CO₂‐responsiveness of leaf isoprene emission: Why do species differ?

Abstract

Leaf isoprene emission rate, I, decreases with increasing atmospheric CO 2 concentration with major implications for global change. There is a significant interspecific variability in [CO 2 ]-responsiveness of I, but the extent of this variation is unknown and its reasons are not understood. We hypothesized that the magnitude of emission reduction reflects the size and changeability of precursor pools responsible for isoprene emission (dimethylallyl diphosphate, DMADP and 2-methyl-erythritol 2,4-cyclodiphosphate, MEcDP). Changes in I and intermediate pool sizes upon increase of [CO 2 ] from 400 to 1500 μmol/mol were studied in nine woody species spanning boreal to tropical ecosystems. I varied 10-fold, total substrate pool size 37-fold and the ratio of DMADP/MEcDP pool sizes 57-fold. At higher [CO 2 ], I was reduced on average by 65%, but [CO 2 ]-responsiveness varied an order of magnitude across species. The increase in [CO 2 ] resulted in concomitant reductions in both substrate pools. The variation in [CO 2 ]-responsiveness across species scaled with the reduction in pool sizes, the substrate pool size supported and the share of DMADP in total substrate pool. This study highlights a major interspecific variation in [CO 2 ]responsiveness of isoprene emission and conclusively links this variation to interspecific variability in [CO 2 ] effects on substrate availability and intermediate pool size.

Hydroxymethylbutenyl diphosphate accumulation reveals MEP pathway regulation for high CO 2 -induced suppression of isoprene emission

“…While CO 2 responsiveness of isoprene emission varies among the species, we observed a 42% decrease in isoprene emission from poplar leaves at elevated CO 2 ( Fig. 1 B ), which corresponded to previous findings ( 31 – 33 ). Our data also indicate that CO 2 -mediated suppression of isoprene emission is independent of stomatal conductance ( SI Appendix , Fig.…”

“…ATP and NADPH levels are reduced during feedback inhibition of photosynthesis by high CO 2 due to triose phosphate utilization (TPU) limitation of photosynthesis ( 27 , 28 ), which could result in lower DMADP levels, reducing the rate of isoprene emission. This is supported by multiple studies showing that isoprene emission is correlated with the DMADP levels in plant tissues ( 29 – 31 ). However, Lantz et al ( 22 ) demonstrated that suppression of isoprene emission at high CO 2 is not correlated with TPU limitation.…”

“…Previous studies demonstrated that isoprene emission is correlated with the in vivo pool size of DMADP ( 29 , 46 , 47 ). In fact, Niinemets et al ( 31 ) showed that DMADP levels decrease at high CO 2 in some species, resulting in reduction of isoprene emission. However, the reason behind the reduced level of DMADP at high CO 2 was not identified.…”

The same boat, different storm: stress volatile emissions in response to biotrophic fungal infections in primary and alternate hosts

“…Surprisingly, in R. frangula , the impact of P. coronata on mono- and sesquiterpenes was minor, but the emissions of isoprene were enhanced ( Figure 1B ). This might indicate both the overall upregulation of the chloroplastic methyl-D-erythritol phosphate (MEP) pathway for isoprenoid synthesis or isoprene synthase activity 7 , 17 , 41 , 42 . Apparently, the stress threshold for elicitation of terpene synthesis was not exceeded in the alternate host, or R. frangula has an overall low capacity for induction of terpene emissions.…”

“…In addition, host differences in the expression of constitutive defenses and capacity to induce defense responses can result in divergent elicitation of volatile emissions in different host species 2 . Regarding volatiles, variations in the degree of constitutive isoprene emissions can give rise to differences in the induction of emissions of stress-elicited isoprenoids in different hosts 5 , 7 , 14 , 17 .…”