Shin-nosuke Hashida

The role of NAD biosynthesis in plant development and stress responses

BACKGROUND Pyridine nucleotides are essential for electron transport and serve as co-factors in multiple metabolic processes in all organisms. Each nucleotide has a particular role in metabolism. For instance, the NAD/NADP ratio is believed to be responsible for sustaining the functional status of plant cells. However, since enzymes involved in the synthesis and degradation of NAD and NADP have not been fully identified, the physiological functions of these co-enzymes in plant growth and development are largely unknown. SCOPE This Botanical Briefing covers progress in the developmental and stress-related roles of genes associated with NAD biosynthesis in plants. Special attention will be given to assessments of physiological impacts through the modulation of NAD and NADP biosynthesis. CONCLUSIONS The significance of NAD biosynthesis in plant development and NADP biosynthesis in plant stress tolerance is summarized in this Briefing. Further investigation of cells expressing a set of NAD biosynthetic genes would facilitate understanding of regulatory mechanisms by which plant cells maintain NAD homeostasis.

Radiocesium contaminations of 20 wood species and the corresponding gamma-ray dose rates around the canopies at 5 months after the Fukushima nuclear power plant accident

Radiocesium ((134)Cs + (137)Cs) deposition from the Fukushima nuclear power plant accident was measured in 20 woody plants (12 evergreen and 8 deciduous species) grown in Abiko (approximately 200 km SSW from the NPP). Leaves (needles) and twigs were sampled from each of three foliar positions (top, middle, and bottom) in the plant canopy in early August 2011. At the time, soils around the plants were also sampled, and gamma radiation dose rates were measured at each sampling position. The average radiocesium activity in the observed leaves of the evergreen species was 7.7 times that in the leaves of the deciduous species. Among the observed evergreen coniferous species, the activity in pre-fallout-expanded leaves was 2.4 times that in the post-fallout-expanded leaves. Notably, a distinct variation in the activity among the evergreen coniferous species could be observed for the post-fallout-expanded leaves but not for the pre-fallout-expanded leaves. Although these differences depend on whether the leaves had expanded at the time of the fallout, it is probable that a considerable amount of radiocesium was translocated to newly developed leaves at a species-specific rate. In addition, it was demonstrated that dose rates around woody plants were not consistent with the prevailing prediction that general dose rates correspondingly decrease with monitoring height from the ground. Thus, the dose rates in the top foliar layer of the deciduous species decreased more than predicted, whereas those in the top foliar layer of the coniferous species did not decrease. This may be due to differences in the balance between the attenuation resulting from a shielding effect of the plant bodies and the higher radiocesium accumulation in the leaves.

Nicotinate/nicotinamide mononucleotide adenyltransferase-mediated regulation of NAD biosynthesis protects guard cells from reactive oxygen species in ABA-mediated stomatal movement in Arabidopsis

Nicotinamide adenine dinucleotide (NAD) and its derivative nicotinamide adenine dinucleotide phosphate (NADP) are indispensable co-factors in broad-spectrum metabolic events for the maintenance of cellular homeostasis in all living organisms. In this study, the cellular expression levels of NAD biosynthesis genes in Arabidopsis were investigated. A very high expression of nicotinate/nicotinamide mononucleotide adenyltransferase (NMNAT) was observed in the differentiated stomatal guard cells of the leaf surface. Transcriptional analysis confirmed that several genes in the biosynthesis pathway were also highly expressed in stomatal guard cells. In fact, NAD and NADP metabolisms were investigated during stomatal movement. Importantly, the generation of phytohormone ABA-induced reactive oxygen species, which acts as a signal for stomatal closure, was accompanied by markedly decreased levels of NAD. The ABA-induced oxidative stress caused stomatal cell death in the nmnat mutant. Furthermore, stomata partially lost their ability to close leading to drought susceptibility. The stomata were less responsive to opening cues as well. These results indicate that NAD biosynthesis is involved in protecting guard cells from ABA-induced local oxidative stress via the regulation of NMNAT activity. In this study, it is demonstrated that NMNAT is essential for the maintenance of NAD homeostasis enabling sustainable stomatal movement.

NAD+ Accumulation during Pollen Maturation in Arabidopsis Regulating Onset of Germination

Although the nicotinamide nucleotides NAD(H) and NADP(H) are essential for various metabolic reactions that play major roles in maintenance of cellular homeostasis, the significance of NAD biosynthesis is not well understood. Here, we investigated the dynamics of pollen nicotinamide nucleotides in response to imbibition, a representative germination cue. Metabolic analysis with capillary electrophoresis electrospray ionization mass spectrometry revealed that excess amount of NAD+ is accumulated in freshly harvested dry pollen, whereas it dramatically decreased immediately after contact with water. Importantly, excess of NAD+ impaired pollen tube growth. Moreover, NAD+ accumulation was retained after pollen was imbibed in the presence of NAD+-consuming reaction inhibitors and pollen germination was greatly retarded. Pollen deficient in the nicotinate/nicotinamide mononucleotide adenyltransferase (NMNAT) gene, encoding a key enzyme in NAD biosynthesis, and a lack of NAD+ accumulation in the gametophyte, showed precocious pollen tube germination inside the anther locule and vigorous tube growth under high-humidity conditions. Hence, the accumulation of excess NAD+ is not essential for pollen germination, but instead participates in regulating the timing of germination onset. These results indicate that NAD+ accumulation acts to negatively regulate germination and a decrease in NAD+ plays an important role in metabolic state transition.

