Super roots in Lotus corniculatus: A unique tissue culture and regeneration system in a legume species

Super roots (SR) of Lotus corniculatus are a fast growing legume root culture that allows continuous root cloning, direct somatic embryogenesis and mass regeneration of plants under entirely growth regulator-free culture conditions. These features are unique to root cultures and are now stably expressed since the culture was isolated more than 5 years ago. Super roots switch from exclusive root proliferation to shoot production upon transfer to illuminated and stationary conditions. Lateral root formation continues at a reduced rate while embryos and shoots are forming. When infected with Rhizobium loti, SR carrying somatic embryos and small shoots, form root nodules. Regenerating SR provide evidence that root-derived somatic embryos and shoots of L. corniculatus begin to form in a process related to the development of lateral roots or root nodules. When treated with a low concentration of benzylamino purine (BAP), shoots form at an increased rate, matching the spacing typical for lateral roots. The super-growing root culture of L. corniculatus regenerates plants that show no morphological differences as compared to regenerants from wild-type roots or seedlings. Roots dissected from plantlets derived from SR or from super root-derived protoplasts express all super root qualities again when cultured in vitro. Super roots provide a favorable experimental system for developmental studies that are sensitive to exogenous hormones, such as lateral root formation or nodulation in vitro.