Somatic cybridization in citrus: potential for improving disease resistance/tolerance in existing cultivars without altering cultivar integrity

We learned unexpectedly decades ago that we can produce cybrid citrus plants as a byproduct of protoplast fusion experiments designed to generate somatic hybrids.
Cybrid plants are diploid and have the nuclear genome of one parent combined with the mitochondrial genome and/or chloroplast genome of a second parent.
Our somatic hybridization model fuses embryogenic suspension culture protoplasts (source of totipotency) of one parent with leaf-derived protoplasts of a complementary second parent (non-regenerable). Regenerated diploid plants that exhibit the leaf-protoplast parent phenotype are always cybrids that exhibit the mitochondrial genome of the embryogenic parent combined with the nucleus of the leaf-parent.
The chloroplast genome is randomly inherited from one or the other parent.
Following this discovery, we began making targeted cybrid plants.
The first cybrid citrus cultivar released for commercial production was our N2-28 ‘Summer Gold’ grapefruit, a unique grapefruit that has an extended harvest window and higher Brix than standard ‘Ruby’. We have recently discovered a putative cybrid of ‘Dancy’ tangerine with Satsuma cytoplasm that is not showing Alternaria susceptibility, a serious problem with traditional ‘Dancy’. We also produced three populations of targeted grapefruit cybrids containing cytoplasm from ‘Meiwa’ kumquat, with a goal of improving tolerance to citrus canker.
Greenhouse assay results for canker tolerance showed that cybrids containing the ‘Meiwa’ mitochondrial genome along with the grapefruit chloroplast genome exhibited typical grapefruit susceptibility to citrus canker; whereas cybrids that contained both the mitochondrial and chloroplast genomes from ‘Meiwa’ exhibited variable increased levels of canker tolerance.
The increased canker tolerance of these selections is now being observed in the field, and now a few of these grapefruit cybrids are unexpectedly showing increased tolerance (and possibly resistance) to HLB in the field after several years of strong HLB pressure.
Further research is underway to validate these preliminary results as necessary to expedite the commercial release of any grapefruit cultivars with improved disease resistance.

XV International Citrus Congress

J.W. Grosser, A.A. Omar, F.G. Gmitter, C. Chase, M. Murata, J. Graham, F.T. Zambon

chloroplast, citrus canker, grapefruit, HLB (Huanglongbing), mitochondria, protoplast fusion

https://doi.org/10.17660/ActaHortic.2026.1448.34