Are changes in sulfate assimilation pathway needed for evolution of C(4) photosynthesis?

C(4) photosynthesis characteristically features a cell-specific localization of enzymes involved in CO(2) assimilation in bundle sheath cells (BSC) or mesophyll cells. Interestingly, enzymes of sulfur assimilation are also specifically present in BSC of maize and many other C(4) species. This localization, however, could not be confirmed in C(4) species of the genus Flaveria. It was, therefore, concluded that the bundle sheath localization of sulfate assimilation occurs only in C(4) monocots. However, recently the sulfate assimilation pathway was found coordinately enriched in BSC of Arabidopsis, opening new questions about the significance of such cell-specific localization of the pathway. In addition, next generation sequencing revealed expression gradients of many genes from C(3) to C(4) species and mathematical modeling proposed a sequence of adaptations during the evolutionary path from C(3) to C(4). Indeed, such gradient, with higher expression of genes for sulfate reduction in C(4) species, has been observed within the genus Flaveria. These new tools provide the basis for reexamining the intriguing question of compartmentalization of sulfur assimilation. Therefore, this review summarizes the findings on spatial separation of sulfur assimilation in C(4) plants and Arabidopsis, assesses the information on sulfur assimilation provided by the recent transcriptomics data and discusses their possible impact on understanding this interesting feature of plant sulfur metabolism to find out whether changes in sulfate assimilation are part of a general evolutionary trajectory toward C(4) photosynthesis.