Cyanogenesis in rubber tree: Post-translational regulation of the cyanogenic response

Background/Question/Methods

The release of hydrogen cyanide (HCN) from injured plant tissue affects multiple ecological interactions. Plant derived HCN can act as a defence against herbivores and also plays an important role in plant pathogen interactions. Crucial for the activity as feeding deterrent is the amount of HCN generated per unit time, referred to as cyanogenic capacity (HCNc). Strong intraspecific variation in HCNc has been observed among cyanogenic plants. This variation, in addition to genotypic variability (e.g. Trifolium repens), can result from modifications in the expression level of the enzymes involved in either cyanogenic precursor formation or HCN release (e.g. Sorghum bicolor and Phaseolus lunatus). Thus, a modification of the HCNc in reaction to the environment can only be achieved from one to the next generation when under genetic control and within days or hours when transcriptional regulations are involved. In the present study we show that in rubber tree (Hevea brasiliensis) the HCNc is modulated by post-translational activity regulation of the key enzymes for cyanide release. Linamarase (LIN) and hydroxynitrile lyase (HNL) activity was determined by colorimetric assays utilizing dissociation of the substrates para-Nitrophenyl-β-D-glucopyranosid and acetone cyanohydrin, respectively.

Results/Conclusions

In rubber tree leaves LIN and HNL reveal up to 10-fold increased activity in response to tissue damage. This enzyme activation occurs within seconds and results in accelerated HCN formation. It is restricted to the damaged leaf area and depends on the severity or type of tissue damage. At molecular level the activation goes along with a distinct increase of soluble LIN and HNL.
Linamarase and HNL activation - in contrast to genetic and transcriptional regulations - allows an immediate, local and damage type dependent modulation of the cyanogenic response. Accordingly, this post-translational activation, apparently mediated by de-polymerization of preformed enzyme, plays a decisive role for the defence of H. brasiliensis against herbivores as well as pathogens. It allows more flexible reactions in response to these different antagonists.