abscisic acid, apple proliferation, auxin, jasmonates, phloem,
Candidatus Phytoplasma mali, phytohormones, plant defence,
plant-phytoplasma interaction, salicylic acid,
Apple proliferation (AP) is caused by a psyllid-transmitted phytoplasma and is one of the most economically important diseases on apple in Europe. AP was first reported in northern Italy and in the following years from many European countries. Previous research on phytoplasma-induced diseases of rosaceous fruit trees (mainly AP, pear decline and European stone fruit yellows), exhibits strong evidence that phloem injuries play a central and probably universal role in phytoplasma pathogenicity. The effect of phytoplasma infections on phloem function and the resulting diseases received little attention in the past. Phytoplasma infection severely impairs assimilate translocation in host plants and might be responsible for massive changes in phloem physiology including signalling components. As shown for other phytoplasma species, infection brings about Ca2+ influx into sieve tubes, leading to sieve-tube occlusion by callose deposition or protein plugging, which is assumed also for AP phytoplasma. Effectors may cause gating of sieve-element Ca2+ channels leading to sieve-tube occlusion with presumptive dramatic effects on phytoplasma spread, photoassimilate distribution and the whole phloem physiology. However, there is indication that phloem loading is affected by phytoplasma infection. As sieve elements need a permanent input of energy to ensure their viability, sugar metabolism and the associate energy production of the companion cells have a dramatic impact on the physiological fitness of phloem function. It is presumptive that signalling substances are produced prior sieve-element occlusion to ensure the spread throughout the plant body. Analyses of diverse phytohormones in response to challenge with AP phytoplasma show a strong increase of salicylic acid accompanied by a decrease of jasmonic acid. This supports the idea that phytohormones have a role in plant defence signalling against phytoplasma infection. It is a matter of debate whether mechanisms involved in phloem impairment could differ between pathosystems and vary with the plant susceptibility to infection.