Blue-light receptor phototropin 1 suppresses immunity to promote Phytophthora infestans infection.

Blue-light (BL) phototropin receptors (phot1 and phot2) regulate plant growth by activating NPH3/RPT2-like (NRL) family members. Little is known about roles for BL and phots in regulating plant immunity. We showed previously that Phytophthora infestans RXLR effector Pi02860 targets potato (St)NRL1, promoting its ability to enhance susceptibility by facilitating proteasome-mediated degradation of the immune regulator StSWAP70. This raises the question: do BL and phots negatively regulate immunity? We employed coimmunoprecipitation, virus-induced gene silencing, transient overexpression and targeted mutation to investigate contributions of phots to regulating immunity. Whereas transient overexpression of Stphot1 and Stphot2 enhances P. infestans colonization of Nicotiana benthamiana, silencing endogenous Nbphot1 or Nbphot2 reduces infection. Stphot1, but not Stphot2, suppressed the INF1-triggered cell death (ICD) immune response in a BL- and NRL1-dependent manner. Stphot1, when coexpressed with StNRL1, promotes degradation of StSWAP70, whereas Stphot2 does not. Kinase-dead Stphot1 fails to suppress ICD, enhance P. infestans colonization or promote StSWAP70 degradation. Critically, BL enhances P. infestans infection, which probably involves phots but not other BL receptors such as cryptochromes and F-box proteins ZTL1 and FKF1. We demonstrate that Stphot1 and Stphot2 play different roles in promoting susceptibility, and Stphot1 kinase activity is required for BL- and StNRL1-mediated immune suppression.

Evolutionarily distinct resistance proteins detect a pathogen effector through its association with different host targets.

Knowledge of the evolutionary processes which govern pathogen recognition is critical to understanding durable disease resistance. We determined how Phytophthora infestans effector PiAVR2 is recognised by evolutionarily distinct resistance proteins R2 and Rpi-mcq1. We employed yeast two-hybrid, co-immunoprecipitation, virus-induced gene silencing, transient overexpression, and phosphatase activity assays to investigate the contributions of BSL phosphatases to R2- and Rpi-mcq1-mediated hypersensitive response (R2 HR and Rpi-mcq1 HR, respectively). Silencing PiAVR2 target BSL1 compromises R2 HR. Rpi-mcq1 HR is compromised only when BSL2 and BSL3 are silenced. BSL1 overexpression increases R2 HR and compromises Rpi-mcq1. However, overexpression of BSL2 or BSL3 enhances Rpi-mcq1 and compromises R2 HR. Okadaic acid, which inhibits BSL phosphatase activity, suppresses both recognition events. Moreover, expression of a BSL1 phosphatase-dead (PD) mutant suppresses R2 HR, whereas BSL2-PD and BSL3-PD mutants suppress Rpi-mcq1 HR. R2 interacts with BSL1 in the presence of PiAVR2, but not with BSL2 and BSL3, whereas no interactions were detected between Rpi-mcq1 and BSLs. Thus, BSL1 activity and association with R2 determine recognition of PiAVR2 by R2, whereas BSL2 and BSL3 mediate Rpi-mcq1 perception of PiAVR2. R2 and Rpi-mcq1 utilise distinct mechanisms to detect PiAVR2 based on association with different BSLs, highlighting central roles of these effector targets for both disease and disease resistance.

AVR2 Targets BSL Family Members, Which Act as Susceptibility Factors to Suppress Host Immunity.

To be successful plant pathogens, microbes use "effector proteins" to manipulate host functions to their benefit. Identifying host targets of effector proteins and characterizing their role in the infection process allow us to better understand plant-pathogen interactions and the plant immune system. Yeast two-hybrid analysis and coimmunoprecipitation were used to demonstrate that the Phytophthora infestans effector AVIRULENCE 2 (PiAVR2) interacts with all three BRI1-SUPPRESSOR1-like (BSL) family members from potato (Solanum tuberosum). Transient expression of BSL1, BSL2, and BSL3 enhanced P. infestans leaf infection. BSL1 and BSL3 suppressed INFESTIN 1 elicitin-triggered cell death, showing that they negatively regulate immunity. Virus-induced gene silencing studies revealed that BSL2 and BSL3 are required for BSL1 stability and show that basal levels of immunity are increased in BSL-silenced plants. Immune suppression by BSL family members is dependent on the brassinosteroid-responsive host transcription factor CIB1/HBI1-like 1. The P. infestans effector PiAVR2 targets all three BSL family members in the crop plant S. tuberosum These phosphatases, known for their role in growth-promoting brassinosteroid signaling, all support P. infestans virulence and thus can be regarded as susceptibility factors in late blight infection.

