The mutated CYTOKININ OXIDASE/DEHYDROGENASE 7 promotes cell division in pith and plays a critical role in the development of stem lettuce

Lettuce has developed into several horticultural types, varying dramatically in plant architecture. Stem lettuce, the major type worldwide, has a succulent swollen stem. The genetic and molecular mechanisms underlying the change from small stem in leafy lettuce to large stem in stem lettuce remain largely unknown. In this study, we cloned a major QTL controlling succulent swollen stem in lettuce. The QTL encodes cytokinin oxidase/dehydrogenase 7 (LsCKX7), which degrades cytokinin. The LsCKX7 gene has a nonsense mutation or 2-bp frame-shift insertion in most cultivars of stem lettuce. The non-functional LsCKX7 led to increased concentration of cytokinins in pith and consequently at least 9.6-25.3% increase in stem diameter in different seasons. The two loss-of-function mutations in LsCKX7 occurred only in stem lettuce, suggesting its critical roles in the development of stem lettuce. The mutated LsCKX7 gene can be directly applied in breeding programs of stem lettuce. Our results also shed light on the development of stem pith and provide references for studies on other stem vegetables.