Expression and inheritance of hypersensitive resistance to rice hoja blanca virus mediated by the viral nucleocapsid protein gene in transgenic rice.

Rice hoja blanca virus (RHBV) is a major virus disease of economic importance affecting rice in northern South America, Central America and the Caribbean. This is the first report of transgenic resistance to RHBV and the transformation of an indica rice variety from Latin America. Rice transformed with the RHBV nucleocapsid protein ( N) gene had a significant reduction in disease development. Several reactions were observed that ranged from susceptible to completely resistant plants (immunity). The resistant reactions were characterized by the production of local lesions like a hypersensitive reaction or a recovery phenotype with the emergence of symptom-less new leaves. These transgenic RHBV-resistant rice lines expressed the N gene RNA at low levels that were below the detection limit by Northern blots and only resolved by RT-PCR. The nucleocapsid protein could not be detected in any of the transgenic plants either by Western or ELISA tests. These results suggest that the resistance encoded by the N gene in these plants appears to be mediated by RNA. When challenged with RHBV, the resistant transgenic lines showed a significant increased performance for important agronomic traits including the number of tillers, the number of grains per plant and the yield as compared to the susceptible control. Furthermore, upon inoculation some of the most-resistant transgenic lines showed agronomic traits similar to the uninoculated non-transgenic Cica 8 control. Using both agronomic traits and disease severity as criteria, several of the most-resistant lines were followed through the R(4) generation and demonstrated that the N gene and RHBV resistance was inherited in a stable manner. These transgenic rice lines could become a new genetic resource in developing RHBV-resistant cultivars.

In vitro tuberization of potato clones from different maturity groups.

In vitro tuberization on shoot cultures of early, mid-season, late and very late potatoes was compared. Shoots were grown at 12, 16, or 20 h photoperiods; tuberization was then induced at 0, 8 or 16 h light. In the dark, shoots from early plants initially grown at 16 h consistently set tubers earlier than the other types, whereas the very late line tuberized later and produced significantly fewer tubers. Tuber setting of mid-season plants could not be distinguished from the late type. Tuberization of the very late line was significantly hastened by shortening the photoperiod from 20 h to 12 h during the shoot growth period. Light during tuber induction delayed tuberization. This system may be useful to screen callus-derived plants for maturity, and may also be suitable for in vitro study of the photoperiodic control of tuberization.

Insect-resistant plants with improved horticultural traits from interspecific potato hybrids grown in vitro.

Plants were regenerated from petiole calli of interspecific hybrids of Solanum tuberosum x S. berthaultii, an insect-resistant wild species. Callus culture was used to generate genetic changes to overcome the restricted recombination between the two genomes. Two plants out of 58 (3.5%) from calli of hybrid J114-1 showed stable and heritable differences from the hybrid over two cycles of evaluations in the field. Replicated trials were conducted in 1987 and 1988, using two populations of plants propagated by nodal cuttings from the original regenerates maintained in vitro. One regenerate showed insect resistance and increased marketable yield (approximately two fold) in the field. The other had higher levels of phenolic exudate in one of the two types of foliar trichomes associated with the insect resistance mechanism. Some desirable changes were discernible only in sexual progeny of regenerates, not in the regenerates themselves. In a backcross to S. tuberosum, 7 of 14 (50%) regenerates from hybrid F743-4 showed more progeny (up to 15-fold) with improved trichome traits and horticultural characteristics than the original hybrid. The variations were not associated with changes in ploidy. Fifteen plants obtained from these crosses are currently being incorporated into breeding lines. These results suggest that a period of callus culture followed by plant regeneration may aid in the introgression of desirable traits from wild species into crop plants.