Revisiting SA biosynthesis: new post-PAL route in plant immunity

Area of Science:

• Plant Biology
• Biochemistry
• Plant Immunity

Background:

• Salicylic acid (SA) is a key plant defense hormone.
• SA biosynthesis was thought to occur via two main pathways: isochorismate synthase (dominant in Brassicaceae) and phenylalanine ammonia-lyase (PAL) (common in other plants).
• The PAL pathway's biochemical steps remained incomplete due to an unidentified benzoic acid 2-hydroxylase.

Purpose of the Study:

• To elucidate the missing biochemical steps in the PAL-dependent salicylic acid (SA) biosynthesis pathway.
• To identify the molecular basis for SA production in non-Brassicaceae plants.
• To establish the evolutionary conservation of SA biosynthesis mechanisms in plant immunity.

Main Methods:

• Investigated the post-phenylalanine ammonia-lyase (PAL) metabolic route.
• Identified key intermediates including benzoyl-coenzyme A (CoA).
• Characterized the multi-compartmental nature of the pathway.

Main Results:

• Identified a conserved post-PAL pathway converting trans-cinnamic acid to SA via benzoyl-CoA intermediates.
• This pathway operates across multiple subcellular compartments.
• The findings explain pathogen-induced SA accumulation in plants outside the Brassicaceae family.

Conclusions:

• The post-PAL pathway represents an ancestral and conserved mechanism for plant immunity.
• This pathway is crucial for salicylic acid (SA) biosynthesis in a wide range of plant species.
• The discovery completes our understanding of SA biosynthesis and its role in plant defense.