Enzymic and substrate basis for the anaplerotic step in guard cells

Area of Science:

• Plant physiology
• Biochemistry

Background:

• Guard cells regulate stomatal opening and closing, crucial for plant gas exchange and water balance.
• Organic anion synthesis is a key process in guard cell function, but the underlying enzymatic mechanisms and rates are not fully understood.

Purpose of the Study:

• To investigate the rate of organic anion synthesis in Vicia faba L. guard cells.
• To determine the activity and kinetic properties of phosphoenolpyruvate (PEP) carboxylase in guard cells.
• To assess the role of PEP carboxylation as an anaplerotic step in guard cells.

Main Methods:

• Calculating the maximum rate of organic anion synthesis based on malate accumulation.
• Estimating the minimum rate of the PEP carboxylase-catalyzed reaction in guard cells.
• Determining the apparent K(mpep) and concentration of PEP in guard cells.

Main Results:

• The maximum rate of organic anion synthesis was calculated to be 200 millimoles per kilogram dry weight per hour.
• The minimum estimate for PEP carboxylase activity in guard cells was 650 millimoles per kilogram dry weight per hour, significantly higher than in other leaf tissues.
• The apparent K(mpep) was 60 µM at pH 8.7, and PEP concentration was 270 µM during anion synthesis.

Conclusions:

• PEP carboxylase activity in guard cells is exceptionally high.
• The carboxylation of PEP is a likely anaplerotic step in guard cells, supporting stomatal function.