Knocking off seed germination in the dark.
Timing of seed germination is determined precisely by several environmental factors including light. In the dark, Phytochrome-Interacting Factor 1 (PIF1) inhibits seed germination by coordinating hormone signaling and suppressing cell wall loosening, whereas, in the light, phytochromes, red/far-red light absorbing-photoreceptors, promote seed germination by inhibiting PIF1. Although PIF1 is known as a master transcription factor for seed germination, it is not known if PIF1 acts alone to regulate seed germination. In the present paper, authors show that LEUNIG_HOMOLOG (LUH), a Groucho family transcription co-regulator, physically interacts with PIF1 protein, binds to promoters of various genes together with PIF1, and regulates the expression of target genes in the dark, causing hormone signaling and cell wall unfavorable for germination. The discovery indicates that PIF1 is not a so-called “lone wolf,” but has a friend!
Light is one of the most important environmental signals. It regulates every aspect of the plant life cycle starting from seed germination and ending with leaf senescence. Sophisticated photoreceptor systems continuously monitor light conditions and adjust developmental processes according to available light conditions. Phytochromes are plant photoreceptors sensing red and far-red light. Upon sensing red light, phytochromes induce various light responses including promotion of seed germination and the suppression of leaf senescence.
About 10 years ago, a KAIST research team headed by professor Giltsu Choi identified PIF1 as a master transcription factor inhibiting seed germination in the dark. Wild type seeds germinate only when phytochromes are activated by red light. However, if PIF1 gene is mutated, all seeds germinate, even in the dark. The team later found that PIF1 inhibits seed germination by setting hormone signaling and cell wall hardness unfavorable for germination. It is not known, however, if PIF1 does it all by itself.
The team hypothesized that PIF1 might need transcription co-regulators to do the job. By systematically searching, the team found that LEUNIG_HOMOLOG (LUH), a Groucho family transcription co-regulator, physically interacts with the PIF1 protein, binds to promoters of various genes together with PIF1, and regulates the expression of target genes together with PIF1 in the dark, causing hormone signaling and cell wall unfavorable for germination. Consistently, if LUH gene is mutated, luh mutant seeds, like pif1 mutant seeds, germinate even in the dark. The results indicate that even the master PIF1 needs a friend LUH to inhibit seed germination in the dark.
Nayoung Lee, Jeongmoo Park, Keunhwa Kim, and Giltsu Choi. (2015) The Transcriptional Coregulator LEUNIG_HOMOLOG Inhibits Light-Dependent Seed Germination in Arabidopsis. The Plant Cell DOI 10.1105/tpc.15.00444
* Laboratory website of the author
Lab webpage; https://sites.google.com/a/kaist.edu/phytochrome