Photoperiod is one of the most reliable environmental cues for plants to regulate flowering timing. In Arabidopsis thaliana, CONSTANS (CO) transcription factor plays a central role in regulating photoperiodic flowering. In contrast to posttranslational regulation of CO protein, still little was known about CO transcriptional regulation. Here we show that the CINCINNATA (CIN) clade of class II TEOSINTE BRANCHED 1/ CYCLOIDEA/ PROLIFERATING CELL NUCLEAR ANTIGEN FACTOR (TCP) proteins act as CO activators. Our yeast one-hybrid analysis revealed that class II CIN-TCPs, including TCP4, bind to the CO promoter. TCP4 induces CO expression around dusk by directly associating with the CO promoter in vivo. In addition, TCP4 binds to another flowering regulator, GIGANTEA (GI), in the nucleus, and induces CO expression in a GI-dependent manner. The physical association of TCP4 with the CO promoter was reduced in the gi mutant, suggesting that GI may enhance the DNA-binding ability of TCP4. Our tandem affinity purification coupled with mass spectrometry (TAP-MS) analysis identified all class II CIN-TCPs as the components of the in vivo TCP4 complex, and the gi mutant did not alter the composition of the TCP4 complex. Taken together, our results demonstrate a novel function of CIN-TCPs as photoperiodic flowering regulators, which may contribute to coordinating plant development with flowering regulation.

Summary CYCLING DOF FACTOR 1 (CDF1) and its homologs play an important role in the floral transition by repressing the expression of floral activator genes such as CONSTANS (CO) and FLOWERING LOCUS T (FT) in Arabidopsis. The day-length specific removal of CDF1-dependent repression is a critical mechanism in photoperiodic flowering. However, the mechanism by which CDF1 represses CO and FT transcription remained elusive. Here we demonstrate that Arabidopsis CDF proteins contain non-EAR motif-like conserved domains required for interaction with the TOPLESS (TPL) co-repressor protein. This TPL interaction confers a repressive function on CDF1, as mutations of the N-terminal TPL binding domain largely impair the ability of CDF1 protein to repress its targets. TPL proteins are present on specific regions of the CO and FT promoters where CDF1 binds during the morning. In addition, TPL binding increases when CDF1 expression is elevated, suggesting that TPL is recruited to these promoters in a time-dependent fashion by CDFs. Moreover, reduction of TPL activity induced by expressing a dominant negative version of TPL (tpl-1) in phloem companion cells results in early flowering and a decreased sensitivity to photoperiod in a manner similar to a cdf loss-of-function mutant. Our results indicate that the mechanism of CDF1 repression is through the formation of a CDF-TPL transcriptional complex, which reduces the expression levels of CO and FT during the morning for seasonal flowering.

The plant‐specific protein GIGANTEA (GI) controls many developmental and physiological processes, mediating rhythmic post‐translational regulation. GI physically binds several proteins implicated in the circadian clock, photoperiodic flowering, and abiotic stress responses. To understand GI's multifaceted function, we aimed to comprehensively and quantitatively identify potential interactors of GI in a time‐specific manner, using proteomics on Arabidopsis plants expressing epitope‐tagged GI. We detected previously identified (in)direct interactors of GI, as well as proteins implicated in protein folding, or degradation, and a previously uncharacterized transcription factor, CYCLING DOF FACTOR6 (CDF6). We verified CDF6's direct interaction with GI, and ZEITLUPE/FLAVIN‐BINDING, KELCH REPEAT, F‐BOX 1/LIGHT KELCH PROTEIN 2 proteins, and demonstrated its involvement in photoperiodic flowering. Extending interaction proteomics to time series provides a data resource of candidate protein targets for GI's post‐translational control.

Plant regeneration is an element of natural and horticultural plant propagation, and a key step in the production of transgenic plants. However, regeneration capacity varies widely among genotypes and species, the molecular basis of which is largely unknown. To shed light on the causes of variation in natural regeneration capacity, we undertook a GWAS of shoot regeneration from dormant cut stems in Populus trichocarpa. Using estimates of callus and shoot regeneration provided by a novel computer vision system, and using a variety of GWAS pipelines and statistical approaches, our analyses revealed over 200 candidate genes. The candidates each explained small fractions of the total genetic variance and many appeared to be members of genetic regulatory networks, showing regeneration to be a highly polygenic trait. The top candidates included regulators of cell adhesion, stress signaling, and hormone signaling pathways, as well as other diverse functions. These candidates provide new insights into the biological complexity of plant regeneration, and may serve as new reagents for improving regeneration and transformation of recalcitrant genotypes and species.

Because of the limitations inherent in conventional breeding of trees and clonally propagated crops, gene editing is of great interest. Dozens of published papers attest to the high efficiency of CRISPR-based systems in clonal crops and trees. The opportunity for “clean” edits is expected to avoid or reduce regulatory burdens in many countries and may improve market acceptance. To date, however, nearly all studies in trees and clonal crops retained all of the gene editing machinery in the genome. Despite high gene editing efficiency, technical and regulatory obstacles are likely to greatly limit progress toward commercial use. Technical obstacles include difficult and slow transformation and regeneration, delayed onset of flowering or clonal systems that make sexual segregation of CRISPR-associated genes difficult, inefficient excision systems to enable removal of functional (protein- or RNA-encoding) transgenic DNA, and narrow host range or limited gene-payload viral systems for efficient transient editing. Regulatory obstacles include those such as in the EU where gene-edited plants are regulated like GMO crops, and the many forms of method-based systems that regulate stringently based on the method vs. product novelty and thus are largely applied to each insertion event. Other major obstacles include the provisions of the Cartagena Protocol with respect to international trade and the need for compliance with the National Environmental Policy Act in the USA. The USDA SECURE act has taken a major step toward a more science- and risk-based—vs. method and insertion event based—system, but much further regulatory and legal innovation is needed in the USA and beyond.