Identification of two new trichome-specific promoters of Nicotiana tabacum

Cell-type-specific metabolism in plants

Every plant organ contains tens of different cell types, each with a specialized function. These functions are intrinsically associated with specific metabolic flux distributions that permit the synthesis of the ATP, reducing equivalents and biosynthetic precursors demanded by the cell. Investigating such cell-type-specific metabolism is complicated by the mosaic of different cells within each tissue combined with the relative scarcity of certain types. However, techniques for the isolation of specific cells, their analysis in situ by microscopy, or modeling of their function in silico have permitted insight into cell-type-specific metabolism. In this review we present some of the methods used in the analysis of cell-type-specific metabolism before describing what we know about metabolism in several cell types that have been studied in depth; (i) leaf source and sink cells; (ii) glandular trichomes that are capable of rapid synthesis of specialized metabolites; (iii) guard cells that must accumulate large quantities of the osmolytes needed for stomatal opening; (iv) cells of seeds involved in storage of reserves; and (v) the mesophyll and bundle sheath cells of C4 plants that participate in a CO₂ concentrating cycle. Metabolism is discussed in terms of its principal features, connection to cell function and what factors affect the flux distribution. Demand for precursors and energy, availability of substrates and suppression of deleterious processes are identified as key factors in shaping cell-type-specific metabolism.

Engineering Nicotiana tabacum trichomes for triterpenic acid production

In this work, we aimed at implementing the biosynthesis of triterpenic acids in Nicotiana tabacum glandular trichomes. Although endogenous genes coding for enzymes involved in such biosynthetic pathway are found in the Nicotiana tabacum genome, implementing such pathway specifically in glandular trichomes required to boost endogenous enzymatic activities. Five transgenes coding for a farnesyl-diphosphate synthase, a squalene synthase, a squalene epoxidase, a beta-amyrin synthase and a beta-amyrin 28-monooxygenase were introduced in N.tabacum, their expression being driven by pMALD1, a trichome-specific transcriptional promoter. This study aimed at testing whether sinking isoprenoid precursors localized in plastids, by exploiting potential cross-talks allowing the exchange of terpenoid pools from the chloroplast to the cytosol, could be a way to improve overall yield. By analyzing metabolites extracted from entire leaves, a low amount of ursolic acid was detected in plants expressing the five transgenes. Our study shows that the terpene biosynthetic pathway could be, in part, redirected in N.tabacum glandular trichomes with no deleterious phenotype at the whole plant level (chlorosis, dwarfism,…). In light of our results, possible ways to improve the final yield are discussed.

Two homeologous MATE transporter genes, NtMATE21 and NtMATE22, are involved in the modulation of plant growth and flavonol transport in Nicotiana tabacum

The multidrug and toxic compound extrusion (MATE) protein family has been implicated in the transport of a diverse range of molecules, including specialized metabolites. In tobacco (Nicotiana tabacum), only a limited number of MATE transporters have been functionally characterized, and no MATE transporter has been studied in the context of flavonoid transport in this plant species so far. In the present study, we characterize two homeologous tobacco MATE genes, NtMATE21 and NtMATE22, and demonstrate their role in flavonol transport and in plant growth and development. The expression of these two genes was reported to be up-regulated in trichomes as compared with the trichome-free leaf. The transcript levels of NtMATE21 and NtMATE22 were found to be higher in flavonol overproducing tobacco transgenic lines as compared with wild type tobacco. The two transporters were demonstrated to be localized to the plasma membrane. Genetic manipulation of NtMATE21 and NtMATE22 led to altered growth phenotypes and modulated flavonol contents in N. tabacum. The β-glucuronidase and green fluorescent protein fusion transgenic lines of promoter regions suggested that NtMATE21 and NtMATE22 are exclusively expressed in the trichome heads in the leaf tissue and petals. Moreover, in a transient transactivation assay, NtMYB12, a flavonol-specific MYB transcription factor, was found to transactivate the expression of NtMATE21 and NtMATE22 genes. Together, our results strongly suggest the involvement of NtMATE21 and NtMATE22 in flavonol transport as well as in the regulation of plant growth and development.

Overexpression of TcCHS Increases Pyrethrin Content When Using a Genotype-Independent Transformation System in Pyrethrum (Tanacetum cinerariifolium)

Pyrethrum (Tanacetum cinerariifolium) is one of the most important industrial crops for the extraction of pyrethrins, which are natural insecticidal compounds. Progress in pyrethrum molecular breeding with the objective of increasing pyrethrin content has been slow for lack of a suitable gene transfer system. Regeneration recalcitrance is a crucial barrier to establishing a genetic transformation system in pyrethrum. Therefore, in this study, an Agrobacterium-mediated transformation system in pyrethrum was developed using shoot apical meristems from germinated seedlings. Factors affecting transformation efficiency were optimized. Optimal conditions included explants at the "no true leaf" stage with a half apical meristem, an Agrobacterium tumefaciens cell density of OD₆₀₀ = 0.5, two days of cocultivation, and the incorporation of 1.5 mg L^-1 6-BA and 30 mg L^-1 kanamycin into the selection medium. Under the optimized conditions, two expression cassettes (proTcCHS-GUS and proRbcS-TcCHS) were successfully transformed into pyrethrum. Polymerase chain reaction (PCR), Southern blotting, reverse-transcription quantitative PCR (RT-qPCR), and histochemical staining confirmed the identity of proTcCHS-GUS transgenic plants. PCR and RT-qPCR analyses confirmed the identity of proRbcS-TcCHS transgenic plants. The transformation efficiency was 0.83% (5 transgenic lines/600 infected explants). The relative concentration of pyrethrins in proRbcS-TcCHS transformants (OX T0-1: 1.50% or OX T0-2: 1.24%) was higher than that in nontransformed plants (WT: 0.76%). Thus, the genetic transformation system overcame the low regeneration efficiency and integrated a foreign gene into the pyrethrum genome. The new system is a suitable and effective tool for creating high-yielding cultivars of pyrethrum.

