The plant hormones strigolactones and smoke-derived karrikins are butenolide signals that control distinct aspects of plant development. Perception of both molecules in Arabidopsis thaliana requires the F-box protein MORE AXILLARY GROWTH2 (MAX2). Recent studies suggest that the homologous SUPPRESSOR OF MAX2 1 (SMAX1) in Arabidopsis and DWARF53 (D53) in rice (Oryza sativa) are downstream targets of MAX2. Through an extensive analysis of loss-of-function mutants, we demonstrate that the Arabidopsis SMAX1-LIKE genes SMXL6, SMXL7, and SMXL8 are co-orthologs of rice D53 that promote shoot branching. SMXL7 is degraded rapidly after treatment with the synthetic strigolactone mixture rac-GR24. Like D53, SMXL7 degradation is MAX2-and D14-dependent and can be prevented by deletion of a putative P-loop. Loss of SMXL6,7,8 suppresses several other strigolactone-related phenotypes in max2, including increased auxin transport and PIN1 accumulation, and increased lateral root density. Although only SMAX1 regulates germination and hypocotyl elongation, SMAX1 and SMXL6,7,8 have complementary roles in the control of leaf morphology. Our data indicate that SMAX1 and SMXL6,7,8 repress karrikin and strigolactone signaling, respectively, and suggest that all MAX2-dependent growth effects are mediated by degradation of SMAX1/SMXL proteins. We propose that functional diversification within the SMXL family enabled responses to different butenolide signals through a shared regulatory mechanism.

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Karrikins and strigolactones are two classes of butenolide molecules that have diverse effects on plant growth. Karrikins are found in smoke and strigolactones are plant hormones, yet both molecules are likely recognized through highly similar signaling mechanisms. Here we review the most recent discoveries of karrikin and strigolactone perception and signal transduction. Two paralogous α/β hydrolases, KAI2 and D14, are respectively karrikin and strigolactone receptors. D14 acts with an F-box protein, MAX2, to target SMXL/D53 family proteins for proteasomal degradation, and genetic data suggest that KAI2 acts similarly. There are striking parallels in the signaling mechanisms of karrikins, strigolactones, and other plant hormones, including auxins, jasmonates, and gibberellins. Recent investigations of host perception in parasitic plants have demonstrated that strigolactone recognition can evolve following gene duplication of KAI2.

Soluble amyloid-beta (Aβ) oligomers are hypothesized to be the pathogenic species in Alzheimer's disease (AD), and increased levels of oligomers in the brain subsequent to traumatic brain injury (TBI) may exacerbate secondary injury pathways and underlie increased risk of developing AD in later life. To determine whether TBI causes Aβ aggregation and oligomerization in the brain, we exposed triple transgenic AD model mice to controlled cortical impact injury and measured levels of soluble, insoluble, and oligomeric Aβ by enzyme-linked immunosorbent assay (ELISA) at 1, 3, and 7 days postinjury. TBI rapidly increased levels of both soluble and insoluble Aβ40 and Aβ42 in the injured cortex at 1 day postinjury. We confirmed previous findings that identified damaged axons as a major site of Aβ accumulation using both immunohistochemistry and biochemistry. We also report that soluble Aβ oligomers were significantly increased in the injured cortex, as demonstrated by both ELISA and Western blot. Interestingly, the mouse brain is able to rapidly clear trauma-induced Aβ, with both soluble and insoluble Aβ species returning to sham levels by 7 days postinjury. In conclusion, we demonstrate that TBI causes acute accumulation and aggregation of Aβ in the brain, including the formation of low- and high-molecular-weight Aβ oligomers. The formation and aggregation of Aβ into toxic species acutely after injury may play a role in secondary injury cascades after trauma and, chronically, may contribute to increased risk of developing AD in later life.

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Protein neddylation is involved in a wide variety of cellular processes. Here we show that the DNA damage response is perturbed in cells inactivated with an E2 Nedd8 conjugating enzyme UBE2M, measured by RAD51 foci formation kinetics and cell based DNA repair assays. UBE2M knockdown increases DNA breakages and cellular sensitivity to DNA damaging agents, further suggesting heightened genomic instability and defective DNA repair activity. Investigating the downstream Cullin targets of UBE2M revealed that silencing of Cullin 1, 2, and 4 ligases incurred significant DNA damage. In particular, UBE2M knockdown, or defective neddylation of Cullin 2, leads to a blockade in the G1 to S progression and is associated with delayed S-phase dependent DNA damage response. Cullin 4 inactivation leads to an aberrantly high DNA damage response that is associated with increased DNA breakages and sensitivity of cells to DNA damaging agents, suggesting a DNA repair defect is associated. siRNA interrogation of key Cullin substrates show that CDT1, p21, and Claspin are involved in elevated DNA damage in the UBE2M knockdown cells. Therefore, UBE2M is required to maintain genome integrity by activating multiple Cullin ligases throughout the cell cycle.

Auxin critically regulates plant growth and development. Auxin-driven transcriptional responses are mediated through the AUXIN RESPONSE FACTOR (ARF) family of transcription factors. ARF protein condensation attenuates ARF activity, resulting in dramatic shifts in the auxin transcriptional landscape. Here, we perform a forward genetics screen for ARF hypercondensation, identifying an F-box protein, which we named AUXIN RESPONSE FACTOR F-BOX1 (AFF1). Functional characterization of SCFAFF1 revealed that this E3 ubiquitin ligase directly interacts with ARF19 and ARF7 to regulate their accumulation, condensation, and nucleo-cytoplasmic partitioning. Mutants defective in AFF1 display attenuated auxin responsiveness, and developmental defects, suggesting that SCFAFF1 -mediated regulation of ARF protein drives aspects of auxin response and plant development.