Relevance of a decrease in the amount of magnesium in the guard cells for light-induced stomatal opening and guard cell photosynthesis in field bean (Vicia faba L.)

• Applicant: Professor Dr. Christoph-Martin Geilfus
• Subject Area: Plant Cultivation, Plant Nutrition, Agricultural Technology
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 471624304

The plant nutrient magnesium (Mg2+) has many functions. For instance, it is relevant for photosynthetic electron transport, for extrusion of protons (H+) by contributing to the activity of the plasma membrane (PM) H+-translocating ATPase, or for sugar partitioning. Under Mg2+-deficiency, mesophyll cells show a reduced operating efficiency of the photosystem (PS) II and accumulate sugars in photosynthetic sources. There is a wealth of information witnessing the relevance of Mg2+ for these processes in the mesophyll tissue. However, current research has not yet achieved to clarify whether the same processes are affected within guard cells (GCs), should these cells contain not enough Mg2+. After all, there is a pressing need to clarify this because evidence is increasing that these Mg2+-dependent processes, i.e. GC-photosynthesis, GC-sugar partitioning, and GC-H+-extrusion are pivotal for regulating stomatal pore size. In GCs at post-dawn, the PM-H+-ATPase energizes stomatal opening upon illumination via hydrolysing ATP for acidifying the apoplast, while GC photosynthesis is a source of this ATP. Disturbed sugar partitioning changes osmotic potential in the GCs with implication on GC swelling. The pressing question is: Is there a link between the amount of Mg2+ in the GCs and light-induced stomatal opening? This research program seeks elucidation if a reduced amount of Mg2+ in GCs of field bean (Vicia faba) obstructs light-induced stomatal opening. As soon as there is not enough Mg2+ in the GCs, it is hypothesized that light-induced stomatal opening is delayed because of a reduced GC PS II operating efficiency (i.e. reduced production of ATP) and a reduced PM-H+-ATPase–mediated extrusion of H+. Moreover, it is anticipated that sugar partitioning is disturbed within GCs that are characterized by a reduced amount of Mg2, which may change GC solute concentration, influencing GC swelling. The sensitivity of the GC PM-H+-ATPase to light, in dependency of the GC Mg2+ content, is tested using microscopy-based ratiometric pH quantification on intact GCs. GC PS II operating efficiency is determined on isolated intact GCs using a microscopy–pulse amplitude modulation chlorophyll fluorometer. For judging on GC-internal carbohydrate partitioning, both GCs and GC chloroplasts will be subjected to GC-EI/TOF-MS or UPLC-MS/MS for the quantification of Calvin cycle-derived sugars and enzymes. A comparison between both fractions (GC v. GC chloroplast) will reveal if carbohydrate partitioning is affected in GCs that contain not enough Mg2+. The outcome of this research programme is an advanced understanding of the relevance of GC-internal Mg2+ for GC photosynthesis and stomatal opening.

• DFG Programme: Research Grants