Phytoplasma-Triggered Ca2+ Influx Is Involved in Sieve-Tube Blockage

Phytoplasmas are obligate, phloem-restricted phytopathogens that are disseminated by phloem-sap-sucking insects. Phytoplasma infection severely impairs assimilate translocation in host plants and might be responsible for massive changes in phloem physiology. Methods to study phytoplasma- induced changes thus far provoked massive, native occlusion artifacts in sieve tubes. Hence, phytoplasma-phloem relationships were investigated here in intact Vicia faba host plants using a set of vital fluorescent probes and confocal laser-scanning microscopy. We focused on the effects of phytoplasma infection on phloem mass-flow performance and evaluated whether phytoplasmas induce sieve-plate occlusion. Apparently, phytoplasma infection brings about Ca2+ influx into sieve tubes, leading to sieve-plate occlusion by callose deposition or protein plugging. In addition, Ca2+ influx may confer cell wall thickening of conducting elements. In conclusion, phytoplasma effectors may cause gating of sieve-element Ca2+ channels leading to sieve-tube occlusion with presumptive dramatic effects on phytoplasma spread and photoassimilate distribution.