Transmission of phytoplasmas to plants occurs when leafhoppers feed from plant phloem. The bacteria are introduced into plant phloem with insect saliva. In plants, phytoplasmas remain restricted to the phloem tissue (Fig. 7) where they systemically spread throughout the plant.

Fig. 7. Aster Yellows isolate Witches' Broom (AY-WB) phytoplasmas (arrows) in two adjacent sieve elements (se1 and se2) of an infected aster leaf; note the rudimentary nucleus (n) in a maturing sieve element (se1), next to a companion cell (cc). Other abbreviations used in figure: pl, plastid; spl, sieve plate. Scale bar = 1 mm.

Phytoplasmas cause severe symptoms (Fig. 8), such as, stunting, phyllody, witches broom, yellowing, and yield losses in over 300 economically important plant species worldwide. These organisms are not transmitted via plant seeds, but may be transovarially transmitted to next-generation leafhoppers. The molecular and genetic mechanisms underlying the diverse interactions between plant pathogenic phytoplasmas and their plant or insect hosts remain unknown.

Fig. 8. Nicotiana benthamiana plants infected with Aster Yellows strain Witches' Broom phytoplasma (AY-WB). The inset shows a close-up of one of the AY-WB-infected plants.