Singlet oxygen (¹O₂) refers to the lowest excited electronic state of molecular oxygen. It easily oxidizes biological molecules and, therefore, is cytotoxic. In plant cells, ¹O₂ is formed mostly in the light in thylakoid membranes by reaction centers of photosystem II. In high concentrations, ¹O₂ destroys membranes, proteins and DNA, inhibits protein synthesis in chloroplasts leading to photoinhibition of photosynthesis, and can result in cell death. However, ¹O₂ also acts as a signal relaying information from chloroplasts to the nucleus, regulating expression of nuclear genes. In spite of its extremely short lifetime, ¹O₂ can diffuse from the chloroplasts into the cytoplasm and the apoplast. As shown by recent studies, ¹O₂-activated signaling pathways depend not only on the levels but also on the sites of ¹O₂ production in chloroplasts, and can activate two types of responses, either acclimation to high light or programmed cell death. ¹O₂ can be produced in high amounts also in root cells during drought stress. This review summarizes recent advances in research on mechanisms and sites of ¹O₂ generation in plants, on ¹O₂-activated pathways of retrograde- and cellular signaling, and on the methods to study ¹O₂ production in plants.

To determine the driving forces for symplastic sugar flux between mesophyll and phloem, gradients of sugar concentrations and osmotic pressure were studied in leaf tissues of two Scrophulariaceae species, Alonsoa meridionalis and Asarina barclaiana. A. meridionalis has a typical symplastic configuration of minor-vein phloem, i.e. intermediary companion cells with highly developed plasmodesmal connections to bundle-sheath cells. In A. barclaiana, two types of companion cells, modified intermediary cells and transfer cells, were found in minor-vein phloem, giving this species the potential to have a complex phloem-loading mode. We identified all phloem-transported carbohydrates in both species and analyzed the levels of carbohydrates in chloroplasts, vacuoles, and cytoplasm of mesophyll cells by nonaqueous fractionation. Osmotic pressure was measured in single epidermal and mesophyll cells and in whole leaves and compared with calculated values for phloem sap. In A. meridionalis, a 2-fold concentration gradient for sucrose between mesophyll and phloem was found. In A. barclaiana, the major transported carbohydrates, sucrose and antirrhinoside, were present in the phloem in 22- and 6-fold higher concentrations, respectively, than in the cytoplasm of mesophyll cells. The data show that diffusion of sugars along their concentration gradients is unlikely to be the major mechanism for symplastic phloem loading if this were to occur in these species. We conclude that in both A. meridionalis and A. barclaiana, apoplastic phloem loading is an indispensable mechanism and that symplastic entrance of solutes into the phloem may occur by mass flow. The conditions favoring symplastic mass flow into the phloem are discussed.

In land plants, at least five groups of photoreceptors perceive information about light conditions and diurnal rhythm, as well as about ambient temperature, presence of pathogens or competing...