No abstract

Zinc (Zn) deficiency is a widespread phenomenon in agricultural soils worldwide and has a major impact on crop yield and quality, and hence on human nutrition and health. Although dicotyledonous crops represent more than 30% of the human plant-based nutrition, relatively few efforts have been dedicated to the investigation of Zn deficiency response mechanisms in dicots in contrast to monocotyledonous crops, such as rice or barley. Here, we describe the Zn requirement and impact of Zn deficiency in several economically important dicot crops, Phaseolus vulgaris, Glycine max, Brassica oleracea and Solanum lycopersicum. We briefly review our current knowledge of the Zn deficiency response in Arabidopsis and outline how this knowledge is translated in dicot crops. We highlight commonalities and differences between dicot species (and with monocots) regarding the function and regulation of Zn transporters and chelators, as well as the Zn sensing mechanisms and the role of hormones in the Zn deficiency response. We moreover show how the Zn homeostatic network intimately interacts with other nutrients, such as iron or phosphate. Finally, we outline how variation in Zn deficiency tolerance and Zn use efficiency among cultivars of dicot species can be leveraged for the design of Zn biofortification strategies.

Metallic micronutrients are essential throughout the plant life cycle. Maintaining metal homeostasis in plant tissues requires a highly complex and finely tuned network controlling metal uptake, transport, distribution and storage. Zinc and cadmium hyperaccumulation, such as observed in the model plant Arabidopsis halleri, represents an extreme evolution of this network. Here, non‐ectopic overexpression of the A. halleri ZIP6 (AhZIP6) gene, encoding a zinc and cadmium influx transporter, in Arabidopsis thaliana enabled examining the importance of zinc for flower development and reproduction. We show that AhZIP6 expression in flowers leads to male sterility resulting from anther indehiscence in a dose‐dependent manner. The sterility phenotype is associated to delayed tapetum degradation and endothecium collapse, as well as increased magnesium and potassium accumulation and higher expression of the MHX gene in stamens. It is rescued by the co‐expression of the zinc efflux transporter AhHMA4, linking the sterility phenotype to zinc homeostasis. Altogether, our results confirm that AhZIP6 is able to transport zinc in planta and highlight the importance of fine‐tuning zinc homeostasis in reproductive organs. The study illustrates how the characterization of metal hyperaccumulation mechanisms can reveal key nodes and processes in the metal homeostasis network.

Plant decaying biomass is the most abundant provider of carbon sources for soil-dwelling microorganisms. To optimally evolve in such environmental niches, microorganisms possess an arsenal of hydrolytic enzymatic complexes to feed on the various types of polysaccharides, oligosaccharides, and monosaccharides.

Cellulose being the most abundant polysaccharide on earth, beta-glucosidases hydrolyzing cello-oligosaccharides are key enzymes to fuel glycolysis in microorganisms developing on plant material. In Streptomyces scabiei, the causative agent of common scab in root and tuber crops, a genetic compensation phenomenon safeguards the loss of the gene encoding the cello-oligosaccharide hydrolase BglC by awakening the expression of alternative beta-glucosidases. Here we reveal that the BglC compensating enzyme BcpE2 is the GH3-family beta-glucosidase that displays the highest reported substrate promiscuity able to release the glucose moiety of all tested types of plant-derived heterosides (aryl β-glucosides, monolignol glucosides, cyanogenic glucosides, anthocyanosides, and coumarin heterosides). BcpE2 structure analysis highlighted a large cavity in the PA14 domain that covers the active site, and the high flexibility of this domain would allow proper adjustment of this cavity for disparate heterosides. The exceptional substrate promiscuity of BcpE2 provides microorganisms a versatile tool for scavenging glucose from plant-derived nutrients that widely vary in size and structure. Importantly, scopolin is the only substrate commonly hydrolyzed by both BglC and BcpE2 thereby generating the potent virulence inhibitor scopoletin. Next to fueling glycolysis, both enzymes thus also interfere with the plant defense mechanisms to fine-tune the strength of virulence.