Variegated plants: an excellent model system for studying carbon and nitrogen reallocation in source and sink tissues within the same leaf

Marija Vidovic
(Laboratory for Plant Molecular Biology, Institute of Molecular Genetics and Genetic Engineering, University of Belgrade, Serbia)

In the global warming era, we are facing extreme changes in environmental conditions such as high and low temperatures, high light and ultraviolet radiation intensity, as well as drought and increased CO₂ levels. The effects of these factors on plants are often interrelated and usually result in a disturbed balance between the amount of energy received and the ability to process it. In order to balance energy input and prevent photooxidative damage, plants have evolved multiple mechanisms for energy dissipation and photoprotection. To efficiently dissipate the excess excitation energy (EEE), the additional electron sinks, such as photorespiration, biosynthesis of phenolics and nitrate reduction are stimulated.

Variegated leaves of Pelargonium zonale containing photosynthetically active green leaf (GL) and non-active white leaf (WL) provide an excellent model system for studying processes associated with photosynthesis and sink-source interactions, enabling the same microenvironmental conditions, unlike common root-shoot studies. WL present a carbon sink and depends on photosynthetic and energy-generating processes in GL. The upregulated nitrogen metabolism in WL compensates for the insufficient energy from carbon metabolism by providing alternative respiratory substrates. At the same time, WL serves as nitrogen storage. Natural leaf variegation, as a unique system, provides a distinct combination of spatially separated antioxidants, but capable of communicating. WL exhibited upregulated H₂O₂ scavenging network, based on ascorbate–glutathione cycle, Mn and Cu/Zn superoxide dismutase and glutathione (mostly located in mitochondria). GL antioxidant defence is relied on thylakoid-bound ascorbate peroxidase (tAPX), ascorbate (predominantly localised in the nucleus, peroxisomes, plastids and mitochondria) and catalase.

Overall, our research provides a new molecular data resource for further research with this excellent model system. Furthermore, it contributes to uncovering molecular genetic mechanisms underlying foliar variegation and understanding its adaptive value, as no consistent conclusions on its ecological benefits have been proposed so far.