Two genetic brakes help regulate periwinkle’s alkaloid pathway

Catharanthus roseus produces monoterpenoid indole alkaloids, or MIAs, through a pathway that supplies precursors for the chemotherapy drugs vinblastine and vincristine. A new study identifies two proteins that help restrain expression of this pathway—but also shows why removing that restraint is not a straightforward recipe for increasing production.

In a bioRxiv preprint, Samuel Breselge and colleagues characterize the transcription factors CrJAM2 and CrJAM3 using gene silencing, promoter-reporter assays and genetically modified root cultures. The work has not been peer reviewed. Its strongest evidence concerns regulation of gene expression, rather than improved alkaloid yields.

Testing the brakes from several directions

The researchers examined CrJAM2 and CrJAM3 because related proteins restrain responses to jasmonate, a plant defense hormone, in Arabidopsis. In C. roseus, jasmonate activates an alkaloid-regulatory network involving the transcription factor MYC2a and downstream activators called BIS and ORCA.

To test the two candidate repressors, the team first used virus-induced gene silencing in leaves. In the immature leaves analyzed, CrJAM2 expression fell by 36% and CrJAM3 by 62%. Several pathway-related genes, including BIS1, BIS2, TDC and T16H1, showed roughly twofold or greater increases in expression.

Those downstream changes require caution: the figure caption describes the increases in most regulatory and pathway genes as not statistically significant. The alkaloid measurements were similarly suggestive rather than conclusive. Across five plants per condition, vindoline, ajmalicine and serpentine measurements rose by 33%, 86% and 51%, respectively, while catharanthine was unchanged. None of those metabolite differences reached statistical significance.

A separate set of experiments provided clearer evidence of repression. In seedlings, the researchers linked selected gene promoters—the DNA regions controlling transcription—to a light-producing reporter. Increasing CrJAM2 or CrJAM3 reduced activity from several promoters, including those of key pathway activators and biosynthetic enzymes. Not every promoter responded, however, and the T16H2 promoter showed unexpected activation, particularly with CrJAM3.

These interventions demonstrate effects on promoter activity, but do not by themselves establish whether either protein directly binds each affected promoter.

Root cultures reveal unequal roles

The team then generated genetically modified hairy root cultures, either overexpressing individual CrJAM genes or carrying CRISPR-Cas9 mutations targeting CrJAM2 and CrJAM3 together. Additional cultures also targeted the related repressor CrRMT1.

RNA sequencing showed that CrJAM2 overexpression broadly reduced MIA-pathway gene expression before hormone treatment or 30 minutes afterward, relative to corresponding controls. Several genes were suppressed by more than 75% in both selected CrJAM2-overexpressing lines.

CrJAM3 had a narrower effect. Some root-associated pathway genes actually increased in its overexpression lines, suggesting that the two proteins have overlapping but distinct regulatory roles.

Conversely, the two double-mutant lines generally showed unchanged or modestly elevated pathway-gene expression under basal conditions and at the early sampling point. The triple-mutant lines were less consistent with one another. Thus, reducing these repressors did not uniformly activate every part of the pathway.

A hormone-dependent reversal

The most striking complication emerged 24 hours after treatment with 250 micromolar methyl jasmonate. Three of the four CrJAM-overexpressing lines then showed higher expression of many pathway genes than hormone-treated controls. Meanwhile, the double-mutant lines showed unchanged or lower expression of most pathway genes relative to their controls.

The authors propose that feedback within the regulatory network could explain this reversal. CrJAM overexpression also suppressed expression of JAZ repressors; CrJAM mutations increased it. At 24 hours, differences in the balance of activator and repressor transcripts tracked the contrasting pathway responses.

That explanation remains a model supported by expression patterns, not a demonstrated causal sequence. Transcript measurements alone do not establish the corresponding protein activities or show that JAZ changes caused the reversal.

Regulation is not yet production

The root-culture evidence also has practical limits. Expression profiling used two selected lines per altered genotype, and some edited alleles retained potentially functional protein regions. Most importantly, the modified cultures lost viability during prolonged maintenance and died before alkaloid profiling could be completed. The cause of that decline was not established.

Taken together, the experiments support CrJAM2 and CrJAM3 as regulators that broadly restrain alkaloid-pathway gene expression in the tested settings. They do not establish increased chemotherapy-drug production or a durable alkaloid-yield benefit. The hormone-dependent reversal instead highlights a feedback-controlled network whose response depends on the regulator, tissue and sampling time.