Knocking Out MAX1 Rewires Leaf Senescence in Lettuce Toward a Stress State

Deleting a single hormone biosynthesis gene in lettuce does not simply slow the aging of leaves in the dark, according to a new study published in BMC Plant Biology. Instead, loss of the gene MORE AXILLARY GROWTH1 (MAX1) redirects the senescence program itself, pushing the plant’s response toward a transcriptional state that is largely absent from wild-type leaves. The finding reframes how strigolactones, a class of plant hormones long implicated in senescence, may act in leafy vegetable crops.

Strigolactones regulate diverse aspects of plant development and have been implicated in promoting leaf senescence, a process that ultimately limits the shelf life of harvested greens. Yet senescence phenotypes associated with strigolactone deficiency have not been consistently observed across species, suggesting that this hormonal function may be species- or context-dependent. The contribution of endogenous strigolactone biosynthesis to senescence in leafy vegetable crops had also remained unclear.

To address these questions, the research team, led by Adi Kedem and Guy Azrieli with Lior Tal as senior author, investigated the role of MAX1 in dark-induced leaf senescence in lettuce (Lactuca sativa). MAX1 encodes a key enzyme in the strigolactone biosynthetic pathway, making it a central point of control over the plant’s own production of these hormones.

The evidence gathered in the study points consistently in one direction: endogenous strigolactone biosynthesis plays a major role in dark-induced senescence in lettuce. The researchers observed that strigolactone pathway genes were induced during dark storage, indicating that the biosynthetic machinery is activated precisely when leaves are deprived of light. In parallel, applying exogenous GR24, a synthetic strigolactone analog, accelerated senescence in the experimental system.

Genetic complementation provided further support. Lettuce MAX1 (LsMAX1) was able to rescue the delayed-senescence phenotype of the Arabidopsis max1 mutant, demonstrating functional conservation of the gene across these two species. This cross-species result strengthens the conclusion that the observed effects in lettuce stem from the loss of MAX1-dependent strigolactone production rather than from an unrelated function of the gene.

The decisive experiment used CRISPR/Cas9 genome editing to generate Lsmax1 mutants in lettuce. When these mutants were subjected to prolonged darkness, they exhibited a pronounced stay-green phenotype, retaining their chlorophyll and visible leaf integrity far longer than wild-type plants. This visible delay was accompanied by a strongly reduced induction of key senescence-associated genes, the molecular markers that normally rise as the senescence program executes.

Strikingly, however, the mutants were not transcriptionally silent. Despite the delayed visible senescence, Lsmax1 leaves retained a substantial transcriptional response to dark storage. The loss of LsMAX1 did not simply weaken the wild-type senescence program; instead, it redirected part of the response toward a distinct stress-associated transcriptional state that was largely absent from wild type. In other words, the mutant leaves were not failing to respond to darkness, but responding along a different trajectory altogether.

The study also examined the broader developmental consequences of losing LsMAX1. Vegetative rosette architecture was unaffected, meaning the edible leafy stage of the plant looked normal.

Taken together, the authors conclude that MAX1-dependent strigolactone biosynthesis is an important regulator of leaf senescence in lettuce, with a role that extends beyond controlling the rate of senescence. Rather than simply delaying the wild-type program, loss of LsMAX1 alters the transcriptional trajectory of senescence, favoring an alternative stress-associated state during prolonged darkness. The strong stay-green phenotype, achieved without detectable changes to vegetative rosette architecture, highlights strigolactone biosynthesis as a potential target for extending postharvest longevity in lettuce and other leafy crops.

As with any early-access study, some caveats apply. The findings concern dark-induced senescence under controlled conditions, and the published version shared ahead of final formatting is subject to further edits. The authors describe strigolactone biosynthesis as a potential target for postharvest improvement, a direction that will require further work to translate from controlled experiments into practical applications for leafy vegetable production and storage.

Subject of Research: The role of the strigolactone biosynthesis gene MAX1 in dark-induced leaf senescence in lettuce

Article Title: Loss of MAX1 redirects, rather than delays, the leaf senescence program in lettuce

Article References: Kedem, A., Azrieli, G., Ron, M., Ozeri, N., Reeves, M., Russ, D., Michelmore, R., & Tal, L. (2026). Loss of MAX1 redirects, rather than delays, the leaf senescence program in lettuce. BMC Plant Biology. https://doi.org/10.1186/s12870-026-10206-4

Image Credits: AI Generated

DOI: 10.1186/s12870-026-10206-4

Keywords: lettuce, MAX1, strigolactones, leaf senescence, CRISPR, stay-green, postharvest longevity, Lactuca sativa, transcriptomics, dark-induced senescence, GR24, LsMAX1