Gluconeogenesis and nitrogen metabolism in maize

Area of Science:

• Plant Biochemistry
• Molecular Biology
• Agricultural Science

Background:

• Gluconeogenesis in plants occurs via two main pathways: phosphoenolpyruvate carboxykinase (PEPCK) and pyruvate orthophosphate dikinase (PPDK).
• Understanding the spatial and metabolic roles of these enzymes is crucial for plant nitrogen assimilation and energy metabolism.

Purpose of the Study:

• To determine the occurrence and location of PEPCK and PPDK in developing maize kernels and roots.
• To investigate the relationship between the abundance of these enzymes and the metabolism of specific nitrogenous compounds.
• To elucidate the broader role of gluconeogenesis in coordinating nitrogen metabolism in maize.

Main Methods:

• Enzyme assays were performed on extracts from developing maize kernels and roots.
• Localization studies were conducted to pinpoint the specific tissues where PEPCK and PPDK are found.
• Enzyme abundance was analyzed under conditions of ammonium feeding, anoxia, and in relation to specific nitrogenous compound metabolism (alanine, asparagine).

Main Results:

• PPDK was more abundant than PEPCK in whole kernel extracts, with PPDK localized in the peripheral endosperm and PEPCK in the pedicel and basal endosperm transfer cells.
• In maize roots, PEPCK abundance increased with ammonium feeding and was localized in vascular-associated tissues, while PPDK abundance increased under anoxia, correlating with alanine accumulation.
• Both enzymes' abundance was significantly altered by nitrogen metabolism factors, with PPDK linked to alanine and PEPCK to ammonium or asparagine metabolism.

Conclusions:

• Gluconeogenesis occurs in diverse maize tissues, with distinct localization patterns for PPDK and PEPCK.
• The abundance of PPDK and PEPCK is closely associated with the metabolism of specific nitrogenous compounds, suggesting a role in nitrogen coordination.
• These enzymes may be integral to a mechanism coordinating nitrogen import/mobilization with utilization, potentially contributing to the evolution of C4 photosynthesis and other metabolic pathways.