A bunch of bric-à-brac (Curios) no more: on the importance of BTF proteins in mutually assured destruction in blue light

When buried seeds germinate, they have one goal: to reach the soil surface and initiate photosynthesis. Exposure to light will cause a massive transcriptional reprogramming that will make it very difficult for changes in temperature to affect seedling and later plant development (Salomé et al., 2008). However, mounting evidence has provided another mechanism by which changes in temperature can influence light-mediated plant development: by targeting the photoreceptors themselves!

In response to higher temperatures (typically 28ºC), plants elongate their hypocotyls and petioles more than they would at more normal temperatures like 22ºC. Why, you ask? The changes in plant architecture observed at higher temperatures are accompanied by greater transpiration and water loss, which cools down leaves, possibly to mitigate heat exposure (Crawford et al., 2012). Plants grown at higher temperatures also tend to flower faster to insure population survival in a fight-or-flight response (without the possibility of flight, of course).

The red light photoreceptor phytochrome B is a temperature sensor and modulates hypocotyl elongation by interacting with members of the transcription factor family PHYTOCHROME INTERACTING PROTEIN (PIF). PIF and phyB protein abundance is also regulated by degradation in a mutually-assured destruction; that is, both proteins are degraded via the recruitment of E3 ubiquitin ligases from the Light-Response Bric-à-Brac/Tramtrack/Broad (LRB) family (Ni et al., 2014). As it turns out, Libang Ma and colleagues show here that the blue light photoreceptor cryptochrome 2 (cry2) may also be the subject of a similar regulation in white light and blue light as a function of temperature (Ma et al., 2021).

CRY2 transcript levels are not affected by temperature, but the authors showed that cry2 protein abundance changes with temperature, as cry2 accumulated in light-grown seedlings shifted from 16ºC to 28ºC. Conversely, cry2 abundance dropped in white light-grown seedlings transferred from 28ºC to 16ºC (see Figure). Of note to those interested in entrainment, daily temperature cycles of 16ºC/28ºC were sufficient to drive a rhythm in cry2 abundance in constant light! The authors then demonstrated that cry2 is degraded at lower temperatures by the 26S proteasome, as pre-treating seedlings with the proteasome inhibitor MG132 abrogated cry2 degradation. Importantly, only photoactivated cry2 was degraded, as cry2 abundance remained high in seedlings transferred from 28ºC to 16ºC and exposed to red light or kept in the dark.

Figure.

The blue light photoreceptor cry2 is stabilized at higher temperatures (28°C) but becomes targeted for degradation via the 26S proteasome by recruiting LRB E3 ubiquitin ligases. Figure adapted from Ma et al. (2021).

Which E3 ubiquitin ligase might be responsible for cry2 degradation? We know that phyB and PIF3 are both targeted for degradation via their interaction with LRB E3 ubiquitin ligases, with the lrb1 lrb2 lrb3 triple mutant preventing PIF3 and phyB degradation upon exposure to red light (Ni et al., 2014). Since cry1 and cry2 were previously shown to physically interact with PIF4 and PIF5 in low blue light, the blue light photoreceptors might similarly recruit LRBs, thus sealing their fate in blue light. Indeed, the authors observed that the lrb1 lrb2 lrb3 triple mutant no longer degrades cry2 upon transfer to lower temperatures. In addition, cry2 failed to become ubiquitinated in the triple mutant, in contrast to the high ubiquitination levels seen in wild type.

Do LRBs exert their effect on cry2 via direct physical interaction? The authors provided a clear positive answer to this question, as demonstrated in vitro with pull-down assays and in vivo with co-immunoprecipitation and bimolecular fluorescence complementation assays, supporting the view that LRBs degrade cry2 at lower temperatures.

Does the proposed model hold together? The authors turned to genetics to test its mettle: the lrb1 lrb2 lrb3 triple mutant displayed the same short hypocotyl phenotype from 16ºC to 28ºC, but the lrb1 lrb2 lrb3 cry2 quadruple mutant exhibited the same long hypocotyl as the cry2 mutant at all temperatures. The authors concluded that cry2 is thus targeted for degradation via the 26S proteasome by LRB E3 ligases at lower temperatures. These results paint a vastly more complex and intricate picture of light and temperature signaling in plants than even I feared. This information is also critical to our understanding of responses to climate change, and how we might mitigate some of these effects by engineering more tolerant plants.