Differences and similarities in the photoregulation of gibberellin metabolism between rice and dicots

Abstract

In rice seedlings, elongation of leaf sheaths is suppressed by light stimuli. The response is mediated by two classes of photoreceptors, phytochromes and cryptochromes. However, it remains unclear how these photoreceptors interact in the process. Our recent study using phytochrome mutants and novel cryptochrome RNAi lines revealed that cryptochromes and phytochromes function cooperatively, but independently to reduce active GA contents in seedlings in visible light. Blue light captured by cryptochrome 1 (cry1a and cry1b) induces robust expression of GA 2-oxidase genes (OsGA2ox4-7). In parallel, phytochrome B with auxiliary action of phytochrome A mediates repression of GA 20-oxidase genes (OsGA20ox2 and OsGA20ox4). The independent effects cumulatively reduce active GA contents, leading to a suppression of leaf sheath elongation. These regulatory mechanisms are distinct from phytochrome B function in dicots. We discuss reasons why the distinct system appeared in rice, and advantages of the rice system in early photomorphogenesis.

To monitor the surrounding light conditions, plants use a variety of photoreceptors, including cryptochrome, phytochrome, phototropin and others. Cryptochrome and phytochrome are essential for the adjustment of growth strategies to the light environment. Cryptochrome is a blue/UV-A (B/UV-A) photoreceptor, while phytochrome mediates various responses to red/far-red (R/FR) light.¹ Both photoreceptors form small gene families in angiosperms; rice possesses three cryptochrome (CRY1a, CRY1b and CRY2) and three phytochrome genes (PHYA, PHYB and PHYC).^2-4

Physiological analyses of phytochrome-deficient mutants and cryptochrome overexpressers suggested that these photoreceptors regulate leaf sheath elongation, leaf blade inclination and leaf greening in rice. Notably, leaf sheath elongation was suppressed through cry1 species (cry1a and cry1b) and phyB in response to B and R light, respectively.^2,3 However, it remained unclear whether signals from these photoreceptors are integrated into a common pathway or processed on independent routes.

The phytohormone gibberellin (GA) promotes germination, stem elongation, flower induction and a variety of other responses.⁵ The relationship between the light-dependent reduction of the levels of active GAs and the suppression of stem elongation has been thoroughly studied in dicots including lettuce,⁶ pea⁷ and Arabidopsis,⁸ but not in monocots. Therefore, we investigated the photoregulation of GA metabolism in rice seedlings, focusing on changes in endogenous GA contents and in the expression of genes encoding enzymes involved in GA biosynthesis and inactivation. We found that B and R light reduced active GA levels with consistent changes in the expression of some genes for GA metabolism. Next, we identified the responsible photoreceptors using known phytochrome mutants^3,4,9 and novel cryptochrome-deficient lines. Our analyses demonstrated that cry1s are involved in the induction of GA inactivation genes (OsGA2ox4-7) under B light (Fig. 1A)¹⁰ which seems consistent with observations in Arabidopsis (Fig. 1B).⁸ However, our findings also indicated that the regulation of GA biosynthesis genes in rice differs from that in Arabidopsis, in which cryptochromes suppress the expression of AtGA20ox1 and AtGA3ox1 under B light (Fig. 1B).⁸ In rice, GA biosynthesis genes (OsGA20ox2 and OsGA20ox4) were also suppressed under B light, but this effect was observed also in all cryptochrome-deficient lines available to date. Surprisingly, phytochrome triple mutants could not suppress the expression of GA biosynthesis genes under B light, indicating that the B light-induced suppression is mediated by phytochromes rather than cryptochromes in rice (Fig. 1A). This was not surprising since rice phytochromes mediate various B light responses, especially at the seedling stage.^3,11 Moreover, detailed analyses of phytochrome mutants exposed to R light showed that phyB with synergistic support by phyA did not only suppress GA biosynthesis genes but also weakly induced GA inactivation genes (OsGA2ox4, OsGA2ox6, OsGA2ox7 and OsGA2ox9) (Fig. 1A).