Plant pathogen effector utilizes host susceptibility factor NRL1 to degrade the immune regulator SWAP70.

Plant pathogens deliver effectors into plant cells to suppress immunity. Whereas many effectors inactivate positive immune regulators, other effectors associate with negative regulators of immunity: so-called susceptibility (S) factors. Little is known about how pathogens exploit S factors to suppress immunity. Phytophthora infestans RXLR effector Pi02860 interacts with host protein NRL1, which is an S factor whose activity suppresses INF1-triggered cell death (ICD) and is required for late blight disease. We show that NRL1 interacts in yeast and in planta with a guanine nucleotide exchange factor called SWAP70. SWAP70 associates with endosomes and is a positive regulator of immunity. Virus-induced gene silencing of SWAP70 in Nicotiana benthamiana enhances P. infestans colonization and compromises ICD. In contrast, transient overexpression of SWAP70 reduces P. infestans infection and accelerates ICD. Expression of Pi02860 and NRL1, singly or in combination, results in proteasome-mediated degradation of SWAP70. Degradation of SWAP70 is prevented by silencing NRL1, or by mutation of Pi02860 to abolish its interaction with NRL1. NRL1 is a BTB-domain protein predicted to form the substrate adaptor component of a CULLIN3 ubiquitin E3 ligase. A dimerization-deficient mutant, NRL1NQ, fails to interact with SWAP70 but maintains its interaction with Pi02860. NRL1NQ acts as a dominant-negative mutant, preventing SWAP70 degradation in the presence of effector Pi02860, and reducing P. infestans infection. Critically, Pi02860 enhances the association between NRL1 and SWAP70 to promote proteasome-mediated degradation of the latter and, thus, suppress immunity. Preventing degradation of SWAP70 represents a strategy to combat late blight disease.

RXLR Effector AVR2 Up-Regulates a Brassinosteroid-Responsive bHLH Transcription Factor to Suppress Immunity.

An emerging area in plant research focuses on antagonism between regulatory systems governing growth and immunity. Such cross talk represents a point of vulnerability for pathogens to exploit. AVR2, an RXLR effector secreted by the potato blight pathogen Phytophthora infestans, interacts with potato BSL1, a putative phosphatase implicated in growth-promoting brassinosteroid (BR) hormone signaling. Transgenic potato (Solanum tuberosum) plants expressing the effector exhibit transcriptional and phenotypic hallmarks of overactive BR signaling and show enhanced susceptibility to P. infestans Microarray analysis was used to identify a set of BR-responsive marker genes in potato, all of which are constitutively expressed to BR-induced levels in AVR2 transgenic lines. One of these genes was a bHLH transcription factor, designated StCHL1, homologous to AtCIB1 and AtHBI1, which are known to facilitate antagonism between BR and immune responses. Transient expression of either AVR2 or CHL1 enhanced leaf colonization by P. infestans and compromised immune cell death activated by perception of the elicitin Infestin1 (INF1). Knockdown of CHL1 transcript using Virus-Induced Gene Silencing (VIGS) reduced colonization of P. infestans on Nicotiana benthamiana Moreover, the ability of AVR2 to suppress INF1-triggered cell death was attenuated in NbCHL1-silenced plants, indicating that NbCHL1 was important for this effector activity. Thus, AVR2 exploits cross talk between BR signaling and innate immunity in Solanum species, representing a novel, indirect mode of innate immune suppression by a filamentous pathogen effector.

Beta Interferon Production Is Regulated by p38 Mitogen-Activated Protein Kinase in Macrophages via both MSK1/2- and Tristetraprolin-Dependent Pathways.

