Reporter‐based screen to identify bundle sheath anatomy mutants ($) (Plant J)

Installation of C4 photosynthesis into C3 crops appears a realistic way to boost crop yields.  A key aspect of C4 photosynthesis is an enlarged bundle sheath volume and an increase in bundle sheath chloroplast number.  To identify the regulators of this phenotype, Döring et al. subjected Arabidopsis seeds to EMS mutagenesis, and employed a high-throughput screen to identify plants with C4-like bundle sheath anatomy.  The screen employed two independent reporter cassettes (luciferase and chloroplast-targetted GFP) to identify signal changes in bundle sheath cells or chloroplasts. Fifty-seven stable mutant lines were isolated, five of which showed an increase in bundle sheath cell number.  A SHOREmap approach (mapping by sequencing) was used to identify a shortlist of candidate SNPs responsible for the phenotypes in each of these five mutant lines. Interestingly, these mutants were not only affected in bundle sheath anatomy, but also in the anatomy of their vascular tissue.  This was probably due to a limitation of the GLDPA (P-subunit of glycine decarboxylase) promoter employed in the reporter cassettes used for screening, as it is active in both tissues – there are currently no known dicot promoters that exclusively drive expression in bundle sheath cells.  No mutants with increased bundle sheath chloroplast number or size were isolated.  (Summary by Mike Page) Plant J 10.1111/tpj.14165