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Disengagement of light responses in Arabidopsis by localized developmental factors

Plant development and growth are profoundly influenced by environmental cues such as light intensity and composition. In particular, changes in red (600 nm to 700 nm) and far-red (700 nm to 750 nm) light inform about the threat of competing plants nearby, which deplete red light and generate an unfavorable shade environment enriched in far-red light (1). Plants detect red and far-red light using an evolutionarily conserved family of photoreceptors named phytochromes, which are activated by red light and inactivated by far-red light (2). Phytochromes play an important role in shaping plant architecture, in part, by restricting the rate of stem growth (3). When plants encounter a canopy of neighboring plants, the far-red light-enriched shade environment inactivates phytochromes to promote stem growth, thereby allowing the plants to escape shade via the so-called shade avoidance response (4, 5). In the plant model species Arabidopsis thaliana (Arabidopsis), phytochromes are ubiquitously expressed in all tissue/organ types throughout the life cycle (6). This expression pattern enables all tissues/organs to continuously monitor and respond to changes in the local light environment. However, intriguingly, shade does not elicit growth in all stem tissues: While the embryonic stem (hypocotyl) and the leaf petiole are exquisitely sensitive to shade, the internodes—which are located at the boundary region connecting the hypocotyl, the bases of the leaves, and the shoot apical meristem—are almost completely unresponsive (Fig. 1A) (7). The lack of a light response in the internodes keeps Arabidopsis in the rosette habit, which is thought to provide adaptive advantages against grazing, drought, and cold (8, 9). The distinct responses to shade between different Arabidopsis stem tissues suggest that phytochrome signaling can be modulated or gated by tissue/ organ-specific developmental signals. However, the molecular mechanisms underlying the developmental control of light signaling remain poorly understood. In PNAS, Ejaz et al. (10) uncover fascinating mechanisms by which locally expressed developmental factors in the Arabidopsis internode disconnect phytochromes from the downstream shade avoidance response, providing a glimpse into the mechanisms by which developmental factors gate light signaling. The shade avoidance response in Arabidopsis is mediated predominantly by phytochrome B (phyB) and basic helix–loop–helix transcription factors named PHYTOCHROME INTERACTING FACTORs (PIFs), which directly interact with active phyB (11). In general, PIFs play antagonistic roles in phyB signaling: Photoactivated phyB binds to PIFs and inhibits their functions by promoting their degradation and/or repressing their transcriptional activity (12). For example, PIF4 exerts essential functions in stem growth under shade conditions and warm temperatures by directly activating growth-relevant genes, such as those involved in the biosynthesis and signaling of the plant growth hormone auxin (13–16). Active phyB restricts stem growth by promoting PIF4’s ubiquitin-dependent proteolysis via the proteasome (17, 18). In shaderesponsive stem tissues, such as the hypocotyl and leaf petiole, inactivation of phyB by shade reduces phyB accumulation in the nucleus and releases the function of PIF4 in promoting stem growth (Fig. 1B) (18–20). Ejaz et al. (10) unveil that, in the internodes, shadeinduced tissue growth is repressedby a locally expressed transcriptional regulator, ARABIDOPSIS THALIANA HOMEOBOX GENE 1 (ATH1). During the vegetative phase, ATH1 is expressed in the shoot apical meristem, the subapical region (including the internodes), and the leaf primordia, and it is required for the development of the boundaries between shoot organs and the stem (21). Interestingly, a loss-of-function ath1-3 mutant regains the growth response to shade (10). By analyzing genome-wide ATH1 target genes, Ejaz et al. (10) reveal that ATH1 directly activates two BLADE-ON-PETIOLE (BOP) genes, BOP1 and BOP2. BOP1 and BOP2 are Bric-a-Brack/Tramtrack/Broad domain-containing proteins that act as the substrate recognition subunits of a Cullin 3-based E3 ubiquitin ligase to mediate PIF4 degradation (22). Therefore, these findings indicate