HY5 Contributes to Light-Regulated Root System Architecture Under a Root-Covered Culture System

Abstract

Light is essential for plant organogenesis and development. Light-regulated shoot morphogenesis has been extensively studied; however, the mechanisms by which plant roots perceive and respond to aboveground light are largely unknown, particularly because the roots of most terrestrial plants are usually located underground in darkness. To mimic natural root growth conditions, we developed a root-covered system (RCS) in which the shoots were illuminated and the plant roots could be either exposed to light or cultivated in darkness. Using the RCS, we observed that root growth of wild-type plants was significantly promoted when the roots were in darkness, whereas it was inhibited by direct light exposure. This growth change seems to be regulated by ELONGATED HYPOCOTYL 5 (HY5), a master regulator of photomorphogenesis. Light was found to regulate HY5 expression in the roots, while a HY5 deficiency partially abolished the inhibition of growth in roots directly exposed to light, suggesting that HY5 expression is induced by direct light exposure and inhibits root growth. However, no differences in HY5 expression were observed between illuminated and dark-grown cop1 roots, indicating that HY5 may be regulated by COP1-mediated proteasome degradation. We confirmed the crucial role of HY5 in regulating root development in response to light under soil-grown conditions. A transcriptomic analysis revealed that light controls the expression of numerous genes involved in phytohormone signaling, stress adaptation, and metabolic processes in a HY5-dependent manner. In combination with the results of the flavonol quantification and exogenous quercetin application, these findings suggested that HY5 regulates the root response to light through a complex network that integrates flavonol biosynthesis and reactive oxygen species signaling. Collectively, our results indicate that HY5 is a master regulator of root photomorphogenesis.

Disruption of CYCLOPHILIN 38 function reveals a photosynthesis-dependent systemic signal controlling lateral root emergence

“…Together these data suggest that CYP38 and HY5 likely regulate root growth through independent pathways. This is consistent with work showing that HY5 likely mediates its effect on lateral root growth directly through local induction of HY5 expression in light-exposed root tissues [23].…”

“…We found that hy5 mutants showed similar lateral root density to WT under standard light conditions, while they showed a partial rescue of lateral root growth under low light conditions (Figure S4C, D). These data suggest that HY5 mediates light-dependent growth-inhibition of lateral root development, which is similar to findings of a recent study [23]. To determine whether HY5 acts in the same genetic pathway as CYP38 , we compared the phenotypes of hy5 and cyp38 single and double mutants.…”

“…ELONGATED HYPOCOTYL 5 ( HY5 ) acts as a mobile signal mediating root-to-shoot communication in response to light quantity and quality [22–24]. hy5 mutants exhibit root systems with reduced primary root length, however, lateral root development is accelerated, relative to WT [23]. We found that hy5 mutants showed similar lateral root density to WT under standard light conditions, while they showed a partial rescue of lateral root growth under low light conditions (Figure S4C, D).…”

Modulation of plant root traits by nitrogen and phosphate: transporters, long-distance signaling proteins and peptides, and potential artificial traps

“…AtNRT2.1 is a negative regulator of LR initiation in response to a combination of high external sucrose and low nitrate (Little et al 2005). Recently, it has been reported that HY5 suppresses total LR length under N-replete condition regardless if the root is illuminated or not (Zhang et al 2019a). In addition, the activation of the expression of several PHT1 genes and thus Pi uptake require the phytochrome B-HY5 module, and HY5 has been demonstrated to directly bind to the promoter of AtPHT1;1 (Sakuraba et al 2018).…”

“…Development of new methodologies by which plant roots are shielded from light is of significance in future work. In addition to the aluminum foil method utilized in several studies (Chen et al 2016, Zhang et al 2019a, Zheng et al 2019, some other platforms enabling light-shielding and/or in situ imaging of roots (e.g. GLO-Roots) have been demonstrated to be equally feasible (Rellán-Álvarez et al 2015).…”