The aerial epidermis is a major site of quinolizidine alkaloid biosynthesis in narrow-leafed lupin

New Phytol. 2025 Mar;245(5):2052-2068. doi: 10.1111/nph.20384. Epub 2025 Jan 14.

Abstract

Lupins are promising protein crops that accumulate toxic quinolizidine alkaloids (QAs) in the seeds, complicating their end-use. QAs are synthesized in green organs (leaves, stems, and pods) and a subset of them is transported to the seeds during fruit development. The exact sites of biosynthesis and accumulation remain unknown; however, mesophyll cells have been proposed as sources, and epidermal cells as sinks. We investigated the exact sites of QA biosynthesis and accumulation in biosynthetic organs of narrow-leafed lupin (Lupinus angustifolius) using mass spectrometry-based imaging (MSI), laser-capture microdissection coupled to RNA-Seq, and precursor feeding studies coupled to LC-MS and MSI. We found that the QAs that accumulate in seeds ('core' QAs) were evenly distributed across tissues; however, their esterified versions accumulated primarily in the epidermis. Surprisingly, RNA-Seq revealed strong biosynthetic gene expression in the epidermis, which was confirmed in leaves by quantitative real-time polymerase chain reaction. Finally, feeding studies using a stably labeled precursor showed that the lower leaf epidermis is highly biosynthetic. Our results indicate that the epidermis is a major site of QA biosynthesis in narrow-leafed lupin, challenging the current assumptions. Our work has direct implications for the elucidation of the QA biosynthesis pathway and the long-distance transport network from source to seed.

Keywords: LCM; Lupinus angustifolius; MALDI‐MSI; laser‐capture microdissection; lupin alkaloids; plant specialized metabolites.

MeSH terms

  • Alkaloids* / biosynthesis
  • Alkaloids* / metabolism
  • Biosynthetic Pathways / genetics
  • Gene Expression Regulation, Plant*
  • Lupinus* / genetics
  • Lupinus* / metabolism
  • Plant Epidermis* / metabolism
  • Plant Leaves* / genetics
  • Plant Leaves* / metabolism
  • Quinolizidine Alkaloids
  • Quinolizidines* / metabolism
  • Seeds / genetics
  • Seeds / metabolism

Substances

  • Quinolizidines
  • Alkaloids
  • Quinolizidine Alkaloids