The development of leaf disease symptoms and the accumulation of pathogenesis-related (PR) proteins were monitored in leaves of tobacco (Nicotiana tabacum cv. Xanthinc) plants colonized by the arbuscular mycorrhizal fungus Glomus intraradices. Leaves of mycorrhizal plants infected with the leaf pathogens Botrytis cinerea or tobacco mosaic virus showed a higher incidence and severity of necrotic lesions than those of nonmycorrhizal controls. Similar plant responses were obtained at both low (0.1 mM) and high (1.0 mM) nutritional P levels and with mutant plants (NahG) that are unable to accumulate salicylic acid. Application of PR-protein activators induced PR-1 and PR-3 expression in leaves of both nonmycorrhizal and mycorrhizal plants; however, accumulation and mRNA steady-state levels of these proteins were lower, and their appearance delayed, in leaves of the mycorrhizal plants. Application of 0.3 mM phosphate to the plants did not mimic the delay in PR expression observed in the mycorrhizal tobacco. Together, these data strongly support the existence of regulatory processes, initiated in the roots of mycorrhizal plants, that modify disease-symptom development and gene expression in their leaves.

Potato (Solanum tuberosum L.) periderm is composed of the meristematic phellogen that gives rise to an external layer of suberized phellem cells (the skin) and the internal parenchyma-like phelloderm. The continuous addition of new skin layers and the sloughing of old surface layers during tuber maturation results in smooth, shiny skin. However, smooth-skin varieties frequently develop unsightly russeting in response to high soil temperatures. Microscopic observation of microtubers exposed to high temperatures (37°C) suggested heat-enhanced development and accumulation of suberized skin-cell layers. To identify the genes involved in the periderm response to heat stress, skin and phelloderm samples collected separately from immature tubers exposed to high soil temperatures (33°C) and controls were subjected to transcriptome profiling using a potato cDNA array. As expected, the major functional group that was differentially expressed in both skin and phelloderm consisted of stress-related genes; however, while the major up-regulated phelloderm genes coded for heat-shock proteins, many of the skin's most up-regulated sequences were similar to genes involved in the development of protective/symbiotic membranes during plant–microbe interactions. The primary activities regulated by differentially expressed peridermal transcription factors were response to stress (33%) and cell proliferation and differentiation (28%), possibly reflecting the major processes occurring in the heat-treated periderm and implying the integrated activity of the stress response and tissue development. Accumulating data suggest that the periderm, a defensive tissue, responds to heat stress by enhancing the production and accumulation of periderm/skin layers to create a thick protective cover. Skin russeting may be an indirect outcome; upon continued expansion of the tuber, the inflexible skin cracks while new layers are produced below it, resulting in a rough skin texture.