as measures, and quantitatively, using the sorbitol content in annual shoots as a measure. Qualitatively, tree response to fire blight was governed by phenological stage at the time of infection and vigour: low vigour trees inoculated in the autumn (just before entering dormancy) and high vigour trees inoculated in the spring (soon after bloom) were more susceptible than high vigour trees inoculated in the autumn and low vigour trees inoculated in the spring. Quantitatively, the rate of symptom progression in perennial branches (SPR) was significantly (P £ 0.001) correlated to the absolute value of the rate of sorbitol content change (|SCR|). The relationship between hrp genes expression of transformed E. amylovora (estimated according to hrpE and hrpJ expression) and |SCR| was determined on 1 year-old trees. Expression of hrp genes was significantly correlated with |SCR| (P = 0.004) and 63.5% of the variability in the hrp genes expression was attributed to |SCR| values. The expression of hrp genes increased gradually and asymptotically with increasing |SCR| values; further increase in |SCR| did not affect the expression.Abbreviations: SPR -the rate of symptom progression in perennial branches; |SCR| -the absolute value of the rate of sorbitol content change.

Erwinia amylovora Expresses Fast and Simultaneously hrp/dsp Virulence Genes during Flower Infection on Apple Trees

“…Although invasion of flowers is responsible for primary contamination with fire blight and substantially contributes to epidemics [2] , molecular knowledge about the early infection process regarding bacterial virulence gene expression is absent. Regarding responsive plant gene expression, knowledge is limited to studies from leaves, stems, shoots, in vitro plantlets or immature fruits [5] , [6] , [7] , [8] , [9] , [10] , [11] . Only one study investigated the plant gene expression in detached flowers upon E. amylovora inoculation, but not bacterial gene expression [12] .…”

“…Also in E. amylovora the type III secretion system was shown to be essential for floral as well as shoot infections [8] , [15] . Main efforts to understand the early infection process by E. amylovora are based on studies in vegetative plant parts or immature pear, where infection has to be artificially assisted by wounding the plant [6] , [9] , [10] , [11] . However, the time point when E. amylovora genetically activates its type III secretion system, especially if not assisted by wounding and in the presence of the full plant defense such as in infections of flowers attached to the tree, still remains to be elucidated.…”

Levels of fire blight (Erwinia amylovora) susceptibility of native apple, pear and quince germplasm from Lake Van Basin, Turkey

“…The symptoms may be seen on the flowers, shoots, leaves, branches, trunks and fruits, and kill the tree within a single season by causing cankers and severe infections on the shoots, branches and trunks (Thomson 2000;Blachinsky et al 2006;Yom Din et al 2007). Fire blight is therefore a bacterial disease that is difficult to control.…”

Erwinia spp. from pome fruit trees: similarities and differences among pathogenic and non-pathogenic species

“…Rosaceous plants contain sorbitol and sucrose as storage and transport carbohydrates, and the distribution of these carbohydrates is dependent on the environmental conditions, species, and plant tissue (Blachinsky et al 2006;Zhang and Geider 1999). Sorbitol is the dominant sugar alcohol in such plants and is used for carbohydrate transport rather than sucrose, which is utilized in many other plants.…”