Cryptochrome-Related Abiotic Stress Responses in Plants

Abstract

It is well known that light is a crucial environmental factor that has a fundamental role in plant growth and development from seed germination to fruiting. For this process, plants contain versatile and multifaceted photoreceptor systems to sense variations in the light spectrum and to acclimate to a range of ambient conditions. Five main groups of photoreceptors have been found in higher plants, cryptochromes, phototropins, UVR8, zeitlupes, and phytochromes, but the last one red/far red wavelengths photoreceptor is the most characterized. Among the many responses modulated by phytochromes, these molecules play an important role in biotic and abiotic stress responses, which is one of the most active research topics in plant biology, especially their effect on agronomic traits. However, regarding the light spectrum, it is not surprising to consider that other photoreceptors are also part of the stress response modulated by light. In fact, it has become increasingly evident that cryptochromes, which mainly absorb in the blue light region, also act as key regulators of a range of plant stress responses, such as drought, salinity, heat, and high radiation. However, this information is rarely evidenced in photomorphogenetic studies. Therefore, the scope of the present review is to compile and discuss the evidence on the abiotic stress responses in plants that are modulated by cryptochromes.

Keywords: abiotic stress acclimation; blue-light photoreceptor; cryptochromes; drought; heat; high light; salinity.

FIGURE 1

Schematic view of cryptochromes (cry) role in stress acclimation responses. (A) Domain structure of cry consists of Photolyase-Homologous Region (PHR) domain and CRY C-terminal extension (CCE) domain. The cry chromophores methenyltetrahydrofolate (MTHF) and FAD are binding to PHR, a light perception domain. (B) FAD chromophore redox states of cry. FAD is oxidized (FADox) in the darkness (D), with the absorption peak in blue light (BL). On light exposure, FADox changes to the neutral semireduced state (FADH°), which allows the biological activity of cry in the plant cell. With the absorption peak in green light (GL), FADH° is induced to change to the totally reduced form (FAD-). An unclear subject around the reoxidation of FAD is the production of reactive oxygen species (ROS) during the process. Stress signaling pathway regulated by cryptochromes. From the blue light signaling, cry coordinate the negative regulation of COP1-dependent degradation of HY5 and PIF4 transcription factors, triggering changes in stress response target genes expression. Note that these are key pathways in the induction of genes related to signaling and/or biosynthesis of hormones, ROS and stress components. Although the complex signaling of cry in stress acclimation still involves some unknown components, certainly this photoreceptor play a crucial role in BL-dependent stress responses. Arrows means downregulation, T-bars indicate upregulation and dotted lines indicate unknown (?) signaling routes. E3 ubiquitin-ligase; COP1, constitutive photomorphogenic 1; HY5, long hypocotyl 5; PIF4, phytochrome interacting factor 4; ET, ethylene; SA, salicylic acid; ABA, abscisic acid; AUX, auxin.