The Nacreous Sieve-Element Wall in Helthy and in Mlo-Infected Apple Trees

Abstract

The development, ultrastructure, and histochemical composition of the nacreous sieve-element wall were studied in axial and petiolar phloem of young shoots of healthy and mycoplasma-like organism (MLO)-infected apple trees, Malus domestica Borkh. In the protophloem, the sieve-element walls are single-layered and consist of transversely oriented fibrils. In the first-formed sieve elements of the metaphloem, a segregation of the fibrils that constitute the cell wall leads to the formation of a middle wall layer of uneven thickness. The sieve-element walls now appear three-layered, with a thin inner, a thick middle, and a thin outer layer. The outer and the inner layers are rich in cellulose and pectin and consist of densely packed fibrils arranged perpendicular to the cell axis. The middle layer is cellulose- and pectin-poor and consists of an electron-translucent matrix with few, loosely embedded fibrils of variable orientation. The three-layered wall reaches its maximum thickness at sieve-element maturity but diminishes as the sieve element ages. The sieve elements of the axial phloem lose their wall thickenings faster than the sieve elements of the petiolar phloem. In healthy trees, only the youngest sieve tubes of the axial phloem have wall thickenings in autumn, and collapsed sieve tubes never have wall thickenings. In MLO-infected apple trees, however, the shoots that develop during the second flush (witches' brooms) show wall thickenings in autumn in most living sieve tubes, and in some collapsed sieve elements, of the axial phloem. This is a result of the younger developmental stage of the second flush shoots and of disease-related premature sieve-tube collapse, respectively. No differences in the ultrastructure or in the staining properties of the sieve-element wall were found between healthy and diseased apple trees. Therefore, we conclude that MLOs do not influence the sieve-element wall in M. domestica.