A Spotlight on Photobiology

A few weeks ago, in frosty Matsue (Japan), the International Symposium on Plant Photobiology (ISPP2018, organized by Professor Akira Nagatani and colleagues) drew many of the world’s leading photobiologists to discuss their latest research. Here, we report some of the most exciting developments coming from the field, many of which were exemplified in Plant Physiology’s recent focus issue on plant environmental signaling (February 2018).

One emerging concept is that many light signals converge on a core group of transcriptional regulators. Of the five known classes of photoreceptors in Arabidopsis (Arabidopsis thaliana), at least three (phytochromes [phys], cryptochromes [crys], and UVB-resistance 8 [UVR8]) have been shown to inhibit the action of the PHYTOCHROME INTERACTING FACTORs (PIFs) and promote the action of ELONGATED HYPOCOTYL5 (HY5). The PIF-HY5 signaling module (along with COP1, BZR1/BES1, EIN3, and DELLAs) forms a central node for light signal integration in Arabidopsis. Work is currently underway to establish whether this signaling module is conserved across the plant kingdom. In the February 2018 focus issue of Plant Physiology, Pham et al. (2018) provide a thorough review of phy and PIF signaling, outlining phy:PIF partners in moss (Physcomitrella patens), liverwort (Marchantia polymorpha), rice (Oryza sativa), and maize (Zea mays). In the same issue, Burman et al. (2018) demonstrate that an ortholog of HY5 exerts strong developmental control throughout the life cycle of rice. This lends support to the notion that the light signaling module in Arabidopsis can be extended out to other species.

It was clear at ISPP2018 that many questions remain unanswered regarding the function of the module at the molecular level. For example, the binding of PIFs to a promoter does not necessarily lead to changes in transcription. It is likely that in many cases cofactors are required. In the latest issue of Plant Physiology, Zhou et al. (2018) demonstrate that TCP-family transcription factors are required for shade-induced elongation to occur. They show that several genes that are known to be induced by the PIFs under shade are not upregulated in the tcp5 tcp13 tcp17 triple mutant. This raises the possibility that these TCPs act as cofactors for the PIFs and are required for PIF function. Also supporting the hypothesis that PIFs do not act alone at DNA, Peng et al. (2018) show that PIF7 interacts with MORF RELATED GENE2 (MRG2) at the promoter region of shade-induced genes. Once at the DNA, MRG2 presumably promotes the action of histone acetyltransferases (HATs), resulting in histone acetylation and enhanced gene expression. In the past, several HATs and deacetylases have been shown to be involved in light-mediated growth, suggesting that similar protein complexes might be involved in different PIF and HY5-mediated developmental processes.

Another question that is increasingly being asked is where these genes are acting. Often, the site of light perception is found to be distal to the site of the response. Shade cues perceived in the leaf tip enhance hyponasty in the petiole base (Michaud et al., 2017; Pantazopoulou et al., 2017), and root development is strongly affected by aboveground light quality (van Gelderen et al., 2018a, 2018b). Along with inter-organ communication, there is also growing interest in communication between tissues within organs. An interesting case of this is the regulation of endodermal PIF1 by epidermal phyB (Kim et al., 2016). Even more recently in Plant Physiology, Sujung et al. (2018) provide evidence that shade is perceived in the cotyledon mesophyll, with this signal then relayed to the vasculature. Several molecules (auxin, sugars, and HY5) have been shown to mediate inter-organ communication upon light signaling, and it will be interesting to see if any of these are also involved in inter-tissue signaling.

In addition to inter-organ and inter-tissue communication, photobiologists are increasingly looking at communication between organelles. Information about the light status sent from the nucleus to chloroplasts (anterograde signaling) is key for efficient photosynthesis (reviewed in Krahmer et al., 2018). In the reverse direction, retrograde signals from the chloroplasts are sent to the nucleus to signal high light stress and developmental status (reviewed in Hernández-Verdeja and Strand, 2018). A remarkable recent discovery is that light triggers alternative promoter selection for thousands of genes, with the resulting protein isoforms often targeted to different subcellular locations (Ushijima et al., 2017). This finding adds yet another layer of complexity to light signaling in plants.

The list of advances presented here is by no means exhaustive and the plant photobiology field continues to develop in new and exciting directions. The future is certainly bright!