Our suitably erudite – albeit neophyte – botanical generation who knew about the functions of plant stems when quizzed previously (see Plant parts doing unexpected things: Part 1, posted previously) would probably do equally well when asked about the main roles of roots*. However, what they may be surprised to learn is that some roots photosynthesise (yes, like stems or leaves). We’re not talking about ‘typical’ soil-surrounded roots, but the so-called aerial roots of epiphytic plants perched high above the ground on trees – for example certain orchids. These photosynthetic roots dangle in the air that surrounds the epiphyte and its host plant. Whilst a photosynthetic capability is unusual for a root that is typically subterranean, you might expect that gain of this function might be at the expense of another, more typical root role, say absorption. But no, such roots still retain the capacity to absorb water from their surroundings. However, rather than rely on the assistance of root hairs as for their terrestrial, soil-rooted relatives, nature has equipped these aerial roots with an additional tissue, the velamen. The velamen is a remarkable multi-layered epidermis-like structure whose specially thickened cells not only absorb water from the humid air or rain water, but also help to reduce transpiration from the internal root tissues when the velamen cells are dried out. There is still much to uncover about the role of the velamen in the biology of epiphytes, but an interesting discovery has been made by Guillaume Chomicki et al., and one that relates not to the plant’s water relations but to the integrity of the root’s photosynthetic process. Recognising that levels of damaging ultraviolet B (UV-B) radiation are high in the epiphytes’ habitat, and knowing that UV-B screening compounds such as flavonoids help to protect leaves, the team wondered how similarly challenged, photosynthetic roots might be protected from UV-B harm. Using a nice combination of molecular and structural techniques – gene expression analyses, mass spectrometry, histochemistry and chlorophyll fluorescence – they demonstrated that UV-B exposure resulted in inducible production of two UV-B screening flavonoids within the living (i.e. young) velamen of Phalaenopsis × hybrida, but which compounds persist in the cell walls of the functional – dead – velamen tissue. Furthermore, and interestingly, this root mechanism of UV-B protection is apparently different from that employed by leaves. A case of same destination, different routes? Not bad for a dead tissue one could easily write-off as merely acting like a sponge!
* Which, for completeness, are generally assumed to be: anchorage of plant in soil, absorption of water/minerals from the soil, storage of reserve materials, and conduction of water/nutrients to/from the stem – Ed.