Plants are known to synthesize diverse groups of phytochemicals in the different cell types. So, they are claimed as “the great chemists of nature” by Peter J. Facchini – one of the renowned plant molecular biologist from University of Calgary, Alberta, Canada. It is established that the plants manufacture phytochemicals such as proteins, carbohydrates, lipids, flavonoids, alkaloids, polyphenols, sterols etc., and store them in different tissues for special purposes. Among them, alkaloids are one of the important groups of phytochemicals for the defense responses by plants. Recent studies by Facchhini, his co-workers and other independently working scientists suggest that the alkaloids are produced by the involvement of multiple cell types and stored in specific cell types. To recognize the different cell and tissue type in which alkaloids biosynthesis occurs, various studies like study of enzymes, gene transcripts, and immumo-cytochemical studies (see, Immunocytochemistry) have been carried out in model plants like Atropha belladonna, Catarahthus roseus, Eupatorium cannabinum, Hyoscyamus muticus, Senecio jacoba and S. vernalis.

Alkaloids are a diverse group of low-molecular-weight, nitrogen-containing compounds derived mostly from amino acids. Based on the biosynthetic pathways, alkaloids are grouped as monoterperiod indole alkaloids (MIA), Benzylisoquinoline alkaloids (BIA), tropane alkaloids, and pyrrolizidine alkaloids. They are purported to play a defensive role against herbivores and pathogens. Since they pose defense responses they generally accumulate in specific cell types. For example, alkaloids are sequestered to isolated idioblasts and laticifers in Catarahthus roseus, root endodermis and stem cortex/pith in Thalictrum flavum, and laticifers in opium poppy. 

Some enzymes, namely, putrescine N-methyl transferase (PMT) and hyoscyamine 6β – hydroxylase (H6H), which catalyze the biosynthesis of the tropane alkaloid scopolamine were localized to the pericycle in the roots of Atropha belladonna and Hyoscyamus muticus. The PMT which also catalyzes the nicotine biosynthesis is localized to the endodermis, outer cortex, and xylem in Nicotiana sylvestris. The enzyme tropinone reductase II (TRII), which provides pseudotropine for the formation of calystegines, localizes to companion cells of sieve elements in the phloem of potato.

Enzymes involved in MIA biosynthesis localize to several different C. roseus organs and cell types. Enzymes Tabersonine 16-hydroxylase (T16H),  2-oxoglutarate dependent deoxygenase desacetoxyvindoline 4-hydroxylase (D4H), and Acetyl Co-A dependent desacetylvindoline-4-O-acetyltransferase (DAT) are restricted to young leaves and other shoot organs but Tryptophan decarboxylase (TDC) and Strictosidine synthase (STR) are abundant in roots. Cyp7241, TDC, and STR localize to the epidermis of stems, leaves, and flower buds, however, these enxymes occur in cells near the apical meristem in roots. However, D4H and DAT are associated with laticifers and idioblasts of shoots and absent from roots. Since the laticifers and idioblasts are distributed throughout the mesophylls in leaves, often several cell layers away from the epidermis the synthesis of alkaloids like vindoline involves at least two distinct cell types and requires the intercellular translocation of pathway intermediates. Moreover, gene transcripts encoding enzymes in the MEP  pathway (Methylerythritol 4-phosphate pathway) localize to the internal phloem parenchyma of young C. roseus aerial organs. So, the intermediates of vindoline biosynthesis are translocated from the internal phloem to the epidermis and from the epidermis to laticifers and idioblasts. The specific pathway intermediates which undergo intercellular translocation in MIA biosynthesis in C. roseus are not known.

In Senecio species, pyrrolizidine alkaloids are produced in actively growing roots as senecione N-oxides, which are transported via the phloem to above ground organs. Then, senecionine N-oxides are subsequently modified to form pyrrolizidine alkaloids. In Senecio jacoba and S. vernalis, pyrrolizidine alkaloids are accumulated in the inflorescence. HSS, the first committed enzyme in pyrrolizidine alkaloid biosynthesis, is localized to the root endodermis and cortex adjacent to the phloem in S. vernalis. However, in Eupatorium cannabinum, HSS is found throughout the root cortex.

In opium poppy, BIA accumulation occurs in the articulated laticifers found adjacent or proximal to sieve elements of the phloem. The cytoplasms of laticifers or latex contain a full complement of cell organelles and many large vesicles to which alkaloids are sequestered. Experiments like initial in situ hybridization demonstrate that  Tyrosine decarboxylase (TYDC) gene transcripts are localized to the phloem, but not to laticifers. The enzymes of morphinan pathway, namely, salutaridine synthase (Cyp719B1) and salutaridine: NADPH 7-oxidoreductase (SalR) are also not detected in isolated latex. Seven other biosynthetic enzymes – 6OMT, (s)-N-methylcoclaurine 3′-hydroxylase (Cyp80B3), 3′ -hydroxy-N-methylcoclaurine 4′ -O-methyltransferase (4’OMT), Berberine bridge enzyme (BBE), Salutaridinol 7-O-methyltransferase (SalAT), and COR (Codeinone reductase) localize to sieve elements in opium poppy and their corresponding gene transcripts localize to associated companion cells. Previously, sieve elements were not known to support complex metabolism, and were assumed to possess only a limited number of protein required for cell maintenance and solute transport.

Reference: Ziegle J and Facchini P J. 2008. Alkaloid Biosyntheisi: Metabolism and Trafficking. Annual Review of Plant Biology, 59:735-69.

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