SMAX1-dependent seed germination bypasses GA signalling in Arabidopsis and Striga (Nature Plants)

Strigolactones (SL) are germination cues for parasitic plants, as their seeds will not germinate until they perceive SL exuded from the host plant.  In contrast, gibberellins (GA) are the dominant germination hormone in non-parasitic plants; GA-mediated degradation of DELLA repressors permits germination.  Similarities between SL and GA signaling logic have been observed, suggesting a probable crosstalk between both pathways but this understanding remain elusive. Bunsick et al. demonstrated that Arabidopsis thaliana was able to evade GA requirement for germination in the presence of a synthetic SL, through the ectopic expression of ShHTL4 & 7, SL receptors from Striga hermonthica. Arabidopsis has an HTL receptor also, which can promote germination in response to smoke-derived karrikins, but usually plays a minor role in germination. HTL acts through the F-box protein MAX2, which inactivates SMAX1 repressor proteins to promote germination. Interestingly, SL-activated germination in GA-depleted arabidopsis seeds was lost in a loss-of-function mutation of MAX2 in ShHTL4 & 7 lines, suggesting that ShHTL receptors require MAX2 for signaling in Arabidopsis. Furthermore, loss of SMAX1 function allows germination in the presence of DELLA repressors. As the authors conclude, “Our data suggest that ligand-dependent inactivation of SMAX1 in Striga and Arabidopsis can bypass GA-dependent germination in these species.” (Summary by Toluwase Olukayode @toluxylic) Nature Plants 10.1038/s41477-020-0653-z