The glycine decarboxylase complex (GDC) plays a critical role in the photorespiratory C2 cycle of C3 species by recovering carbon following the oxygenation reaction of ribulose-1,5-bisphosphate carboxylase/oxygenase. Loss of GDC from mesophyll cells (MCs) is considered a key early step in the evolution of C4 photosynthesis. To assess the impact of preferentially reducing GDC in rice MCs, we decreased the abundance of OsGDCH (Os10g37180) using an artificial microRNA (amiRNA) driven by a promoter that preferentially drives expression in MCs. GDC H- and P-proteins were undetectable in leaves of gdch lines. Plants exhibited a photorespiratory-deficient phenotype with stunted growth, accelerated leaf senescence, reduced chlorophyll, soluble protein and sugars, and increased glycine accumulation in leaves. Gas exchange measurements indicated an impaired ability to regenerate ribulose 1,5-bisphosphate in photorespiratory conditions. In addition, MCs of gdch lines exhibited a significant reduction in chloroplast area and coverage of the cell wall when grown in air, traits that occur during the later stages of C4 evolution. The presence of these two traits important for C4 photosynthesis and the non-lethal, down-regulation of the photorespiratory C2 cycle positively contribute to efforts to produce a C4 rice prototype.

We compared how stomatal morphology and physiology control intrinsic leaf water use efficiency (iWUE) in two C3 and six C4 grasses grown at ambient (400 µmol mol -1) or glacial CO2 (180 µmol mol -1) and high (1000 µmol m -2 s -1) or low light intensity (200 µmol m -2 s -1). C4 grasses tended to have higher iWUE and CO2 assimilation rates, and lower stomatal conductance (gs), operational stomatal aperture (aop) and guard cell K + influx rate relative to C3 grasses, while stomatal size (SS) and stomatal density (SD) did not vary according to the photosynthetic type. Overall, iWUE and gs depended most on aop and density of open stomata. In turn, aop correlated with K + influx, stomatal opening speed on transition to high light and SS. Species with higher SD had smaller and faster-opening stomata. Although C4 grasses operated with lower gs and aop at ambient CO2, they showed a greater potential to open stomata relative to maximal stomatal conductance (gmax), indicating heightened stomatal sensitivity and control. We uncovered promising links between aop, gs, iWUE and K + influx among C4 grasses, and differential K + influx responses of C4 guard cells to low light, revealing molecular targets for improving iWUE in C4 crops.

Tolerance of anaerobic germination (AG) is a key trait in the development of direct seeded rice. Through rapid and sustained coleoptile elongation, AG tolerance enables robust seedling establishment under flooded conditions. Previous attempts to fine map and characterize AG2 (qAG7.1), a major centromere-spanning AG tolerance QTL, derived from the indica variety Ma-Zhan Red, have failed. Here, a novel approach of “enriched haplotype” genome-wide association study based on the Ma-Zhan Red haplotype in the AG2 region was successfully used to narrow down AG2 from more than 7 Mb to less than 0.7 Mb. The AG2 peak region contained 27 genes, including the Rc gene, responsible for red pericarp development in pigmented rice. Through comparative variant and transcriptome analysis between AG tolerant donors and susceptible accessions several candidate genes potentially controlling AG2 were identified, among them several regulatory genes. Genome-wide comparative transcriptome analysis suggested differential regulation of sugar metabolism, particularly trehalose metabolism, as well as differential regulation of cell wall modification and chloroplast development to be implicated in AG tolerance mechanisms.

We investigated how stomatal morphology and physiology control intrinsic leaf water use efficiency (iWUE) in grasses. Two C3 and six C4 grasses were grown at ambient (400 μl L-1) or glacial CO2 (180 μl L-1) and high (1000 μmol m-2 s-1) or low light intensity (200 μmol m-2 s-1). C4 grasses tended to have higher iWUE and CO2 assimilation rates, and lower stomatal conductance (gs), operational stomatal aperture (aop) and guard cell K+ influx rate relative to C3 grasses, while stomatal size (SS) and stomatal density (SD) did not vary according to the photosynthetic type. Overall, iWUE and gs depended most on aop and density of open stomata. In turn, aop correlated with K+ influx, stomatal opening speed on transition to high light and SS. Species with higher SD had smaller and faster-opening stomata. Although C4 grasses operated with lower gs and aop at ambient CO2, they showed a greater potential to open stomata relative to maximal stomatal conductance (gmax), indicating heightened stomatal sensitivity and control. We uncover novel links between aop, gs, iWUE and K+ influx amongst grasses and differential K+ influx responses of C4 guard cells to low light, revealing molecular targets for breeding crops with high iWUE.