Identification of C₄ responsive genes in the facultative C ₄ plant Hydrilla verticillata

The aquatic monocot Hydrilla verticillata (L.f.) Royle is a well-documented facultative C₄ NADP-malic enzyme species in which the C₄ and Calvin cycles operate in the same cell with the specific carboxylases confined to the cytosol and chloroplast, respectively. Several key components had already been characterized at the molecular level, thus the purpose of this study was to begin to identify other, less obvious, elements that may be necessary for a functional single-cell C₄ system. Using differential display, mRNA populations from C₃ and C₄ H. verticillata leaves were screened and expression profiles compared. From this study, 65 clones were isolated and subjected to a customized macroarray analysis; 25 clones were found to be upregulated in C₄ leaves. Northern and semi-quantitative RT-PCR analyses were used for confirmation. From these screenings, 13 C₄ upregulated genes were identified. Among these one encoded a previously recognized C₄ phosphoenolpyruvate carboxylase, and two encoded distinct pyruvate orthophosphate dikinase isoforms, new findings for H. verticillata. Genes that encode a transporter, an aminotransferase and two chaperonins were also upregulated. Twelve false positives, mostly housekeeping genes, were determined from the Northern/semi-quantitative RT-PCR analyses. Sequence data obtained in this study are listed in the dbEST database (DV216698 to DV216767). As a single-cell C₄ system that lacks Kranz anatomy, a better understanding of how H. verticillata operates may facilitate the design of a transgenic C₄ system in a C₃ crop species.