A kinetic analysis of cadmium accumulation in a Cd hyper-accumulator fern, Athyrium yokoscense and tobacco plants

Cadmium (Cd) accumulations in a Cd hyper-accumulator fern, Athyrium yokoscense (Ay), and tobacco, Nicotiana tabacum (Nt), were kinetically analysed using the positron-emitting tracer imaging system under two medium conditions (basal and no-nutrient). In Ay, maximumly 50% and 15% of the total Cd accumulated in the distal roots and the shoots under the basal condition, respectively. Interestingly, a portion of the Cd in the distal roots returned to the medium. In comparison with Ay, a little fewer Cd accumulations in the distal roots and clearly higher Cd migration to the shoots were observed in Nt under the basal condition (maximumly 40% and 70% of the total Cd, respectively). The no-nutrient condition down-regulated the Cd migration in both species, although the regulation was highly stricter in Ay than in Nt (almost no migration in Ay and around 20% migration in Nt). In addition, the present work enabled to estimate physical and physiological Cd accumulation capacities in the distal roots, and demonstrated condition-dependent changes especially in Ay. These results clearly suggested occurrences of species-/condition-specific regulations in each observed parts. It is probable that integration of these properties govern the specific Cd tolerance/accumulation in Ay and Nt.

Management of nitrogen fertilizer application, rather than functional gene abundance, governs nitrous oxide fluxes in hydroponics with rockwool

AimsNitrous oxide (N2O) is a strong greenhouse effective gas (GHG); the primary human source of N2O is agricultural activities. Excessive nitrogen (N) fertilization of agricultural land is now widely recognized as a major contributor. In soil, the microbial processes of nitrification and denitrification are the principal sources of N2O. However, it remains poorly understood how conventional hydroponics influences GHG emission. Here, we compared GHG fluxes from soil and rockwool used for hydroponics under identical nutrient conditions.MethodsTomato plants (Solanum lycopersicum, momotaro) were grown in soil or by hydroponics using rockwool. In situ emissions of CH4, CO2, and N2O, and the abundance of genes involved in nitrification and denitrification were measured during cultivation.ResultsHydroponics with rockwool mitigated CO2 emission by decreasing the microbial quantity in the rhizosphere. Dilution of the nutrient solution significantly decreased N2O emission from rockwool. Although proliferation of nitrifiers or denitrifiers in the rhizosphere did not induce N2O emission, reuse or long-term use of rockwool induced a 3.8-fold increase in N2O emission.ConclusionsOur data suggest that hydroponics has a lower environmental impact and that adequate fertilizer application, rather than bacterial control, governs N2O fluxes in hydroponic cultivation using rockwool.

Influence of Nitrogen Limitation and Long-Term Use of Rockwool on Nitrous Oxide Emissions in Hydroponic Systems

To mitigate Nitrous Oxide (N2O) emissions derived from Nitrogen (N) fertilizer of agroecosystems, establishment of best management protocols for cultivation is necessary. Hydroponic systems using rockwool have the potential to reduce N2O emissions; however, the effects of nutrient condition and retained N compounds in rockwool on N2O emissions remain unclear. The primary objective of our study was to understand the crucial factors behind emissions of N2O. Tomato cultivation with low levels of nutrient showed reduced growth and yield, but increased N2O emission. In contrast, growth and N2O emissions were increased by cultivation with normal levels of nutrient and used (1-yold) rockwool containing excess N compounds from the previous years cultivation. Though the long-term use of rockwool significantly enhanced seasonal N2O emission, the availability of N2O precursors NO3 − and NH4 + did not clearly explain the variation in N2O fluxes during cultivation. Rather, environmental factors, such as relative water content of rockwool in the rhizosphere, were significantly correlated to N2O emissions during cultivation under various conditions. We conclude that environmental factors most strongly influence the fate of available environmental substrates remaining in rockwool, and thereby control N2O emissions.

NAD+ accumulation as a metabolic off switch for orthodox pollen

Terrestrial plant pollen is classified into two categories based on its metabolic status: pollen with low-metabolism are termed “orthodox” and pollen with high-metabolism are termed “recalcitrant.” Nicotinamide adenine dinucleotide (NAD) is crucial for a number of metabolisms in all extant organisms. It has recently been shown that NAD homeostasis plays an important role in a broad range of developmental processes and responses to environment. Recently, a reverse genetic approach shed light on the significance of NAD biosynthesis on pollen fate. In orthodox Arabidopsis pollen, NAD+ that was accumulated in excess at dispersal dramatically decreased on rehydration. The lack of a key gene that is involved in NAD biosynthesis compromised the excess accumulation. Moreover, absence of the excess accumulation phenocopied the so-called recalcitrant pollen, as demonstrated by the germination inside anthers and the loss of desiccation tolerance. Upon rehydration, NAD+-consuming inhibitors impaired tube germination. Taken together, our results suggest that accumulation of NAD+ functions as a physiochemical molecular switch for suspended metabolism and that the decrease of NAD+ plays a very important role during transitions in metabolic states. Shifting of the redox state to an oxidizing environment may efficiently control the comprehensive metabolic network underlying the onset of pollen germination.