Autocrine or paracrine signaling by beta interferon (IFN-ß) is essential for many of the responses of macrophages to pathogen-associated molecular patterns. This feedback loop contributes to pathological responses to infectious agents and is therefore tightly regulated. We demonstrate here that macrophage expression of IFN-ß is negatively regulated by mitogen- and stress-activated kinases 1 and 2 (MSK1/2). Lipopolysaccharide (LPS)-induced expression of IFN-ß was elevated in both MSK1/2 knockout mice and macrophages. Although MSK1 and -2 promote the expression of the anti-inflammatory cytokine interleukin 10, it did not strongly contribute to the ability of MSKs to regulate IFN-ß expression. Instead, MSK1 and -2 inhibit IFN-ß expression via the induction of dual-specificity phosphatase 1 (DUSP1), which dephosphorylates and inactivates the mitogen-activated protein kinases p38 and Jun N-terminal protein kinase (JNK). Prolonged LPS-induced activation of p38 and JNK, phosphorylation of downstream transcription factors, and overexpression of IFN-ß mRNA and protein were similar in MSK1/2 and DUSP1 knockout macrophages. Two distinct mechanisms were implicated in the overexpression of IFN-ß: first, JNK-mediated activation of c-jun, which binds to the IFN-ß promoter, and second, p38-mediated inactivation of the mRNA-destabilizing factor tristetraprolin, which we show is able to target the IFN-ß mRNA.

Potato NPH3/RPT2-Like Protein StNRL1, Targeted by a Phytophthora infestans RXLR Effector, Is a Susceptibility Factor.

Plant pathogens deliver effectors to manipulate host processes. We know little about how fungal and oomycete effectors target host proteins to promote susceptibility, yet such knowledge is vital to understand crop disease. We show that either transient expression in Nicotiana benthamiana, or stable transgenic expression in potato (Solanum tuberosum), of the Phytophthora infestans RXLR effector Pi02860 enhances leaf colonization by the pathogen. Expression of Pi02860 also attenuates cell death triggered by the P. infestans microbe-associated molecular pattern INF1, indicating that the effector suppresses pattern-triggered immunity. However, the effector does not attenuate cell death triggered by Cf4/Avr4 coexpression, showing that it does not suppress all cell death activated by cell surface receptors. Pi02860 interacts in yeast two-hybrid assays with potato NPH3/RPT2-LIKE1 (NRL1), a predicted CULLIN3-associated ubiquitin E3 ligase. Interaction of Pi02860 in planta was confirmed by coimmunoprecipitation and bimolecular fluorescence complementation assays. Virus-induced gene silencing of NRL1 in N. benthamiana resulted in reduced P. infestans colonization and accelerated INF1-mediated cell death, indicating that this host protein acts as a negative regulator of immunity. Moreover, whereas NRL1 virus-induced gene silencing had no effect on the ability of the P. infestans effector Avr3a to suppress INF1-mediated cell death, such suppression by Pi02860 was significantly attenuated, indicating that this activity of Pi02860 is mediated by NRL1. Transient overexpression of NRL1 resulted in the suppression of INF1-mediated cell death and enhanced P. infestans leaf colonization, demonstrating that NRL1 acts as a susceptibility factor to promote late blight disease.

A Phytophthora infestans RXLR effector targets plant PP1c isoforms that promote late blight disease.

Plant pathogens deliver effectors to alter host processes. Knowledge of how effectors target and manipulate host proteins is critical to understand crop disease. Here, we show that in planta expression of the RXLR effector Pi04314 enhances leaf colonization by Phytophthora infestans via activity in the host nucleus and attenuates induction of jasmonic and salicylic acid-responsive genes. Pi04314 interacts with three host protein phosphatase 1 catalytic (PP1c) isoforms, causing their re-localization from the nucleolus to the nucleoplasm. Re-localization of PP1c-1 also occurs during infection and is dependent on an R/KVxF motif in the effector. Silencing the PP1c isoforms or overexpression of a phosphatase-dead PP1c-1 mutant attenuates infection, demonstrating that host PP1c activity is required for disease. Moreover, expression of PP1c-1mut abolishes enhanced leaf colonization mediated by in planta Pi04314 expression. We argue that PP1c isoforms are susceptibility factors forming holoenzymes with Pi04314 to promote late blight disease.

CREB phosphorylation at Ser133 regulates transcription via distinct mechanisms downstream of cAMP and MAPK signalling.

