Isolation of three B-box zinc finger proteins that interact with STF1 and COP1 defines a HY5/COP1 interaction network involved in light control of development in soybean.

LONG HYPOCOTYL5 (HY5) and STF1 (Soybean TGACG-motif binding Factor 1) are two related bZIP transcription factors that play a positive role in photomorphogenesis and hormonal signaling. In this study, we compared full length STF1 and truncated STF1 overexpression lines and found that the C-terminal 133 amino acids (194-306) possess all the HY5-like function in Arabidopsis. The STF1-DC1 mutant (1-306), with a 20 amino acid deletion at the carboxy terminus, failed to complement the hy5 mutant phenotype, which suggests an intact C-terminus is required for STF1 function. To understand the role of the C-terminal domain in photomorphogenesis we used a yeast two-hybrid screen to isolate proteins that bind to the STF1 C-terminus. We isolated three soybean cDNAs encoding the zinc-finger proteins GmSTO, GmSTH, and GmSTH2, which interact with STF1. These proteins belong to a family of B-box zinc finger proteins that include Arabidopsis SALT TOLERANCE (STO) and STO HOMOLOG (STH) and STH2, which play a role in light-dependent development and gene expression. The C-terminal 63 amino acids of STF1, containing a leucine zipper and the two N-terminal B-boxes, contains the domain involved in interactions between STF1 and GmSTO. In addition, we identified an interaction between soybean COP1 (GmCOP1) and GmSTO and GmSTH, as well as STF1, which strongly suggests the presence of a similar regulatory circuit for light signaling in soybean as in Arabidopsis. This study shows that photomorphogenic control requires complex molecular interactions among several different classes of transcription factors such as bZIP, B-box factors, and COP1, a ubiquitin ligase.

Differential light-dependent regulation of soybean nodulation by papilionoid-specific HY5 homologs.

Legumes have evolved photosynthesis and symbiotic nitrogen fixation for the acquisition of energy and nitrogen nutrients. During the transition from heterotrophic to autotrophic growth, blue light primarily triggers photosynthesis and low soil nitrogen induces symbiotic nodulation. Whether and how darkness and blue light influence root symbiotic nodulation during this transition is unknown. Here, we show that short-term darkness promotes nodulation and that blue light inhibits nodulation through two soybean TGACG-motif-binding factors (STF1 and STF2), which are Papilionoideae-specific transcription factors and divergent orthologs of Arabidopsis ELONGATED HYPOCOTYL 5 (HY5). STF1 and STF2 negatively regulate soybean nodulation by repressing the transcription of nodule inception a (GmNINa), which is a central regulator of nodulation, in response to darkness and blue light. STF1 and STF2 are not capable of moving from the shoots to roots, and they act both locally and systemically to mediate darkness- and blue-light-regulated nodulation. We further show that cryptochromes GmCRY1s are required for nodulation in the dark and partially contribute to the blue light inhibition of nodulation. In addition, root GmCRY1s mediate blue-light-induced transcription of STF1 and STF2, and intriguingly, GmCRY1b can interact with STF1 and STF2 to stabilize the protein stability of STF1 and STF2. Our results establish that the blue light receptor GmCRY1s-STF1/2 module plays a pivotal role in integrating darkness/blue light and nodulation signals. Furthermore, our findings reveal a molecular basis by which photosensory pathways modulate nodulation and autotrophic growth through an intricate interplay facilitating seedling establishment in response to low nitrogen and light signals.

The nuclear fraction of protein kinase CK2 binds to the upstream stimulatory factors (USFs) in the absence of DNA.

The functions of the upstream stimulatory factors USF1 and USF2 are, like those of other transcription factors, regulated by reversible phosphorylation. Besides many other kinases also protein kinase CK2 phosphorylates USF1 but not USF2. In a yeast-two-hybrid screen, however, the non-catalytic CK2ß subunit of CK2 was identified as a binding partner of USF2. This surprising observation prompted us to investigate the CK2/USF interaction in more detail in the present study. By using immunofluorescence analyses as well as co-immunoprecipitations we found that USF1 and USF2 bound to CK2α and CK2ß exclusively in the nucleus, though CK2ß and to a minor amount CK2α were also present in the cytoplasm. Furthermore, we found that unlike other substrates the phosphorylation of USF1 required the presence of the regulatory CK2ß subunit; the catalytic α-subunit of CK2 alone was not able to phosphorylate USF1. Thus, the correct phosphorylation of USF1 is only guaranteed and strictly controlled in particular by nuclear CK2ß. Although the data indicated that a nuclear subfraction of CK2 subunits associated with USF proteins, DNA pull down experiments revealed that the CK2 subunits did not co-localize with DNA bound USF proteins indicating that the USF/CK2 interaction has a pre- or post DNA binding function.