Isolation and Characterization of cDNAs for Differentially Accumulated Transcripts between Mesophyll Cells and Bundle Sheath Strands of Maize Leaves

To characterize novel genes functioning specifically in mesophyll cells (MCs) or bundle sheath cells (BSCs) of C4 plants, differential screening of a maize cDNA library was conducted using 32P-labeled single-strand cDNAs prepared from MCs and bundle sheath strands (BSS) as probes. Ten genes encoding thylakoid membrane proteins in chloroplasts were identified as MC-abundant genes. These included genes for chlorophyll a/b binding proteins, plastocyanin, PsaD, PsbT, PsbR, PsbO, PsaK, PsaG, PsaN and ferredoxin. Seven genes identified as BSS-abundant genes encoded PEP carboxykinase, salt-inducible SalT homolog, heavy metal-inducible metallothionein-like protein, ABA- and drought-inducible glycine-rich protein, and three proteins of unknown function (one of which was named Bss1). In situ hybridization analyses for several selected genes revealed that mRNAs for the metallothionein-like protein and Bss1 were accumulated specifically in BSCs, and that mRNA for the SalT homolog was accumulated in vascular cells around phloem cells. Results suggest that the functional differentiation of MC chloroplasts accompany preferential expression of these small proteins in photosystem complexes and that BSCs are the major site of stress responses.