CREB (cAMP-response-element-binding protein) is an important transcription factor for the activation of a number of immediate early genes. CREB is phosphorylated on Ser133 by PKA (protein kinase A), promoting the recruitment of the co-activator proteins CBP (CREB-binding protein) and p300; this has been proposed to increase the transcription of CREB-dependent genes. CREB is also phosphorylated on Ser133 by MSK1/2 (mitogen- and stress-activated kinase 1/2) in cells in response to the activation of MAPK (mitogen-activated protein kinase) signalling; however, the relevance of this to gene transcription has been controversial. To resolve this problem, we created a mouse with a Ser133 to alanine residue mutation in the endogenous Creb gene. Unlike the total CREB knockout, which is perinatally lethal, these mice were viable, but born at less than the expected Mendelian frequency on a C57Bl/6 background. Using embryonic fibroblasts from the S133A-knockin mice we show in the present study that Ser133 phosphorylation downstream of PKA is required for CBP/p300 recruitment. The requirement of Ser133 phosphorylation for the PKA-mediated induction of CREB-dependent genes was, however, promoter-specific. Furthermore, we show that in cells the phosphorylation of CREB on Ser133 by MSKs does not promote strong recruitment of CBP or p300. Despite this, MSK-mediated CREB phosphorylation is critical for the induction of CREB-dependent genes downstream of MAPK signalling.

Targeted transcriptomic and metabolic profiling reveals temporal bottlenecks in the maize carotenoid pathway that may be addressed by multigene engineering.

Carotenoids are a diverse group of tetraterpenoid pigments found in plants, fungi, bacteria and some animals. They play vital roles in plants and provide important health benefits to mammals, including humans. We previously reported the creation of a diverse population of transgenic maize plants expressing various carotenogenic gene combinations and exhibiting distinct metabolic phenotypes. Here we performed an in-depth targeted mRNA and metabolomic analysis of the pathway to characterize the specific impact of five carotenogenic transgenes and their interactions with 12 endogenous genes in four transgenic lines representing distinct genotypes and phenotypes. We reconstructed the temporal profile of the carotenoid pathway during endosperm development at the mRNA and metabolic levels (for total and individual carotenoids), and investigated the impact of transgene expression on the endogenous pathway. These studies enabled us to investigate the extent of any interactions between the introduced transgenic and native partial carotenoid pathways during maize endosperm development. Importantly, we developed a theoretical model that explains these interactions, and our results suggest genetic intervention points that may allow the maize endosperm carotenoid pathway to be engineered in a more effective and predictable manner.

Dectin-1 regulates IL-10 production via a MSK1/2 and CREB dependent pathway and promotes the induction of regulatory macrophage markers.

In response to infection by fungal pathogens, the innate immune system recognises specific fungal pathogen associated molecular patterns (PAMPs) via pattern recognition receptors including the C-type lectin dectin-1 and members of the Toll Like Receptor (TLR) family. Stimulation of these receptors leads to the induction of both pro- and anti-inflammatory cytokines. The protein kinases MSK1 and 2 are known to be important in limiting inflammatory cytokine production by macrophages in response to the TLR4 agonist LPS. In this study we show that MSKs are also activated in macrophages by the fungal derived ligand zymosan, as well as the dectin-1 specific agonists curdlan and depleted zymosan, via the ERK1/2 and p38α MAPK pathways. Furthermore, we show that MSKs regulate dectin-1 induced IL-10 production, and that this regulation is dependent on the ability of MSKs to phosphorylate the transcription factor CREB. IL-10 secreted in response to zymosan was able to promote STAT3 phosphorylation via an autocrine feedback loop. Consistent with the decreased IL-10 secretion in MSK1/2 knockout macrophages, these cells also had decreased STAT3 tyrosine phosphorylation relative to wild type controls after stimulation with zymosan. We further show that the reduction in IL-10 production in the MSK1/2 macrophages results in increased secretion of IL-12p40 in response to zymosan relative to wild type controls. The production of high levels of IL-10 but low levels of IL-12 has previously been associated with an M2b or 'regulatory' macrophage phenotype, which was initially described in macrophages stimulated with a combination of immune complexes and LPS. We found that zymosan, via dectin-1 activation, also leads to the expression of SphK1 and LIGHT, markers of a regulatory like phenotype in mouse macrophages. The expression of these makers was further reinforced by the high level of IL-10 secreted in response to zymosan stimulation.

MSK1 and MSK2 inhibit lipopolysaccharide-induced prostaglandin production via an interleukin-10 feedback loop.

