Just as animal monozygotic twins can experience different environmental conditions by being reared apart, individual genetically identical trees of the genus Populus can also be exposed to contrasting environmental conditions by being grown in different locations. As such, clonally propagated Populus trees provide an opportunity to interrogate the impact of individual environmental history on current response to environmental stimuli. To test the hypothesis that current responses to an environmental stimulus, drought, are contingent on environmental history, the transcriptome- level drought responses of three economically important hybrid genotypes—DN34 ( Populus deltoides × Populus nigra ), Walker [ P . deltoides var. occidentalis × ( Populus laurifolia × P . nigra )], and Okanese [Walker × ( P . laurifolia × P . nigra )]—derived from two different locations were compared. Strikingly, differences in transcript abundance patterns in response to drought were based on differences in geographic origin of clones for two of the three genotypes. This observation was most pronounced for the genotypes with the longest time since establishment and last common propagation. Differences in genome-wide DNA methylation paralleled the transcriptome level trends, whereby the clones with the most divergent transcriptomes and clone history had the most marked differences in the extent of total DNA methylation, suggesting an epigenomic basis for the clone history-dependent transcriptome divergence. The data provide insights into the interplay between genotype and environment in the ecologically and economically important Populus genus, with implications for the industrial application of Populus trees and the evolution and persistence of these important tree species and their associated hybrids.

Conifers possess inducible terpenoid defense systems. These systems are associated with the formation of traumatic resin ducts (TRD) and are underpinned by enhanced gene expression and activity of terpene synthases (TPS), enzymes responsible for oleoresin formation. We first determined that Sitka spruce (Picea sitchensis [Bong.] Carriere) had the capacity for TRD formation by mechanically wounding representative trees. We then proceeded to investigate whether the white pine weevil (Pissodes strobi Peck.), a stem-boring insect, can influence the expression of genes encoding monoterpene synthases (mono-tps) in Sitka spruce. We went on to compare this response with the effects of a simulated insect attack by drill wounding. A significant increase in mono-tps transcript level was observed in the leaders of lateral branches of weevil-attacked and mechanically wounded trees. In this study, weevils induced a more rapid enhancement of mono-tps gene expression. A full-length Sitka spruce mono-tps cDNA (PsTPS2) was isolated, expressed in Escherichia coli, and functionally identified as (Ϫ)-pinene synthase. The recombinant (Ϫ)-pinene synthase catalyzes the formation of (Ϫ)-␣-pinene and (Ϫ)-␤-pinene, both of which are known constituents of stem oleoresin in Sitka spruce and increase in abundance after weevil attack. These data suggest that increased (Ϫ)-pinene synthase gene expression is an important element of the direct defense system deployed in Sitka spruce after insect attack.Oleoresin is a complex mixture of monoterpenes, sesquiterpenes, and diterpene resin acids that provide chemical and physical protection of conifer trees against potential herbivores, stem-boring insects, and pathogens (Berryman, 1972; Bohlmann and Croteau, 1999; Phillips and Croteau, 1999; Trapp and Croteau, 2001). In Sitka spruce (Picea sitchensis [Bong.] Carrière), constitutive oleoresin is sequestered in preformed resin ducts in bark, sapwood, and needles. During the initial stages of attack by stem-boring insects, such as weevils (Curculionidae) or bark beetles (Coleopterae), this oleoresin is released and repels insects through intoxication or the formation of physical barriers. Conifers also possess inducible terpenoid defense systems. These include the formation of new traumatic resin ducts (TRD) in phloem and xylem tissue (Cheniclet, 1987; Alfaro, 1995; Nagy et al., 2000; Alfaro et al., 2002) and a hypersensitive response associated with the accumulation of terpenoids, lignin, and phenolics associated with cells surrounding an attack site (Raffa, 1991; Franceschi et al., 1998). Traumatic resinosis can be induced by a range of stimuli, including mechanical wounding, abiotic stress, insect attack, pathogen invasion, elicitor molecules derived from fungal or plant cell walls, or by treatment of trees with methyl jasmonate (MeJA; Croteau et al., 1987; Lieutier and Berryman, 1988; Nagy et al., 2000; Franceschi et al., 2002; Martin et al., 2002). Recent work with Norway spruce (Picea abies) demonstrated that differentiation of TRD in the d...

