Chlorophyll-deficient mutation frequency in barley following continuous gamma irradiation over a wide range of exposures throughout the life cycle

An experiment was conducted to test the influence of continuous γ-irradiation over a wide range of exposures throughout one life cycle on inducing chlorophyll-deficient mutations in barley (Hordeum vulgare). 200 seeds per treatment were planted at various distances from the radiation source. Treatments ranged from 0.17 R/day (16.5 R total exposure) to 23.2 R/day (2240 R total exposure). The plants were exposed 20 h per day from time of emergence until harvest (96 days) and the apical spikes harvested from each surviving plant. 20 seeds (at random) from each spike were removed and bulked with those from other spikes within the same treatment to make up the M₁ population. Seeds from non-irradiated plants were also included as a treatment. Up to 5 spikes were harvested from each M₁ plant, planted in greenhouse benches, and M₂ seedlings scored for chlorophyll-deficient mutants. M₁ plant survival and reduction in seed set were approximately the same regardless of the treatment. The frequency of mutations per 10 000 M₂ seedlings from 1.3 for the non-irradiated population to 4.3 for progeny of plants receiving 0.17 R/day. This frequency remained about the same through 1.45 R/day. At 3.17 R/day, the frequency increased to 7.0 and was 45.4 at 23.2 R/day. Although there appears to be a plateau and the low exposures, it is not possible to determine the exact shape of the dose response. A mathematical model with an equation of the form Y=Be^cx2 and also a linear-linear model were used to summarize the data. The mutation spectrum from all treatments was 70.6% albina, 17.6% viridis and 11.7% others.     

Spaceflight reduces somatic embryogenesis in orchardgrass (Poaceae)

Somatic embryos initiate and develop from single mesophyll cells in in vitro cultured leaf segments of orchardgrass ( Dactylis glomerata L.). Segments were plated at time periods ranging from 21 to 0·9 d (21 h) prior to launch on an 11 d spaceflight (STS-64). Using a paired t -test, there was no significant difference in embryogenesis from preplating periods of 14 d and 21 d. However, embryogenesis was reduced by 70% in segments plated 21 h before launch and this treatment was significant at P = 0·0001. The initial cell divisions leading to embryo formation would be taking place during flight in this treatment. A higher ratio of anticlinal:periclinal first cell divisions observed in the flight compared to the control tissue suggests that microgravity affects axis determination and embryo polarity at a very early stage. A similar reduction in zygotic embryogenesis would reduce seed formation and have important implications for long-term space flight or colonization where seeds would be needed either for direct consumption or to grow another generation of plants.     

In vitro differentiation of multiple shoot clumps from intact seedlings of switchgrass

Summary An efficient and reproduciblein vitro culture system has been developed for regeneration of multiple shoot clumps from intact seedlings of both lowland and upland cultivars of switchgrass (Panicum virgatum L.). The multiple shoots were induced on Murashige and Skoog medium supplemented with various combinations of 2,4-dichlorophenoxyacetic acid (2,4-D) and 1-phenyl-3-(1,2,3-thiadiazol-5YL)-urea (thidiazuron or TDZ). Maximum response was obtained with 4.5 μM 2,4-D and 18.2 μM TDZ. These shoots proliferated and rooted efficiently on MS medium without growth regulators. The developmental pattern of the multiple shoots indicated their origin from the enlarged shoot apex via proliferation of axillary buds and subsequent reprogramming of shoot meristems followed by secondary differentiation of adventitious shoots The simplicity of the protocol and direct production of multiple shoots make this a potential system that is highly attractive and amenable for microprojectile-mediated gene transfer.