Abstract
While host gene expression has been related to symptoms associated with different phytoplasma diseases, it is unknown why some phytoplasmas are associated with different symptoms in genotypes of the same plant species. Pear tree selections showed symptoms of either leaf reddening (selection 8824-1) or leaf curling (selection 9328-1) associated with pear decline (PD) phytoplasma presence. PD populations were similar in leaves and shoots of the two selections, but in the roots, populations were significantly lower in selection 8824-1 than in 9328-1, indicating greater resistance. For host carbohydrate metabolism gene expression in PD-infected tissues, significant up-regulation in selection 8824-1 was observed for a sucrose synthase gene in leaves and an acid invertase gene in leaves and roots. These features have been associated with localized higher sugar levels in phytoplasma-infected tissues, and thus may be related to leaf reddening. For host stress/defense response gene expression in PD-infected tissues, significant up-regulation of a phenylalanine ammonia lyase gene was observed in PD-infected shoots of both selections; however, up-regulation of alcohol dehydrogenase gene in shoots, a chitinase gene in all tissues and a phloem protein 2 gene in roots was only observed for selection 8824-1. These changes indicate greater triggered innate immunity in roots associated with lower PD populations and leaf reddening. Leaf reddening may be related to changes in gene expression associated with increased sugar levels in leaves and stronger immune responses in several tissues, while leaf curling may be due to water stress resulting from dysfunctional root associated with higher PD populations in the roots.
Highlights
Pear decline (PD) disease of pear (Pyrus communis L.) is associated with the presence of Candidatus Phytoplasma pyri, a member of the apple proliferation group (16SrX group), that causes a slow death of pear trees on P. calleryana and P. communis rootstocks
Symptoms of leaf reddening in selection 8824-1 could be related to it being more tolerant than selection 9328-1 as indicated by lower PD phytoplasma populations in its roots
The tolerance may be expressed as anthocyanin accumulation in leaves and higher expression of genes for alcohol dehydrogenase, class III chitinase and phloem protein 2 following infection
Summary
Pear decline (PD) disease of pear (Pyrus communis L.) is associated with the presence of Candidatus Phytoplasma pyri, a member of the apple proliferation group (16SrX group), that causes a slow death of pear trees on P. calleryana and P. communis rootstocks. PD symptoms include poor fruit development, reduced shoot growth, phloem necrosis, leaf curling and premature leaf reddening [1]. Leaf curling begins in late summer and is most noticeable in the fall, when the midrib curls downward resulting in the leaf tip sometimes touching the ventral side of the midrib or petiole, forcing the leaf blades to curl upwards. Curled leaves may become thickened and acquire a deep red purplish hue. Premature foliar reddening contrasts with the leaf yellowing typical of normal leaf senescence on healthy trees [2]. Within P. communis, some cultivars, such as Bartlett, Old Home and Beurre Hardy, show PD-induced leaf curl symptoms with little leaf reddening, while other cultivars, such as Comice, show leaf reddening with only slight leaf curling, indicating that there is a host genotype effect on foliar symptoms [3,4]
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