Figure 1. Models of the photoregulation of GA contents in various plant seedlings. (A) Rice cry1 species (cry1a and cry1b) induce a limited number of OsGA2ox gene family members. Phytochromes mediate the repression of GA biosynthesis genes and weakly induce GA inactivation genes. These independent actions cumulatively reduce the level of active GAs in the cells, leading to the suppression of leaf sheath elongation in rice seedlings exposed to visible light. (B) Arabidopsis cry1 and cry2 act complementarily to induce GA inactivation genes and repress GA biosynthesis genes under B light. Arabidopsis phyA mediates the same responses supplementarily. (C) In pea seedlings, R, FR and B light induce GA inactivation genes and repress GA biosynthesis genes. As a consequence, active GA levels drop in the light. phyA but not phyB appears responsible for the responses to R and FR light stimuli. On the other hand, the B light photoreceptor(s) have not been identified yet. The connections between the photoreceptors and the regulation of the expression of genes encoding enzymes for GA biosynthesis and inactivation remain unclear at this stage. (D) In light-induced germination of Arabidopsis, R light perceived by phyB increases active GA levels through the induction of GA biosynthesis genes and the suppression of GA inactivation genes.

Our finding of phyB-mediated regulation of GA metabolism genes in rice deviates from the consensus view of GA regulation based on observations in pea and Arabidopsis. In pea, phyA, but not phyB, mediates the reduction of active GA levels after both R and FR light irradiation (Fig. 1C), by suppressing a GA biosynthesis gene (PsGA3ox1) and inducing a GA inactivation gene (PsGA2ox2).⁷ On the other hand, pea phyA has no capacity of decreasing active GA levels under B light irradiation; B light photoreceptor(s) that still await identification play this role (Fig. 1C). Zhao (2007) et al. reported that Arabidopsis phyA contributed weakly to the induction of AtGA2ox1 under B light (Fig. 1B).⁸ In contrast, Arabidopsis phyB tends to regulate GA responsiveness rather than GA metabolism during the suppression of hypocotyl elongation.¹² Molecular details of phyB involvement in GA signaling have been gradually elucidated in Arabidopsis.^13,14 A series of orthologs of key factors in GA and phytochrome signaling, DELLA and Phytochrome Interacting Factors (PIFs), have been identified and partly characterized in rice.^15,16 However, the interactions of these factors have not been clarified yet.

In conclusion, photoreceptor activities that regulate GA metabolism in the de-etiolation of rice and Arabidopsis seedlings can be subdivided into two aspects. In both species, B light is a major stimulus that suppresses stem elongation, and cry1 plays a pivotal role for the induction of GA inactivation enzymes. As these enzymes inactivate GAs directly, their effects are rapid. Second, a suppression of GA biosynthetic enzymes occurs that appears caused by distinct processes in the two species. Rice phyB directly contributes to the reduction of GA levels through the suppression of GA biosynthesis genes, but Arabidopsis phyB does not; cryptochromes serve this function in Arabidopsis.

We hypothesize that this difference corresponds to distinct strategies in early photomorphogenesis in rice and Arabidopsis. In the light-induced germination of Arabidopsis, R light perceived by phyB increases active GA levels through the induction of GA biosynthesis genes and the suppression of GA inactivation genes (Fig. 1D).^17,18 These phyB effects on GA levels in germinating Arabidopsis seeds are the exact opposite of those observed in rice seedling development. In contrast to Arabidopsis, no light stimuli are required for the germination of rice seeds, implying that rice phyB plays no role in the regulation of GA metabolism in germination. On the other hand, during seedling development, rice phyB directly regulates GA metabolism through the suppression of GA biosynthesis genes and the weak induction of GA inactivation genes. Our research indicates that in rice, independent photoreceptors separately but cooperatively mediate GA metabolism. The resulting network structure may be advantageous as it increases the robustness of the important light responses. While we have characterized the functional relationship between cryptochromes and phytochromes in the regulation of GA metabolism, it remains unclear how these photoreceptors regulate GA signaling in rice. Further research is required to solve the complex network underlying the photoregulation of GA signaling in rice.

Glossary

Abbreviations:

B

blue

cry

cryptochrome

FR

far-red

GA

gibberellin

GA2ox

GA 2-oxidase

GA20ox

GA 20-oxidase

GA3ox

GA 3-oxidase

phy

phytochrome

R

red

Hirose F, Inagaki N, Hanada A, Yamaguchi S, Kamiya Y, Miyao A, et al. Cryptochrome and phytochrome cooperatively but independently reduce active gibberellin content in rice seedlings under light irradiation. Plant Cell Physiol. 2012;53:1570–82. doi: 10.1093/pcp/pcs097.

Disclosure of Potential Conflicts of Interest

No potential conflicts of interest were disclosed.