Vegetarians might not like hearing this, but plants have stem cells too. Like ours, these stem cells are undifferentiated and grow indefinitely in culture. And also like ours, they hold therapeutic promise, since many drugs and insecticides, not to mention perfumes and pigments, are derived from plants.

These products are currently produced using plant cells grown in suspension cultures that have undergone a process called dedifferentiation, which eliminates any specialized properties (like those of root or leaf cells). But these dedifferentiated cells don’t grow that well. They tend to form clumps in large growing tanks, and the process can induce genetic changes in them. These problems inspired recent work described in Nature Biotechnology, in which scientists have isolated and cultured plant stem cells, called cambial meristematic cells (CMCs).

They started with a twig from a wild yew, Taxus cuspidata, the plant that makes Taxol (paclitaxel), which is used to fight breast, lung, and ovarian cancers. They then isolated actively proliferating cells from the cambium, a structure where secondary branches get their start.

The researchers confirmed that the CMCs are in fact stem cells using a number of methods. They were able to readily form a variety of adult structures, and expressed a distinct set of genes that were consistent with a stem cell fate.

To ensure that these cells could satisfactorily churn out a product (paclitaxel, in this case) at a large scale, the researchers tracked their growth in cultures ranging from a small flask to a 20 liter air-lift bioreactor—traditional plant cells can't even grow in the last container. In every case, CMCs grew better than traditional plant cell cultures and produced more paclitaxel per cell weight. Previously reported values for paclitaxel production range from 20-84mg for each kilogram of cells; CMCs made 264 mg/kg of cells.

The CMCs also grew well in a 3 ton bioreactor, although paclitaxel biosynthesis was not measured.

CMCs from other plant species can also mass produce other commercially important natural products. The scientists used their technique to isolate stem cells from ginseng, ginkgo, and tomato. Ginseng makes ginsenosides, which have a number of clinically relevant biological activities including neuroprotection, antioxidant properties, and angiogenesis modulation. Ginseng CMCs cultured in a 3 liter air-lift bioreactor generated over 20-fold more of both chemicals than standard cultured ginseng cells.

Thus using CMCs may avoid the problems associated with existing culture techniques and may give us an inexpensive and sustainable source of natural products—one that is not subject to the uncertainty inherent in plant cultivation.

Nature Biotechnology, 2010. DOI: 10.1038/nbt.1693 (About DOIs).