"How is plant growth controlled?" and "What is the basis of variation in stress tolerance in plants?" were among the 125 most challenging scientific questions, according to the journal Science in 2016.
Strigolactone (SL) is an important plant hormone that plays essential roles in regulating branch number, a key growth and development trait for plants. Recently, scientists from the Chinese Academy of Sciences (CAS) have uncovered the mechanism behind SL perception and its key role in the tillering response to nitrogen.
The "gas and brake" mechanism of SL perception allows "smart and flexible" regulation of the duration and intensity of SL signaling, thereby regulating rice tiller development in response to nitrogen.
The study was published online in Cell on Nov. 4.
Previous SL signaling studies revealed that DWARF14 (D14) and its homologs are SL receptors that interact with F-box protein D3 and transcriptional repressor D53 in an SL-dependent manner, thus triggering downstream transcriptional events.
To elucidate the key mechanism of SL perception, WANG Bing and his colleagues from Prof. LI Jiayang's team at the CAS Institute of Genetics and Developmental Biology (IGDB) systematically identified the D14 amino acid residues vital for interactions with D3 and D53 and revealed the mechanism underlying activation of SL perception.
They further explored the termination mechanism of SL perception. Through ingenious experimental design, they reported that the ubiquitination and protein degradation of D14 were dependent on D3 and required the D14 N-terminal disordered (NTD) domain, representing an undiscovered mechanism in higher plants.
As a subunit of the E3 ligase that recognizes substrates, D3 first promotes the ubiquitination and degradation of D53 to initiate signal transduction and then promotes the ubiquitination and degradation of D14 to terminate signal sensing. This constitutes a "gas pedal" and "brakes" pair in signal transduction.
More interestingly, the NTD domain of D14 undergoes phosphorylation to inhibit ubiquitination and degradation of D14, thereby regulating tiller development in rice. The low-nitrogen environment increases the phosphorylation of D14, thereby inhibiting protein degradation and enhancing SL perception.
Genetic analysis further confirmed that the phosphorylation of D14 at the N-terminus is an important mechanism in the regulation of rice tillering via low-nitrogen signals.
The researchers proposed that the low-nitrogen signal, on the one hand, enhances SL perception by inducing SL biosynthesis and, on the other hand, inhibits D14 degradation by promoting its own phosphorylation, thereby reducing termination of SL perception. These mechanisms synergistically enhance the function of the SL pathway to reduce tiller number.
By changing the phosphorylation status of D14, a reduction in nitrogen fertilizer input without a reduction in tillering can be realized, which is important for precisely improving crop architecture and the molecular design of crops with higher yields and less fertilizer need.
"This study reveals exciting novelties, for instance, with respect to a post-translational modification (phosphorylation) of D14 and its role in the response to nitrogen availability," said a reviewer of the paper.
Their findings provide a new view of SL perception regulation and address the controversial views of some different models of SL perception, according to the reviewers.
This study was supported by the National Key R&D Program, the National Natural Science Foundation of China, the Young Scientists Project for Basic Research of CAS, and the Youth Promotion Association of CAS.