Prostaglandin production is catalyzed by cyclooxygenase 2 (cox-2). We demonstrate here that MSK1 and MSK2 (MSK1/2) can exert control on the induction of cox-2 mRNA by Toll-like receptor (TLR) agonists. In the initial phase of cox-2 induction, MSK1/2 knockout macrophages confirmed a role for MSK in the positive regulation of transcription. However, at later time points both lipopolysaccharide (LPS)-induced prostaglandin and cox-2 protein levels were increased in MSK1/2 knockout. Further analysis found that while MSKs promoted cox-2 mRNA transcription, following longer LPS stimulation MSKs also promoted degradation of cox-2 mRNA. This was found to be the result of an interleukin 10 (IL-10) feedback mechanism, with endogenously produced IL-10 promoting cox-2 degradation. The ability of IL-10 to do this was dependent on the mRNA binding protein TTP through a p38/MK2-mediated mechanism. As MSKs regulate IL-10 production in response to LPS, MSK1/2 knockout results in reduced IL-10 secretion and therefore reduced feedback from IL-10 on cox-2 mRNA stability. Following LPS stimulation, this increased mRNA stability correlated to an elevated induction of both of cox-2 protein and prostaglandin secretion in MSK1/2 knockout macrophages relative to that in wild-type cells. This was not restricted to isolated macrophages, as a similar effect of MSK1/2 knockout was seen on plasma prostaglandin E2 (PGE2) levels following intraperitoneal injection of LPS.

PGE(2) induces macrophage IL-10 production and a regulatory-like phenotype via a protein kinase A-SIK-CRTC3 pathway.

The polarization of macrophages into a regulatory-like phenotype and the production of IL-10 plays an important role in the resolution of inflammation. We show in this study that PGE(2), in combination with LPS, is able to promote an anti-inflammatory phenotype in macrophages characterized by high expression of IL-10 and the regulatory markers SPHK1 and LIGHT via a protein kinase A-dependent pathway. Both TLR agonists and PGE(2) promote the phosphorylation of the transcription factor CREB on Ser(133). However, although CREB regulates IL-10 transcription, the mutation of Ser(133) to Ala in the endogenous CREB gene did not prevent the ability of PGE(2) to promote IL-10 transcription. Instead, we demonstrate that protein kinase A regulates the phosphorylation of salt-inducible kinase 2 on Ser(343), inhibiting its ability to phosphorylate CREB-regulated transcription coactivator 3 in cells. This in turn allows CREB-regulated transcription coactivator 3 to translocate to the nucleus where it serves as a coactivator with the transcription factor CREB to induce IL-10 transcription. In line with this, we find that either genetic or pharmacological inhibition of salt-inducible kinases mimics the effect of PGE(2) on IL-10 production.

Transgenic rice grains expressing a heterologous ρ-hydroxyphenylpyruvate dioxygenase shift tocopherol synthesis from the γ to the α isoform without increasing absolute tocopherol levels.

We generated transgenic rice plants overexpressing Arabidopsis thaliana ρ-hydroxyphenylpyruvate dioxygenase (HPPD), which catalyzes the first committed step in vitamin E biosynthesis. Transgenic grains accumulated marginally higher levels of total tocochromanols than controls, reflecting a small increase in absolute tocotrienol synthesis (but no change in the relative abundance of the α and γ isoforms). In contrast, there was no change in the absolute tocopherol level, but a significant shift from the γ to the α isoform. These data confirm HPPD is not rate limiting, and that increasing flux through the early pathway reveals downstream bottlenecks that act as metabolic tipping points.

Combinatorial genetic transformation of cereals and the creation of metabolic libraries for the carotenoid pathway.

Combinatorial nuclear transformation is used to generate populations of transgenic plants containing random selections from a collection of input transgenes. This is a useful approach because it provides the means to test different combinations of genes without the need for separate transformation experiments, allowing the comprehensive analysis of metabolic pathways and other genetic systems requiring the coordinated expression of multiple genes. The principle of combinatorial nuclear transformation is demonstrated in this chapter through protocols developed in our laboratory that allow combinations of genes encoding enzymes in the carotenoid biosynthesis pathway to be introduced into rice and a white-endosperm variety of corn. These allow the accumulation of carotenoids to be screened initially by the colour of the endosperm, which ranges from white through various shades of yellow and orange depending on the types and quantities of carotenoids present. The protocols cover the preparation of DNA-coated metal particles, the transformation of corn and rice plants by particle bombardment, the regeneration of transgenic plants, the extraction of carotenoids from plant tissues, and their analysis by high-performance liquid chromatography.

