Since graduation, I’ve been on a bit of a plant kick–not just because I needed to learn plant physiology and ecology for my GRE subject test and for my exploration of the flora and fauna near my new home near a suburban conservation area–but also because of my newfound appreciation for horticulture as art and discipline distinct from the botanical and ecological sciences, and for the therapeutic effect that gardening has had for my recent depression. As a side to my experimentation with tropical hibiscus propagation this summer, I was recently able to use my hibiscus calli in class in addition to the plant roots required for the day’s lab protocol. We performed assays to demonstrate areas of high enzymatic activity, and therefore identify zones of rapid cell growth and division, in these plant tissues. One of these was an assay for peroxidase, which neutralizes reactive oxygen species generated by metabolism that can destroy cell machinery and DNA.
Callus tissues are essentially stem cells in the somatic tissues of land plants. These stem cells are derived from more specialized cells that undergo rapid growth and division in the presence of growth hormones in the correct proportion. These callus form disorganized masses before differentiating into new roots and shoots, depending again on the appropriate mixture of hormones. In nature, callus tissue is encountered in plant wounds, where they grow and divide rapidly to heal over the injury. Calli are most frequently studied in vitro, however, since they are commonly used in plant tissue culture to produce numerous clones of a desired plant from single mature cells. In such situations, calli are the plant analogs of bacterial colonies in agar dishes.
I personally find it fascinating that such tissues can be generated on a hibiscus cutting in a jar of water, without sterile technique or specialized tools or media. Someday, I’d like to see if such in vivo callus cells are capable of forming plant embryos in culture, without having to induce calli in vitro. In fact, hibiscus cuttings are quite unique in how they develop such extensive masses of callus tissue all along the submerged part of the stem, not just in wounds. This disorganized tissue differentiates into functional roots in time, even without rooting hormone. This propensity to produce callus prior to rooting makes tropical hibiscus as a group rather easy to propagate, a trait that has made them so amenable to cultivation by enamored enthusiasts around the world. Soft green stem cuttings, taken any time of year, are relatively easy to root in water or soil, as long as adequate warmth and high humidity are provided.