Abstract
Gibberellins (GAs) modulate diverse developmental processes throughout the plant life cycle. However, the interaction between GAs and the circadian rhythm remains unclear. Here, we report that MUT9p-LIKE KINASE1 (MLK1) and MLK2 mediate the interaction between GAs and the circadian clock to regulate hypocotyl elongation in Arabidopsis thaliana DELLA proteins function as master growth repressors that integrate phytohormone signaling and environmental pathways in plant development. MLK1 and MLK2 interact with the DELLA protein REPRESSOR OF ga1-3 (RGA). Loss of MLK1 and MLK2 function results in plants with short hypocotyls and hyposensitivity to GAs. MLK1/2 and RGA directly interact with CIRCADIAN CLOCK ASSOCIATED1 (CCA1), which targets the promoter of DWARF4 (DWF4) to regulate its roles in cell expansion. MLK1/2 antagonize the ability of RGA to bind CCA1, and these factors coordinately regulate the expression of DWF4 RGA suppressed the ability of CCA1 to activate expression from the DWF4 promoter, but MLK1/2 reversed this suppression. Genetically, MLK1/2 act in the same pathway as RGA and CCA1 in hypocotyl elongation. Together, our results provide insight into the mechanism by which MLK1 and MLK2 antagonize the function of RGA in hypocotyl elongation and suggest that MLK1/2 coordinately mediate the regulation of plant development by GAs and the circadian rhythm in Arabidopsis.
Highlights
In seedlings that germinate underground, hypocotyl elongation helps the shoot to reach the surface of the soil, pushing the cotyledons into the light and enabling the switch to autotrophy
Short hypocotyls were observed under long-day (LD) conditions and in the dark, suggesting that MUT9p-LIKE KINASE1 (MLK1) and MLK2 might be involved in the GA pathway (Figure 1F)
We demonstrated that MLK1/2, REPRESSOR OF ga1-3 (RGA), and CLOCK ASSOCIATED1 (CCA1) coordinate plant development, providing insight into the mechanism by which the circadian clock protein CCA1 combines with RGA and MLK1/2 to regulate hypocotyl length
Summary
In seedlings that germinate underground, hypocotyl elongation helps the shoot to reach the surface of the soil, pushing the cotyledons into the light and enabling the switch to autotrophy. In Arabidopsis thaliana, brassinosteroids can overcome the lack of GAs and promote elongation in darkness and in the light (Bai et al, 2012; Gallego-Bartolomé et al, 2012). The Arabidopsis genome encodes five DELLA proteins, namely, GA INSENSITIVE (GAI), REPRESSOR OF ga (RGA), RGA-LIKE1 (RGL1), RGL2, and RGL3, and the rice (Oryza sativa) genome encodes one DELLA protein, SLENDER RICE1 (SLR1) (Ikeda et al, 2001). All of these DELLA proteins function as negative regulators of GA signaling (Olszewski et al, 2002). RGA and GAI redundantly repress elongation growth (Dill and Sun, 2001), whereas RGL1 and RGL2 primarily function in seed germination and floral development (Lee et al, 2002; Wen and Chang, 2002; Cheng et al, 2004)
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