Nicotine Biosynthesis in Nicotiana: A Metabolic Overview

Alkaloids are important compounds found in Nicotiana plants, essential in plant defense against herbivores. The main alkaloid of Nicotiana tabacum, nicotine, is produced in roots and translocated to the leaves. Nicotine is formed by a pyrrolidine and a pyridine ring in a process involving several enzymes. The pyridine ring of nicotine is derived from nicotinic acid, whereas the pyrrolidine ring originates from polyamine putrescine metabolism. After synthesis in root cortical cells, a set of transporters is known to transport nicotine upward to the aerial part and store it in leaf vacuoles. Moreover, nicotine can be metabolized in leaves, giving rise to nornicotine through the N-demethylation process. Some Nicotiana wild species produce acyltransferase enzymes, which allow the plant to make N-acyl-nornicotine, an alkaloid with more potent insecticidal properties than nicotine. However, although we can find a wealth of information about the alkaloid production in Nicotiana spp., our understanding about nicotine biosynthesis, transport, and metabolism is still incomplete. This review will summarize these pathways on the basis on recent literature, as well as highlighting questions that need further investigation.

Analysis of Variability in Curing Conditions and Tobacco-Specific Nitrosamines Within Barns of Dark Air-Cured Tobacco

Significant variability in cured-leaf tobacco-specific nitrosamine (TSNA) content is commonly observed when sampling within dark air-curing barns. This variability may be due to inconsistency in the curing environment within different areas of the barn. A study was initiated in 2012, through support from a CORESTA Study Grant, to evaluate if cured-leaf TSNA content is related to microenvironmental conditions in the barn. Low-converter (TRsc) and high-converter (TRHC) selections of TR Madole dark tobacco were air cured in barns near Princeton and Lexington, KY. Temperature and relative humidity were measured with data loggers placed at 27 different locations within each barn for the duration of curing. There were no significant effects of individual data logger placement in either variety selection on hours above 24°C temperature, hours above 80% relative humidity, or TSNA; therefore, we investigated these data within the 3-dimensional aspects of tier, room, and bent within each barn. There were various effects of tier, room, and bent on temperature, relative humidity, and TSNA. Temperature data followed an understandable pattern across tiers in the barn within each year and location; however, relative humidity and TSNA were more difficult to characterize adequately. There was a significant relationship between hours above 24°C and TSNA, but not hours above 80% relative humidity. This study has shown that the effect of within-barn position on TSNA cannot be easily predicted.

CYANTRANILIPROLE AND SPINOSAD RESIDUES IN FLUE-CURED TOBACCO

From 2013 to 2015, research was conducted to estimate the maximum expected residue levels for the insecticides cyantraniliprole and spinosad following application to flue-cured tobacco. Data were generated in order to assist industry in establishing Guidance Residue Limits for both compounds. The insecticides were applied to fields of tobacco at maximum rates in accordance with the labeled rates and the harvested/cured leaf was analyzed in a lab for chemical residues. The findings indicated that the expected residues on cured leaf would be low or not quantifiable under existing detection techniques.

VERIFICATION OF NITROGEN AND PHOSPHORUS APPLICATION RATES TO FLUE-CURED TOBACCO

With rising input costs, flue-cured tobacco producers must consider modern fertility programs that focus on reduced application rates of alternative nutrient sources. To demonstrate the usability of these fertility programs, research was conducted in 2012, 2013, and 2014 to assess the impacts of reduced input fertilizer programs on flue-cured tobacco produced in the North Carolina Piedmont. Treatments evaluated included all possible combinations of 2 rates of liquid nitrogen (72 and 95 kg N/ha) and 3 rates of phosphorus (0, 25, and 56 kg P2O5/ha). Treatments were arranged in a randomized complete block design with a factorial treatment arrangement and replicated 4 times within each environment. Results confirm that lower application rates of nitrogen and phosphorus are acceptable for tobacco growth and development in the North Carolina Piedmont, as there were no differences in early-season tobacco growth or final leaf yield, quality, and value among the treatments imposed. In addition, the application of liquid nitrogen is suitable for the production of tobacco with acceptable leaf yield and quality. Producers in this region should consider the nutrient sources and application rates evaluated in this study in order to remain economically and environmentally sustainable.