No abstract

Heterotrimeric G proteins have been shown to transmit ultraviolet B (UV-B) signals in mammalian cells, but whether they also transmit UV-B signals in plant cells is not clear. In this paper, we report that 0.5 W m 22 UV-B induces stomatal closure in Arabidopsis (Arabidopsis thaliana) by eliciting a cascade of intracellular signaling events including Ga protein, hydrogen peroxide (H 2 O 2 ), and nitric oxide (NO). UV-B triggered a significant increase in H 2 O 2 or NO levels associated with stomatal closure in the wild type, but these effects were abolished in the single and double mutants of AtrbohD and AtrbohF or in the Nia1 mutants, respectively. Furthermore, we found that UV-B-mediated H 2 O 2 and NO generation are regulated by GPA1, the Ga-subunit of heterotrimeric G proteins. UV-B-dependent H 2 O 2 and NO accumulation were nullified in gpa1 knockout mutants but enhanced by overexpression of a constitutively active form of GPA1 (cGa). In addition, exogenously applied H 2 O 2 or NO rescued the defect in UV-B-mediated stomatal closure in gpa1 mutants, whereas cGa AtrbohD/AtrbohF and cGa nia1 constructs exhibited a similar response to AtrbohD/ AtrbohF and Nia1, respectively. Finally, we demonstrated that Ga activation of NO production depends on H 2 O 2 . The mutants of AtrbohD and AtrbohF had impaired NO generation in response to UV-B, but UV-B-induced H 2 O 2 accumulation was not impaired in Nia1. Moreover, exogenously applied NO rescued the defect in UV-B-mediated stomatal closure in the mutants of AtrbohD and AtrbohF. These findings establish a signaling pathway leading to UV-B-induced stomatal closure that involves GPA1-dependent activation of H 2 O 2 production and subsequent Nia1-dependent NO accumulation.

Allocation of limiting resources, such as nutrients, is an important adaptation strategy for plants. Plants may allocate different nutrients within a specific organ or the same nutrient among different organs. In this study, we investigated the allocation strategies of nitrogen (N) and phosphorus (P) in leaves, stems and roots of 126 shrub species from 172 shrubland communities in Northern China using scaling analyses. Results showed that N and P have different scaling relationships among plant organs. The scaling relationships of N concentration across different plant organs tended to be allometric between leaves and non-leaf organs, and isometric between non-leaf organs. Whilst the scaling relationships of P concentration tended to be allometric between roots and non-root organs, and isometric between non-root organs. In arid environments, plant tend to have higher nutrient concentration in leaves at given root or stem nutrient concentration. Evolutionary history affected the scaling relationships of N concentration slightly, but not affected those of P concentration. Despite fairly consistent nutrients allocation strategies existed in independently evolving lineages, evolutionary history and environments still led to variations on these strategies.