Plant Cuttings

Where in the epidermis would you find chloroplasts?

There are several answers to this question. I expect all of you to say guard cells. Those of you who’ve read the previous item about chlorophagy might also be tempted to say plant vacuoles – presumably of guard cells. But, how many would have said trichomes? Not me for certain. But, they do contain chloroplasts – in tobacco, as graphically demonstrated by Carolyn Akers et al. p3], and in Stevia rebaudiana at least. And there it seemed the matter lay – another site to add to the catalogue of chloroplast-bearing cells – until Raphaëlle Laterre et al. decided to delve a little deeper and examine aspects of the photosynthetic cycle in those structures.

The Chloroplast. Image: Kelvin Ma / Wikipedia

Investigating specifically RubisCO (Ribulose bis-phosphate Carboylase/Oxygenase, the principal CO2-fixing enzyme in the Calvin-Benson-Bassham cycle of photosynthesis that incorporates atmospherically-sourced, inorganic carbon into carbon-containing, energy-rich, organic compounds) in the glandular trichomes of Nicotiana tabacum, they had a surprise in store. The functioning RubisCO enzyme is made up of eight copies of a large protein chain and eight copies of a smaller chain, i.e. it exhibits an impressive quaternary level of protein structure. Analysis of the trichome enzyme’s small sub-units revealed that they belonged to a previously unknown type, now termed NtRbcS-T [Nt for Nicotiana tabacum and RbcS-T for trichome]. Importantly, NtRbcS-T differs from the small sub-unit cluster RbcS-M, the form associated with mesophyll and bundle sheath cells of leaves, and has different properties.

In particular, compared to NtRbcS-M, the trichome-located enzyme has both higher Vmax (Maximal Velocity, the rate of the reaction at the point where substrate concentration has increased to the point where it completely saturates the enzyme’s active sites. Vmax reflects how fast the enzyme can catalyse the reaction) and Km (Michaelis Constant, the substrate concentration at which half the enzyme’s active sites are occupied by substrate). With a high Km, a lot of substrate must be present to saturate the enzyme, i.e. the enzyme has low affinity for the substrate values, as well as higher activity at more acid pH values. What is the significance of this? Laterre et al. suggest that the RubisCO of tobacco trichomes is uniquely adapted to use the CO2 released intra-cellularly by the cells’ specialized metabolism – such as terpene and sugar-ester synthesis – as a substrate for photosynthesis. Evidence in favour of this interpretation is two-fold. First, internally-produced CO2 would lower the chloroplast pH (i.e. making it more acidic at which pH the RubisCO works well).

Second, concentrating CO2 to values above atmospheric – in a way similar to the CCM (carbon-concentrating mechanism of certain cells of plants with C4 photosynthesis) – would increases the enzyme’s usage of CO2 over O2 (the alternative substrate for RubisCO in the process of photorespiration) * – i.e. enhance photosynthesis. Which study neatly demonstrates another truism of science; you don’t know what you’ll find until you look. Or, just because a cell has chloroplasts it doesn’t necessarily mean they are the same as chloroplasts in other, much better-studied, locations…

* Presumably, (any) photorespiration is low in these cells as a consequence? Might this therefore be an insight that could be exploited in attempts to improve RubisCO in mesophyll cells of leaves to enhance photosynthesis and consequently crop yield..?

[Ed. – For readers whose interest in trichomes – and not just glandular ones – has been piqued by this item, there is a fascinating world to discover, e.g. George Wagner, Anthony Schilmiller et al., Joris Glas et al., Li Hong Zhou et al., and Kaizhuan Xiao et al.. And – lest those items imply trichomes are only features of aerial organs of plants – don’t forget the root hairs, trichomes of an underground organ.]


  1. Somebody hasn’t done my first year BS1070 tobacco tissue culture practical! As well as the tissue culture regeneration/somatic embryogenesis shown being set up in the video, we look at the tobacco leaves, making both thin transverse hand sections of the leaves and peels. The trichomes with their single-cell column and spherical head cell always impress the students, as well as the guard cells (with plastids).

    1. Dear Pat,

      You are quite right: I didn’t take your BS1070 class(!)
      Thank you for this reminder that Leicester University botany/plant science/plant biology students should be more knowledgeable than most in matters of chloroplastic epidermal cells. I now have this image of them en masse screaming “Tobacco trichomes!”in answer to the question posed at the start of this blog item.

      Thank you for taking the time out to raise this.



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