Characterization of the cellular action of the MSK inhibitor SB-747651A.

MSK1 (mitogen- and stress-activated kinase 1) and MSK2 are nuclear protein kinases that regulate transcription downstream of the ERK1/2 (extracellular-signal-regulated kinase 1/2) and p38α MAPKs (mitogen-activated protein kinases) via the phosphorylation of CREB (cAMP-response-element-binding protein) and histone H3. Previous studies on the function of MSKs have used two inhibitors, H89 and Ro 31-8220, both of which have multiple off-target effects. In the present study, we report the characterization of the in vitro and cellular properties of an improved MSK1 inhibitor, SB-747651A. In vitro, SB-747651A inhibits MSK1 with an IC50 value of 11 nM. Screening of an in vitro panel of 117 protein kinases revealed that, at 1 µM, SB-747651A inhibited four other kinases, PRK2 (double-stranded-RNA-dependent protein kinase 2), RSK1 (ribosomal S6 kinase 1), p70S6K (S6K is S6 kinase) (p70RSK) and ROCK-II (Rho-associated protein kinase 2), with a similar potency to MSK1. In cells, SB-747651A fully inhibited MSK activity at 5-10 µM. SB-747651A was found to inhibit the production of the anti-inflammatory cytokine IL-10 (interleukin-10) in wild-type, but not MSK1/2-knockout, macrophages following LPS (lipopolysaccharide) stimulation. Both SB-747651A and MSK1/2 knockout resulted in elevated pro-inflammatory cytokine production by macrophages in response to LPS. Comparison of the effects of SB-747651A, both in vitro and in cells, demonstrated that SB-747651A exhibited improved selectivity over H89 and Ro 31-8220 and therefore represents a useful tool to study MSK function in cells.

Synergistic metabolism in hybrid corn indicates bottlenecks in the carotenoid pathway and leads to the accumulation of extraordinary levels of the nutritionally important carotenoid zeaxanthin.

Lutein and zeaxanthin cannot be synthesized de novo in humans, and although lutein is abundant in fruit and vegetables, good dietary sources of zeaxanthin are scarce. Certain corn varieties provide adequate amounts because the ratio of endosperm ß:ε lycopene cyclase activity favours the ß-carotene/zeaxanthin branch of the carotenoid pathway. We previously described a transgenic corn line expressing the early enzymes in the pathway (including lycopene ß-cyclase) and therefore accumulating extraordinary levels of ß-carotene. Here, we demonstrate that introgressing the transgenic mini-pathway into wild-type yellow endosperm varieties gives rise to hybrids in which the ß:ε ratio is altered additively. Where the ß:ε ratio in the genetic background is high, introgression of the mini-pathway allows zeaxanthin production at an unprecedented 56 µg/g dry weight. This result shows that metabolic synergy between endogenous and heterologous pathways can be used to enhance the levels of nutritionally important metabolites.

High-value products from transgenic maize.

Maize (also known as corn) is a domesticated cereal grain that has been grown as food and animal feed for tens of thousands of years. It is currently the most widely grown crop in the world, and is used not only for food/feed but also to produce ethanol, industrial starches and oils. Maize is now at the beginning of a new agricultural revolution, where the grains are used as factories to synthesize high-value molecules. In this article we look at the diversity of high-value products from maize, recent technological advances in the field and the emerging regulatory framework that governs how transgenic maize plants and their products are grown, used and traded.

Simultaneous expression of Arabidopsis ρ-hydroxyphenylpyruvate dioxygenase and MPBQ methyltransferase in transgenic corn kernels triples the tocopherol content.

The quantity and composition of tocopherols (compounds with vitamin E activity) vary widely among different plant species reflecting the expression, activity and substrate specificity of enzymes in the corresponding metabolic pathway. Two Arabidopsis cDNA clones corresponding to ρ-hydroxyphenylpyruvate dioxygenase (HPPD) and 2-methyl-6-phytylplastoquinol methyltransferase (MPBQ MT) were constitutively expressed in corn to further characterize the pathway and increase the kernel tocopherol content. Transgenic kernels contained up to 3 times as much γ-tocopherol as their wild type counterparts whereas other tocopherol isomers remained undetectable. Biofortification by metabolic engineering offers a sustainable alternative to vitamin E supplementation for the improvement of human health.