Improved nitrogen use efficiency in transgenic sugarcane: phenotypic assessment in a pot trial under low nitrogen conditions.

KEY MESSAGE: Sugarcane lines transformed with an alanine aminotransferase gene demonstrated an improved nitrogen use efficiency compared with untransformed controls in a pot trial under low nitrogen conditions.

A novel means for cryopreservation of germplasm of the recalcitrant-seeded species, Ekebergia capensis.

Cryopreserved zygotic embryonic axes offer the best means of genetic diversity conservation of recalcitrant-seeded species, but frequently shoots fail to develop following processing for, and after, cryostorage. The present work offers a means to overcome this, by generating adventitious shoots from seedling roots produced after axis cryopreservation. Embryonic axes of Ekebergia capensis were exposed to cryoprotectants, flash dried, and rapidly cooled in nitrogen slush. Cryoprotection was an essential step, with both glycerol and DMSO permitting survival after cryogen exposure, but sucrose alone, or in combination with glycerol, was deleterious. Adventitious shoots were formed from seedling roots developed by axes germinated after cryogen exposure, after being subjected to intermittent flushing with a BAP-containing medium for 24 h in a temporary immersion system and subsequent culture on a semi-solid BAP-containing medium. After excision, a high proportion of the adventitious shoots produced roots in vitro, with most of these rooted plantlets being subsequently successfully acclimated.

Aflatoxin B1--its effects on an in vitro plant system.

The phytotoxic effects of aflatoxin B1 (AFB1) on in vitro cultures of differentiating calli and regenerating plantlets of Nicotiana tabacum were assessed. Callus appeared more sensitive to the effects of AFB1, with fresh mass accumulation and callus chlorophyll levels affected at low (approximately 0.5 micrograms/ml) aflatoxin concentrations. Transmission electron microscopy revealed early deteriorative alterations in chloroplast morphology. Inhibitory effects of the toxin (up to and including 10 micrograms/ml) on callus fresh mass accumulation were reversed following a 3 week toxin-free recovery period. In tobacco plantlets, root and leaf development, and root and leaf mass were significantly inhibited in a dose-dependent fashion with increasing AFB1 concentration above 0.5 micrograms/ml. Inhibitory effects on plantlet root development were more pronounced that on leaf development.

Control of nitrate and nitrite assimilation by carbohydrate reserves, adenosine nucleotides and pyridine nucleotides in leaves of Zea mays L. under dark conditions.

The rate of in-vivo nitrate reduction by leaf segments of Zea mays L. was found to decline during the second hour of dark anaerobic treatment. On transfer to oxygen the capacity to reduce nitrate under dark conditions was restored. These observations led to the proposal that nitrate reductase is a regulatory enzyme with ADP acting as a negative effector. The effect of ADP on the invitro activity of nitrate reductase and the changes in the in-vivo adenylate pool under dark-N2 and dark-O2 were investigated. It was found that ADP inhibited the activity of partially purified nitrate reductase. Similarly, the in-vivo anaerobic inhibition of nitrate reduction was associated with a build-up of ADP in the leaf tissue. Under anaerobic conditions nitrite accumulated and on transfer to oxygen the accumulated nitrite was reduced. To explain this phenomenon the following hypothesis was proposed and tested. Under anaerobic conditions the supply of reducing equivalents for nitrite reduction in the plastid becomes restricted and nitrite accumulates as a consequence. On transfer to oxygen this restriction is removed and nitrite disappears. This capacity to reduce accumulated nitrite was found to be dependent on the carbohydrate status of the leaf tissue.

The effect of oxygen on nitrate and nitrite assimilation in leaves of Zea mays L. under dark conditions.

The assimilation of nitrate under dark-N2 and dark-O2 conditions in Zea mays leaf tissue was investigated using colourimetric and (15)N techniques for the determination of organic and inorganic nitrogen. Studies using (15)N indicated that nitrate was assimilated under dark conditions. However, the rate of nitrate assimilation in the dark was only 28% of the rate under non-saturating light conditions. No nitrite accumulated under dark aerobiosis, even though nitrate reduction occurred under these conditions. The pattern of nitrite accumulation in leaf tissue in response to dark-N2 conditions consisted of three phases: an initial lag phase, followed by a period of rapid nitrite accumulation and finally a phase during which the rate of nitrite accumulation declined. After a 1-h period of dark-anaerobiosis, both nitrate reduction and nitrite accumulation declined considerably. However, when O2 was supplied, nitrate reduction was stimulated and the accumulated nitrite was rapidly reduced. Anaerobic conditions stimulated nitrate reduction in leaf tissue after a period of dark-aerobic pretreatment.