This prompted us to hypothesize that this mutant was allelic to open stomata2 ( ost2), which was confirmed upon sequencing of the gene ( Merlot et al., 2007). Segregation analysis revealed that the mutation was dominant. One of them exhibited an extremely cool phenotype, with a rosette temperature up to 2☌ less than in the wild type. The six mutants maintained a stable cooler temperature throughout the nighttime and even when the night period was extended for several hours (tested up to 18 h of darkness Fig. The color scale is adjusted so that zero corresponds to the average rosette temperature of Col-0. B, False-color infrared images of the same plants after 18 h in darkness. A, Top-view pictures of wild-type and mutant ( ost2-2D and opal1 to opal5) mature plants. Six mutants were isolated from a thermography screen in dark conditions. Isolation and characterization of mutants with stomata open all night long. To avoid mutations with pleiotropic effects, we focused on the group of six cool mutants with similar growth as in the wild type ( Fig. Candidates with lower temperature than the wild type were selected, and 37 of them showed a heritable, cool phenotype in darkness ( Supplemental Table S1). Here, we screened a mutagenized population of Arabidopsis seedlings by imaging shoot temperature during the night period. So far, no genetic screen has attempted to isolate mutants insensitive to darkness, a situation that plants encounter every night. Based on this property, thermal imaging has permitted screening for mutants impaired in leaf transpiration, and thus discovering new signaling players implicated in stomatal response to drought, atmospheric CO 2, or light quality ( Merlot et al., 2002 for review, see Negi et al., 2014). Transpiration drives evaporative cooling. We propose that dedicated regulators enforce nighttime stomatal closure. Importantly, stomata of the opal mutants closed normally in response to abscisic acid ( ABA) and atmospheric CO 2. Here, we report the isolation and characterization of five Arabidopsis ( Arabidopsis thaliana) mutants that maintain stomata open the whole night and were named open all night long1 ( opal1) to opal5. Despite the importance of stomatal closure at night for plant fitness and ecosystem water fluxes ( Caird et al., 2007), it remains unclear whether this dark response is simply a passive consequence of the absence of light stimulus, or an active process recruiting other mechanisms of stomatal closure or involving independent signaling events ( Tallman, 2004 Kollist et al., 2014). In leaves, they typically open during the day to favor CO 2 diffusion when light is available for photosynthesis, and close at night to limit transpiration and save water. Stomata are mouth-like cellular complexes at the epidermis that regulate gas transfer between plants and atmosphere.
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