We analyzed the expression pattern of various terpene synthase (TPS) genes in response to a wounding injury applied to the apical leader of Sitka spruce (Picea sitchensis Bong. Carr.) genotypes known to be resistant (R) or susceptible (S) to white pine weevil (Pissodes strobi Peck.) attack. The purpose was to test if differences in constitutive or wound-induced TPS expression can be associated with established weevil resistance. All wounding treatments were conducted on 9-year-old R and S trees growing under natural field conditions within the range of variation for weevil R and S genotypes. Representative cDNAs of the monoterpene synthase (mono-TPS), sesquiterpene synthase (sesqui-TPS), and diterpene synthase (di-TPS) classes were isolated from Sitka spruce to assess TPS transcript levels. Based on amino acid sequence similarity, the cDNAs resemble Norway spruce (Picea abies) (2)-linalool synthase (mono-TPS; PsTPS-Linl) and levopimaradiene/abietadiene synthase (di-TPS; PsTPS-LASl), and grand fir (Abies grandis) d-selinene synthase (sesqui-TPS; PsTPS-Sell). One other mono-TPS was functionally identified as (2)-limonene synthase (PsTPS-Lim). No significant difference in constitutive expression levels for these TPSs was detected between R and S trees. However, over a postwounding period of 16 d, only R trees exhibited significant transcript accumulation for the mono-and sesqui-TPS tested. Both R and S trees exhibited a significant accumulation of PsTPS-LASl transcripts. An assessment of traumatic resin duct formation in wounded leaders showed that both R and S trees responded by forming traumatic resin ducts; however, the magnitude of this response was significantly greater in R trees. Collectively, our data imply that the induced resinosis response is an important aspect of defense in weevil R Sitka spruce trees growing under natural conditions.

The nucleotide sequence of 1e16, a tomato (Lycopersicon esculentum Mill.) gene induced by drought stress and regulated by abscisic acid specifically in aerial vegetative tissue, is presented. The single open reading frame contained within the gene has the capacity to encode a polypeptide of 12.7 kilodaltons and is interrupted by a small intron. The predicted polypeptide is rich in leucine, glycine, and alanine and has an isoelectric point of 8.7. The amino terminus is hydrophobic and characteristic of signal sequences that target polypeptides for export from the cytoplasm. There is homology (47.2% identity) between the amino terminus of the LE16 polypeptide and the corresponding amino terminal domain of the maize phospholipid transfer protein. 1e16 was expressed in drought-stressed leaf, petiole, and stem tissue and to a much lower extent in the pericarp of mature green tomato fruit and developing seeds. No expression was detected in the pericarp of red fruit or in drought-stressed roots. Expression of 1e16 was also induced in leaf tissue by a variety of other abiotic stresses including polyethylene glycol-mediated water deficit, salinity, cold stress, and heat stress. None of these stresses or direct applications of abscisic acid induced the expression of 1e16 in the roots of the same plants. The unique expression characteristics of this gene indicates that novel regulatory mechanisms, in addition to endogenous abscisic acid, are involved in controlling gene expression.During a period of drought stress, a plant undergoes a number of physiological and metabolic changes together with an increase in the biosynthesis of the plant hormone ABA (38). Because the accumulation of ABA is an early event after the onset of drought stress, it has been postulated that ABA acts to coordinate the plant's response to water deficit (16). These responses include the closure of stomata, resulting in a limitation of the transpiration rate, a decrease in the water potential of the plant, and decreased photosynthesis (2). Changes in gene expression have been observed after periods of water deficit as indicated by the accumulation of novel polypeptides (3,12). In addition, a number of ABA-regulated genes have been isolated and characterized (for review, see ref. 32). Whereas the majority of these genes are expressed in embryos of developing seeds when the levels of ABA also rise, a few are also expressed in the wilting vegetative portions of the plant.