Coordination of shoot photosynthetic carbon fixation with root inorganic nitrogen uptake optimizes plant performance in a fluctuating environment [1]. However, the molecular basis of this long-distance shoot-root coordination is little understood. Here we show that Arabidopsis ELONGATED HYPOCOTYL5 (HY5), a bZIP transcription factor that regulates growth in response to light [2, 3], is a shoot-to-root mobile signal that mediates light promotion of root growth and nitrate uptake. Shoot-derived HY5 auto-activates root HY5 and also promotes root nitrate uptake by activating NRT2.1, a gene encoding a high-affinity nitrate transporter [4]. In the shoot, HY5 promotes carbon assimilation and translocation, whereas in the root, HY5 activation of NRT2.1 expression and nitrate uptake is potentiated by increased carbon photoassimilate (sucrose) levels. We further show that HY5 function is fluence-rate modulated and enables homeostatic maintenance of carbon-nitrogen balance in different light environments. Thus, mobile HY5 coordinates light-responsive carbon and nitrogen metabolism, and hence shoot and root growth, in a whole-organismal response to ambient light fluctuations.

The phytohormone gibberellic acid (GA) regulates diverse aspects of plant growth and development. GA responses are triggered by the degradation of DELLA proteins, which function as repressors in GA signaling pathways. Recent studies in Arabidopsis thaliana and rice (Oryza sativa) have implied that the degradation of DELLA proteins occurred via the ubiquitinproteasome system. Here, we developed an Arabidopsis cell-free system to recapitulate DELLA protein degradation in vitro. Using this cell-free system, we documented that Lys-29 of ubiquitin is the major site for ubiquitin chain formation to mediate DELLA protein degradation. We also confirmed the specific roles of GA receptors and multisubunit E3 ligase components in regulating DELLA protein degradation. In addition, blocking DELLA degradation with a PP1/PP2A phosphatase inhibitor in our cell-free assay suggested that degradation of DELLA proteins required protein Ser/Thr dephosphorylation activity. Furthermore, our data revealed that the LZ domain of Arabidopsis DELLA proteins is essential for both their stability and activity. Thus, our in vitro degradation system provides biochemical insights into the regulation of DELLA protein degradation. This in vitro assay system could be widely adapted for dissecting cellular signaling pathways in which regulated proteolysis is a key recurrent theme.

Gibberellic acid (GA) regulates many aspects of plant growth and development. The DELLA proteins act to restrain plant growth, and GA relieves this repression by promoting their degradation via the 26S proteasome pathway. The elucidation of the crystalline structure of the GA soluble receptor GID1 protein represents an important breakthrough for understanding the way in which GA is perceived and how it induces the destabilization of the DELLA proteins. Recent advances have revealed that the DELLA proteins are involved in protein-protein interactions within various environmental and hormone signaling pathways. In this review, we highlight our current understanding of the 'relief of repression' model that aims to explain the role of GA and the function of the DELLA proteins, incorporating the many aspects of cross-talk shown to exist in the control of plant development and the response to stress.