Sphingolipid homeostasis: How do cells know when enough is enough? Implications for plant pathogen responses

Area of Science:

• Plant molecular biology
• Biochemistry
• Cellular homeostasis

Background:

• Sphingolipids are crucial for plant cell membranes and survival.
• Dysregulation of sphingolipid intermediates, like LCBs and ceramides, triggers programmed cell death.
• Elevated sphingolipids are vital for plant immune responses, such as the hypersensitive response.

Purpose of the Study:

• To elucidate the regulatory mechanism of sphingolipid biosynthesis in plants.
• To understand the role of serine palmitoyltransferase (SPT) and its regulator Orosomucoid-like (ORM) in maintaining sphingolipid homeostasis.
• To model how ceramide synthases interact with the ORM-SPT complex to control sphingolipid levels.

Main Methods:

• Analysis of the Arabidopsis SPT complex structure using cryo-electron microscopy.
• Characterization of Arabidopsis mutants affecting sphingolipid biosynthesis.
• Biochemical assays to study enzyme kinetics and protein interactions.

Main Results:

• Ceramides bind to ORM proteins, competitively inhibiting SPT activity and sensing intracellular ceramide levels.
• Two classes of Arabidopsis ceramide synthases produce distinct ceramides (trihydroxy vs. dihydroxy).
• These ceramides differentially interact with ORM, modulating SPT activity and thus sphingolipid biosynthesis.

Conclusions:

• The ORM-SPT complex acts as a critical sensor for sphingolipid homeostasis in plants.
• Ceramide synthases and their products fine-tune SPT activity, balancing cell viability with immune responses.
• This regulatory network allows plants to adapt sphingolipid levels based on internal cues and external stimuli like pathogen attack.