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 (MC s) or bundle sheath cells (BSCs) of C4 plants, differential screening of a maize cDNA library was conducted using P-32-labeled single-strand cDNAs pre pared from;MCs and bundle sheath strands (BSS) as probes. Ten genes encodin g thylakoid membrane proteins in chloroplasts were identified as MC-abundan t genes. These included genes for chlorophyll a/b, binding proteins, plasto cyanin, PsaD, PsbT, PsbR, PsbO, PsaK, PsaG, PsaN and ferredoxin, Seven gene s identified as BSS-abundant genes encoded PEP carboxykinase, salt-inducibl e SalT homolog, heavy metal-inducible metallothionein-like protein, ABA- an d drought-inducible glycine-rich protein, and three proteins of unknown fun ction tone of which was named Bss1), In situ hybridization analyses for sev eral selected genes revealed that mRNAs for the metallothionein-like protei n and Bssl were accumulated specifically in BSCs, and that mRNA for the Sal T homolog was accumulated in vascular cells around phloem cells. Results su ggest that the functional differentiation of MC chloroplasts accompany pref erential expression of these small proteins in photosystem complexes and th at BSCs are the major site of stress responses.