Bacillus cereus AR156 is a plant growth-promoting rhizobacterium that induces resistance against a broad spectrum of pathogens including Pseudomonas syringae pv. tomato DC3000. This study analyzed AR156-induced systemic resistance (ISR) to DC3000 in Arabidopsis ecotype Col-0 plants. Compared with mock-treated plants, AR156-treated ones showed an increase in biomass and reductions in disease severity and pathogen density in the leaves. The defense-related genes PR1, PR2, PR5, and PDF1.2 were concurrently expressed in the leaves of AR156-treated plants, suggesting simultaneous activation of the salicylic acid (SA)- and the jasmonic acid (JA)- and ethylene (ET)-dependent signaling pathways by AR156. The above gene expression was faster and stronger in plants treated with AR156 and inoculated with DC3000 than that in plants only inoculated with DC3000. Moreover, the cellular defense responses hydrogen peroxide accumulation and callose deposition were induced upon challenge inoculation in the leaves of Col-0 plants primed by AR156. Also, pretreatment with AR156 led to a higher level of induced protection against DC3000 in Col-0 than that in the transgenic NahG, the mutant jar1 or etr1, but the protection was absent in the mutant npr1. Therefore, AR156 triggers ISR in Arabidopsis by simultaneously activating the SA- and JA/ET-signaling pathways in an NPR1-dependent manner that leads to an additive effect on the level of induced protection.

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The signaling role of hydrogen gas (H 2 ) has attracted increasing attention from animals to plants. However, the physiological significance and molecular mechanism of H 2 in drought tolerance are still largely unexplored. In this article, we report that abscisic acid (ABA) induced stomatal closure in Arabidopsis (Arabidopsis thaliana) by triggering intracellular signaling events involving H 2 , reactive oxygen species (ROS), nitric oxide (NO), and the guard cell outward-rectifying K + channel (GORK). ABA elicited a rapid and sustained H 2 release and production in Arabidopsis. Exogenous hydrogen-rich water (HRW) effectively led to an increase of intracellular H 2 production, a reduction in the stomatal aperture, and enhanced drought tolerance. Subsequent results revealed that HRW stimulated significant inductions of NO and ROS synthesis associated with stomatal closure in the wild type, which were individually abolished in the nitric reductase mutant nitrate reductase1/2 (nia1/2) or the NADPH oxidasedeficient mutant rbohF (for respiratory burst oxidase homolog). Furthermore, we demonstrate that the HRW-promoted NO generation is dependent on ROS production. The rbohF mutant had impaired NO synthesis and stomatal closure in response to HRW, while these changes were rescued by exogenous application of NO. In addition, both HRW and hydrogen peroxide failed to induce NO production or stomatal closure in the nia1/2 mutant, while HRW-promoted ROS accumulation was not impaired. In the GORK-null mutant, stomatal closure induced by ABA, HRW, NO, or hydrogen peroxide was partially suppressed. Together, these results define a main branch of H 2 -regulated stomatal movement involved in the ABA signaling cascade in which RbohF-dependent ROS and nitric reductase-associated NO production, and subsequent GORK activation, were causally involved.

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