Location matters: from changing a presumption about the Citrus tristeza virus tissue tropism to understanding the stem pitting disease.

Abstract

Stem pitting is a common virus-induced disease phenotype that tremendously impacts growth of perennial woody plants. How stem pitting develops in the infected trees remains unclear. Here, we assessed the development of stem pits upon infection of citrus by Citrus tristeza virus (CTV), which has been regarded as 'phloem-limited'. By taking advantage of a highly susceptible virus host - Citrus macrophylla - and a CTV isolate lacking a viral effector - the p33 protein, the development pattern of stem pitting was revealed via time-course observations and histological analyses. The stem pits result from the virus-colonized nonlignified 'gumming' malformations which are initiated by virus invasion into multiple spatially separated tissue layers - protophloem, metaphloem, and, unexpectedly, metaxylem. Notably, invasion of CTV into the unspecialized metaxylem cells interrupted the differentiation of the xylem tracheary elements. With the radial spread of CTV into the adjacent cells towards the stem periphery, the clusters of virus-colonized immature metaxylem cells extended in size, merging, at a certain stage, with virus-bearing cells in the protophloem and metaphloem layers. Collectively, our data provide a new insight into the process of the stem pitting development and the role of the xylem tissue in the virus pathogenicity.