Regulation of MYB and bHLH Transcription Factors： A Glance at the Protein Level

In complex, constantly changing environments, plants have developed astonishing survival strategies. These elaborated strategies rely on rapid and precise gene regulation mediated by transcription factors （TFs）. TFs represent a large fraction of plant genomes and among them, MYBs and basic helix-loop-helix （bHLHs） have unique inherent properties specific to plants. Proteins of these two TF families can act as homo- or heterodimers, associate with proteins from other protein families, or form MYB/bHLH complexes to regulate distinct cellular processes. The ability of MYBs and bHLHs to interact with multiple protein part- ners has evolved to keep up with the increased metabolic complexity of multi-cellular organisms. Associ- ation and disassociation of dynamic TF complexes in response to developmental and environmental cues are controlled through a plethora of regulatory mechanisms specifically modulating TF activity. Regulation of TFs at the protein level is critical for efficient and precise control of their activity, and thus provides the mechanistic basis for a rapid on-and-off switch of TF activity. In this review, examples of post-translational modifications, protein-protein interactions, and subcellular mobilization of TFs are discussed with regard to the relevance of these regulatory mechanisms for the specific activation of MYBs and bHLHs in response to a given environmental stimulus.

Biotechnological approaches in glucosinolate production

Glucosinolates(GLSs) are sulfur-rich, amino acid-derived defense compounds characteristic of the Brassicales order. In the past, GLSs were mostly known as anti-nutritional factors in fodder, biopesticides in agriculture, and flavors in condiments such as mustard. However, in recent times, GLSs have received increased attention as promoters of human health.This has spurred intensive research towards generating rich sources of health-promoting GLSs. We provide a comprehensive overview of the biotechnologica approaches applied to reach this goal. This includes optimization of GLS production and composition in native, GLS-producing plants, including hairy root and cell cultures thereof, as well as synthetic biology approaches in heterologous hosts, such as tobacco and the microbial organisms Escherichia coli and Saccharomyces cerevisiae. The progress using these different approaches is discussed.

The Glucosinolate Biosynthetic Gene A OP2 Mediates Feed-back Regulation of Jasmonic Acid Signaling in Arabidopsis

Survival in changing and challenging environments requires an organism to efficiently obtain and use its resources. Due to their sessile nature, it is particularly critical for plants to dynamically optimize their meta- bolism. In plant primary metabolism, metabolic fine-tuning involves feed-back mechanisms whereby the output of a pathway controls its input to generate a precise and robust response to environmental changes. By contrast, few studies have addressed the potential for feed-back regulation of secondary metabolism. In Arabidopsis, accumulation of the defense compounds glucosinolates has previously been linked to genetic variation in the glucosinolate biosynthetic gene AOP2. AOP2 expression can increase the transcript levels of two known regulators （MYB28 and MYB29） of the pathway, suggesting thatAOP2 plays a role in positive feed-back regulation controlling glucosinolate biosynthesis. We generated mutants affecting AOP2, MYB28/29, or both. Transcriptome analysis of these mutants identified a so far unrecognized link between AOP2 and jasmonic acid （JA） signaling independent of MYB28 and MYB29. Thus, AOP2 is part of a regulatory feed-back loop linking glucosinolate biosynthesis and JA signaling and thereby allows the glucosinolate pathway to influence JA sensitivity. The discovery of this regulatory feed-back loop provides insight into how plants optimize the use of resources for defensive metabolites.

GTR-Mediated Radial Import Directs Accumulation of Defensive Glucosinolates to Sulfur-Rich Cells in the Phloem Cap of Arabidopsis Inflorescence Stem

In the phloem cap region o i Arabidopsis plants,sulfur-rich cells(S-cells)accumulate>100 mM glucosinolates(GLS),but are biosynthetically inactive.The source and route of S-cell-bound GLS remain elusive.In this study,using single-cell sampling and scanning electron microscopy with energy-dispersive X-ray analysis we show that two GLS importers,NPF2.10/GTR1 and NPF2.11/GTR2,are critical for GLS accumulation in S-cells,although they are not localized in the S-cells.Comparison of GLS levels in S-cells in multiple combinations of homo-and heterografts o lg t r l gtr2,biosynthetic null mutant and wild-type plants indicate that S-cells accumulate GLS via symplasmic connections either directly from neighboring biosynthetic cells or indirectly to non-neighboring cells expressing GTR1/2.Distinct sources and transport routes exist for different types of GLS,and vary depending on the position of S-cells in the inflorescence stem.Based on these findings,we propose a model illustrating the GLS transport routes either directly from biosynthetic cells or via GTR-mediated import from apoplastic space radially into a symplasmic domain,wherein the S-cells are the ultimate sink.Similarly,we observed accumulation of the cyanogenic glucoside defensive compounds in high-turgor cells in the phloem cap of Lotus japonicus,suggesting that storage of defensive compounds in high-turgor cells may be a general mechanism for chemical protection of the phloem cap.