The genome of the recretohalophyte Limonium bicolor provides insights into salt gland development and salinity adaptation during terrestrial evolution

Halophytes have evolved specialized strategies to cope with high salinity. The extreme halophyte sea lavender (Limonium bicolor) lacks trichomes but possesses salt glands on its epidermis that can excrete harmful ions, such as sodium, to avoid salt damage. Here, we report a high-quality, 2.92-Gb, chromosome-scale L. bicolor genome assembly based on a combination of Illumina short reads, single-molecule, real-time long reads, chromosome conformation capture (Hi-C) data, and Bionano genome maps, greatly enriching the genomic information on recretohalophytes with multicellular salt glands. Although the L. bicolor genome contains genes that show similarity to trichome fate genes from Arabidopsis thaliana, it lacks homologs of the decision fate genes GLABRA3, ENHANCER OF GLABRA3, GLABRA2, TRANSPARENT TESTA GLABRA2, and SIAMESE, providing a molecular explanation for the absence of trichomes in this species. We identified key genes (LbHLH and LbTTG1) controlling salt gland development among classical trichome homologous genes and confirmed their roles by showing that their mutations markedly disrupted salt gland initiation, salt secretion, and salt tolerance, thus offering genetic support for the long-standing hypothesis that salt glands and trichomes may share a common origin. In addition, a whole-genome duplication event occurred in the L. bicolor genome after its divergence from Tartary buckwheat and may have contributed to its adaptation to high salinity. The L. bicolor genome resource and genetic evidence reported in this study provide profound insights into plant salt tolerance mechanisms that may facilitate the engineering of salt-tolerant crops.

Control of resource allocation between primary and specialized metabolism in glandular trichomes

Plant specialized metabolites are often synthesized and stored in dedicated morphological structures such as glandular trichomes, resin ducts, or laticifers where they accumulate in large concentrations. How this high productivity is achieved is still elusive, in particular, with respect to the interface between primary and specialized metabolism. Here, we focus on glandular trichomes to survey recent progress in understanding how plant metabolic cell factories manage to balance homeostasis of essential central metabolites while producing large quantities of compounds that constitute a metabolic sink. In particular, we review the role of gene duplications, transcription factors and photosynthesis.

Methyl jasmonate treatment, aphid resistance assay, and transcriptomic analysis revealed different herbivore defensive roles between tobacco glandular and non-glandular trichomes

KEY MESSAGE

Methyl jasmonate treatment and aphid resistance assays reveal different roles in herbivore defensive responses between tobacco glandular and non-glandular trichomes. These roles correlate with trichome gene expression patterns. In plants, trichomes greatly contribute to biotic stress resistance. To better understand the different defensive functions between glandular and non-glandular trichomes, we used Nicotiana tabacum as a model. This species bears three types of trichomes: long and short stalk glandular trichomes (LGT and SGT, respectively), and non-glandular trichomes (NGT). Tobacco accession T.I.1068 (lacking NGT) and T.I.1112 (lacking LGT) were used for the experiment. After methyl jasmonate (MeJA) treatment, LGT formation was promoted not only in T.I.1068, but also in T.I.1112, whereas NGT remained absent in T.I.1068, and was slightly reduced in T.I.1112. Diterpenoids, which play important roles in herbivore resistance, accumulated abundantly in T.I.1068 and were elevated by MeJA; however, they were not found in T.I.1112 but became detectable after MeJA treatment. The aphid resistance of T.I.1068 was higher than that of T.I.1112, and both were enhanced by MeJA, which was closely correlated with LGT density. Trichomes detached from T.I.1068 and T.I.1112 were used for RNA-Seq analysis, the results showed that pentose phosphate, photosynthesis, and diterpenoid biosynthesis genes were much more expressed in T.I.1068 than in T.I.1112, which was consistent with the vigorous diterpenoid biosynthesis in T.I.1068. In T.I.1112, citrate cycle, propanoate, and glyoxylate metabolism processes were enriched, and some defensive protein genes were expressed at higher levels than those in T.I.1068.These results suggested that LGT plays a predominant role in aphid resistance, whereas NGT could strengthen herbivore resistance by accumulating defensive proteins, and the roles of LGT and NGT are associated with their gene expression patterns.

Engineering Metabolism in Nicotiana Species: A Promising Future

Molecular farming intends to use crop plants as biofactories for high value-added compounds following application of a wide range of biotechnological tools. In particular, the conversion of nonfood crops into efficient biofactories is expected to be a strong asset in the development of a sustainable bioeconomy. The 'nonfood' status combined with the high metabolic versatility and the capacity of high-yield cultivation highlight the plant genus Nicotiana as one of the most appropriate 'chassis' for molecular farming. Nicotiana species are a rich source of valuable industrial, active pharmaceutical ingredients and nutritional compounds, synthesized from highly complex biosynthetic networks. Here, we review and discuss approaches currently used to design enriched Nicotiana species for molecular farming using new plant breeding techniques (NPBTs).