A missense mutation in theXan-hgene encoding the Mg-chelatase subunit I leads to a viable pale green line with phenotypic features of potential interest for barley breeding programs

Abstract

AbstractThe pale green trait, i.e. reduced chlorophyll content, has been shown to increase the efficiency of photosynthesis and biomass accumulation when photosynthetic microorganisms and tobacco plants are cultivated at high densities. Thus, thehus1barley mutant is defective in photosystem antenna biogenesis, and exhibits a 50% reduction in leaf chlorophyll content. Nevertheless, its agronomical performance under standard field conditions is comparable to that of the wild-type. This supports the notion that crops can decrease their investment in antenna proteins and chlorophyll biosynthesis without detrimental effects on productivity. Here, we assess the effects of reducing leaf chlorophyll content in barley by altering the chlorophyll biosynthesis pathway (CBP). To this end, we have isolated and characterised the pale green barley mutantxan-h.chli-1, which carries a missense mutation in theXan-hgene for subunit I of Mg-chelatase (HvCHLI), the first enzyme in the CBP. Intriguingly,xan-h.chli-1is the only known viable homozygous mutant at theXan-hlocus in barley. The Arg298Lys amino-acid substitution in the ATP-binding cleft causes a slight decrease inHvCHLI protein abundance, and a marked reduction in Mg-chelatase activity. Under controlled growth conditions, mutant plants display reduced accumulation of antenna and photosystem core subunits, together with reduced photosystem II yield relative to wild type under moderate illumination, and consistently higher than wild-type levels at high light intensities. Moreover, the reduced content of leaf chlorophyll is associated with a stable reduction in daily transpiration rate, and slight decreases in total biomass accumulation and water-use efficiency. These traits are reminiscent of phenotypic features of wild barley accessions and landraces that thrive under arid climatic conditions. Overall, our findings make thexan-h.chli-1allelic variant of potential interest for tailoring barley, and other crop plants, for growth in harsh environments.