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

Abstract

Background Strigolactones (SLs) regulate diverse aspects of plant development and have been implicated in promoting leaf senescence. However, senescence phenotypes associated with SL deficiency have not been consistently observed across species, suggesting that this function may be species- or context-dependent. Moreover, the contribution of endogenous SL biosynthesis to senescence in leafy vegetable crops remains unclear. Here, we investigated the role of the SL biosynthetic gene MORE AXILLARY GROWTH1 (MAX1) in dark-induced leaf senescence in lettuce (Lactuca sativa). Results We found that endogenous SL biosynthesis plays a major role in dark-induced senescence in lettuce. SL pathway genes were induced during dark storage, while exogenous GR24 accelerated senescence and lettuce MAX1 (LsMAX1) complemented the delayed-senescence phenotype of the Arabidopsis max1 mutant. Consistent with these findings, CRISPR/Cas9-generated Lsmax1 mutants exhibited a pronounced stay-green phenotype during prolonged darkness, accompanied by strongly reduced induction of key senescence-associated genes. Despite this delayed visible senescence, Lsmax1 retained a substantial transcriptional response to dark storage. Strikingly, loss of LsMAX1 did not simply weaken the wild-type senescence program, but redirected part of the response toward a distinct stress-associated transcriptional state that was largely absent from wild type. Loss of LsMAX1 did not affect vegetative rosette architecture, although increased branching emerged after bolting. Conclusions Our findings establish MAX1-dependent SL biosynthesis as an important regulator of leaf senescence in lettuce and reveal 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 without detectable changes to vegetative rosette architecture further highlights SL biosynthesis as a potential target for extending postharvest longevity in lettuce and other leafy crops.