MYB proteins regulate carbon flow into the phenylpropanoid pathway for lignin biosynthesis.

Salicylic acid (SA) is a pivotal phytohormone for plant responses to biotic and abiotic stresses. Plants have evolved two pathways to produce SA: the isochorismate synthase and phenylalanine ammonia lyase (PAL) pathways¹. Whereas the isochorismate synthase pathway has been fully identified^2–4, the PAL pathway remains incomplete. Here we report the full characterization of the PAL pathway for SA biosynthesis via functional analysis of rice (Oryza sativa) SA-DEFICIENT GENE 1 (OSD1) to OSD4. The cinnamoyl-coenzyme A (CoA) ligase OSD1 catalyses the conversion of trans-cinnamic acid to cinnamoyl-CoA, which is subsequently transformed to benzoyl-CoA via the β-oxidative pathway in peroxisomes. The resulting benzoyl-CoA is further converted to benzyl benzoate by the peroxisomal benzoyltransferase OSD2. Benzyl benzoate is subsequently hydroxylated to benzyl salicylate by the endoplasmic reticulum membrane-resident cytochrome P450 OSD3, which is ultimately hydrolysed to salicylic acid by the cytoplasmic carboxylesterase OSD4. Evolutionary analyses reveal that the PAL pathway was first assembled before the divergence of gymnosperms and has been conserved in most seed plants. Activation of the PAL pathway in rice significantly enhances salicylic acid levels and plant immunity. Completion of the PAL pathway provides critical insights into the primary salicylic acid biosynthetic pathway across plant species and offers a precise target for modulating crop immunity.

SUMMARY: The phytohormone salicylic acid (SA) regulates biotic and abiotic stress responses in plants. Two distinct biosynthetic pathways for SA have been well documented in plants: the isochorismate (IC) pathway in the chloroplast and the phenylalanine ammonia‐lyase (PAL) pathway in the cytosol. However, there has been no solid evidence that the PAL pathway contributes to SA biosynthesis. Here, we report that feeding Arabidopsis thaliana with Ring‐¹³C‐labeled phenylalanine (¹³C₆‐Phe) resulted in incorporation of the ¹³C label not into SA, but into its isomer 4‐hydroxybenzoic acid (4‐HBA) instead. We obtained similar results when feeding ¹³C₆‐Phe to the SA‐deficient ics1 ics2 mutant and the SA‐hyperaccumulating mutant s3h s5h. Notably, we detected ¹³C₆‐SA when ¹³C₆‐benzoic acid (BA) was provided, suggesting that SA can be synthesized from BA. Furthermore, despite the substantial accumulation of SA upon pathogen infection, we did not observe incorporation of ¹³C label from Phe into SA. We also did not detect ¹³C₆‐SA in PAL‐overexpressing lines in the kfb01 kfb02 kfb39 kfb50 background after being fed ¹³C₆‐Phe, although endogenous PAL levels were dramatically increased. Based on these combined results, we propose that SA biosynthesis is not from Phe in Arabidopsis. These results have important implications for our understanding of the SA biosynthetic pathway in land plants.

Summary: Pigments, as coloured secondary metabolites, endow the world with a rich palette of colours. They primarily originate from plants and microorganisms and play crucial roles in their survival and adaptation processes. In this article, we categorize pigments based on their chemical structure into flavonoids, carotenoids, pyrroles, quinones, azaphilones, melanins, betalains, flavins, and others. We further meticulously describe the colours, sources, and biosynthetic pathways, including key enzymatic steps and regulatory networks that control pigment production, in both plants and microorganisms. In particular, we highlight the role of transport proteins and transcription factors in fine‐tuning these pathways. Finally, we introduce the use of pigments in practical production and research, aiming to provide new insights and directions for the application of coloured compounds in diverse fields, such as agriculture, industry, and medicine.