Identification of tomato introgression lines with enhanced susceptibility or resistance to infection by parasitic giant dodder (Cuscuta reflexa).

The parasitic flowering plant genus Cuscuta (dodder) is a parasitic weed that infects many important crops. Once it winds around the shoots of potential host plants and initiates the development of penetration organs, called haustoria, only a few plant species have been shown to deploy effective defense mechanisms to ward off Cuscuta parasitization. However, a notable exception is Solanum lycopersicum (tomato), which exhibits a local hypersensitive reaction when attacked by giant dodder (Cuscuta reflexa). Interestingly, the closely related wild desert tomato, Solanum pennellii, is unable to stop the penetration of its tissue by the C. reflexa haustoria. In this study, we observed that grafting a S. pennellii scion onto the rootstock of the resistant S. lycopersicum did not change the susceptibility phenotype of S. pennellii. This suggests that hormones, or other mobile substances, produced by S. lycopersicum do not induce a defense reaction in the susceptible tissue. Screening of a population of introgression lines harboring chromosome fragments from S. pennellii in the genome of the recurrent parent S. lycopersicum, revealed that most lines exhibit the same defense reaction as shown by the S. lycopersicum parental line. However, several lines showed different responses and exhibited either susceptibility, or cell death that extended considerably beyond the infection site. These lines will be valuable for the future identification of key loci involved in the perception of, and resistance to, C. reflexa and for developing strategies to enhance resistance to infection in crop species.

Growth responses of Betula pendula ecotypes to red and far-red light

The effect of Red light (R), Far-red light (FR) and R/FR combinations on shoot growth of latitudinal ecotypes of B. pendula was studied using special diodes that emit monochromatic lights. When a 12 hrs PAR (110 µmol m-2 s-1) was extended with R, FR or R/FR ratios, lower intensities of monochromatic lights could not prevent growth cessation. At 25 µmol m-2 s-1, FR compared to R enhanced stem elongation in all ecotypes. This was due to the inhibitive effect of R on internode elongation. When day-length was extended by R/FR at various ratios, there was continuous shoot elongation, but was found to be declining with increasing ratios. The more the R, the shorter were the internodes of each plant. B. pendula ecotypes produced branches when PAR light during the day was extended by incandescent light, but did not do so when the light extensions were made by monochromatic R or FR or their combination. Branching increased with decreasing latitude of the ecotype.