The SCF complex is a widely studied multi-subunit ring E3 ubiquitin ligase that tags targeted proteins with ubiquitin for protein degradation by the ubiquitin 26S-proteasome system (UPS). The UPS is an important system that generally keeps cellular events tightly regulated by purging misfolded or damaged proteins and selectively degrading important regulatory proteins. The specificity of this post-translational regulation is controlled by F-box proteins (FBPs) via selective recognition of a protein-protein interaction motif at the C-terminal domain. Hence, FBPs are pivotal proteins in determining the plant response in multiple scenarios. It is not surprising that the FBP family is one of the largest protein families in the plant kingdom. In this review, the roles of FBPs, specifically in plants, are compiled to provide insights into their involvement in secondary metabolites, plant stresses, phytohormone signalling, plant developmental processes and miRNA biogenesis. Keywords Ubiquitin 26S-proteasome system (UPS) • SCF complex • F-box protein (FBP) • Plant stresses • Phytohormone signalling • Secondary metabolites • Plant development • miRNA

Citral is a mixture of neral and geranial, which are of great interest to the fragrance industry due to its lemon-scented aroma. A newly characterized nerol dehydrogenase of Persicaria minor (PmNeDH) from our recent findings has shown a capacity to convert citral from nerol. Differential gene expression analysis revealed that the expression level of PmNeDH was highly upregulated during early treatment of several stress-related phytohormones i.e. methyl jasmonate (MeJA), salicylic acid (SA) and abscisic acid (ABA). SA and ABA were shown to have a prolonged effect on PmNeDH expression level until second day of treatment. The findings were in agreement with the cisregulatory elements predicted from the gene promoter. The phylogenetic relationship of PmNeDH with its homologs from the medium-chain dehydrogenases/reductases (MDR) superfamily was also mentioned. In this study, we proposed a possible biological function of PmNeDH gene in P. minor, which might play significant roles in plant defence mechanism.

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Protein degradation can occur through Ubiquitin 26S-Proteosome System (UPS). The degradation can be mediated by the SCF E3 ubiquitin ligase complex consisting of Skp1, Cullin, and F-box protein as the main components. The F-box protein at the C-terminal domain functions to recognize the targeted protein to be ubiquitinated and degraded via UPS. A stress-responsive F-box gene, PmF-box1 from Persicaria minor was categorized in the F-box containing kelch repeat (FBK) family; a family that specific to plant kingdom. To identify the targeted protein of PmF-box1, yeast-two hybrid system (Y2H) was used. In the Y2H screening process, mating efficiency is very important to fish out the interacting proteins. Therefore, one modification was conducted to increase the mating efficiency. In this screening, PmF-box1 was used as a bait to screen for the Y2H library which was constructed using RNA from plant samples treated with abscisic acid (ABA) and polyethylene glycol (PEG)-8000 and control sample. Autoactivation and toxicity tests of bait were performed before the Y2H screening. Tests on PmF-box1 showed that it is not toxic to the yeast and cannot autoactivate the yeast reporter genes. Mating efficiency was improved from 2.07% to 9.15% after addition of PEG-4000 in the mating culture compared to the original protocol, which it also increased the colony number in the screening step afterward. Additionally, bands of gene with different sizes were observed on electrophoresis gel after colony PCR analysis from the improved technique. Those genes may code for potential interacting proteins that needs further identification and confirmation.

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Green leaf volatiles (GLVs) play an essential role in plant defence, plant-plant interaction and plant-insect interaction. The plant releases GLVs and inhibits the growth and propagation of plant pathogens. In this study, overexpression of PmF-box1 in wild type A. thaliana showed the downregulation of genes involved in the lipoxygenase-hydroperoxide lyase (LOX-HPL) pathway, which contributes to the biosynthesis of GLVs. It resulted in a marked reduction of hexanal production in the PmF-box1-overexpressing plant. The expression pattern of LOX-HPL branch genes in the kelch-repeat modified PmF-box1 (KMF)-overexpressing plant showed a pattern much closer to the expression of LOX-HPL branch genes in the vector control (VC) plant. It was shown that the functional KMF protein sequence was not responsible for the significant reduction of all GLVs including hexanal, 1-hexanol, (Z)-3-hexen-1-ol, and the carbon 5 (C5) volatile, 1-penten-3-ol, in plants overexpressing KMF. Furthermore, this study also showed that the relative proportion of production of 1-penten-3-ol to hexanal was higher in the PmF-box1-overexpressing plant. Based on the current comparative literature search, PmF-box1 does not appear to interact directly with the proteins or transcription factors of the LOX-HPL pathway. On the other hand, PmF-box1 interacts with SAMS1, which subsequently influences the HPL pathway enzyme genes. Thus, this study highlights the potential roles of PmF-box1 in the manipulation of GLV productions.