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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - LEAF-OF-LIFE (Photosynthetic energy balance, chloroplast integrity, carbon flow and epigenetic regulation of isoprenoid biosynthesis during leaf development and senescence)

Teaser

Summary

The methyl erythritol phosphate (MEP) pathway in the chloroplast synthesizes isoprenoids (a class of hydrocarbons) by almost exclusively utilising carbon and energy supplied by photosynthesis in plant leaves. Isoprene (a five-carbon volatile organic compound) and isoprenoid-type cytokinins (a class of plant hormones that regulate development and senescence) are made at the same metabolic step using the same substrate in the MEP pathway. The Horizon2020 MSCA project LEAF-OF-LIFE was developed on the premise that isoprene and cytokinins interactively influence leaf development, phenotype, and senescence. The project aimed to (1) quantify age-specific changes in photosynthesis in isoprene emitting and non-emitting leaves during leaf senescence and (2) elucidate isoprene (inter) actions with phytohormones (specifically cytokinins), and characterise genomic controls on isoprenoid-mediated regulation of leaf senescence. Using Arabidopsis thaliana (an annual model plant to investigating development, phenotype, and genomics) that expressed an isoprene synthase from Eucalyptus, and hybrid poplar (a globally prominent isoprene-emitting plantation tree) where isoprene emission is suppressed via post-transcriptional gene silencing, we quantified the impact of isoprene emission on cytokinin metabolism, leaf and plant phenotype (including reproduction), photosynthesis and leaf senescence. Leaf RNA-sequencing was envisaged to identify gene regulatory elements and changes in gene expression during the life of a leaf from emergence to senescence (in presence and absence of isoprene emission). The scientific merit of LEAF-OF-LIFE is that it aimed to quantify age-specific changes in photosynthetic energy and carbon status during leaf senescence, which provides finer interpretation of phenology of autumn senescence and volatile hydrocarbon emissions from deciduous forests. The wider societal significance is that the know-how from this project promised to open-up new economically and ecologically sustainable means to manage leaf and plant senescence, by manipulating leaf isoprenoid metabolism. Characterisation of novel gene regulatory cascades involved in age-specific changes in isoprenoid biosynthesis will be spearheading new research intended to target volatile isoprenoid metabolism so as to alter leaf lifespan (delay or induce leaf senescence) in commercial crop plants.

Work performed

Work performed: Project preparatory phase lasted 4 months. Monitoring life of individual leaves in Arabidopsis (life cycle of 80 days x 3 rounds) followed. Leaves were individually tagged, monitored and sampled at 6 life-stages. The two-season back-to-back experiments involving poplars began in spring 2018 and hundreds of leaves were individually tagging and monitored during the poplar growing season (april to dec 2018). Extensive phenotyping, photosynthesis characterisation, isoprene and isoprenoid cytokinin quantification in these lines were repeated and confirmed in independent experiments. Leaves also analysed under high light stress and drought. RNA-seq and post-sequencing analyses of both Arabidopsis and poplar are ongoing.
Main results: (1) isoprene-emitting leaves of Arabidopsis and poplars senesced early (relative to non-emitting lines) (2) isoprene emission significantly altered leaf and plant phenotype, induced different temporal (leaf age-specific) changes in photosynthesis, while showing photosynthetic stability under short-term stresses, (3) Isoprene-emission led to greater abundance of cytokinins in healthy mature leaves (genomic impact is being analysed).
Implications and exploitation of results from LEAF-OF-LIFE:
Faster growth, shortening of leaf lifespan, hastening of flowering and quicker completion of plant lifecycle in isoprene-emitting individuals reveals a significant new role for isoprene (mediated via cytokinin signalling) in altering leaf and plant life-history strategy. This leads to new genomic and metabolic targets to improve plant performance under stress and alter plant phenotype and productivity by manipulating constitutive and exogenous volatile hydrocarbons. The main research paper (to be communicated) will also reassess the evolutionary significance of constitutive volatile hydrocarbon emission, while new independent research projects (emanating from LEAF-OF-LIFE) have begun conducting trials with crop plants.

Final results

LEAF-OF-LIFE shows that introducing isoprene emission capacity in Arabidopsis accelerated plant growth, quickened reproductive senescence, and shortened the leaf and plant lifespan. Similarly, suppressing isoprene emission in poplar (a natural isoprene-emitter) enhanced their leaf lifespan and reduced their overall growth rate. Isoprene, as shown by several experimental studies, may impart photosynthetic stability via membrane interactions and antioxidant activity under transient stressful conditions. However, our results add credence to a novel primary role for foliar isoprene emission in altering leaf and organismal development and lifespan, via isoprene emission-induced alteration in cytokinin metabolism.
(a) LEAF-OF-LIFE is the first attempt to understand the relationship between photosynthesis, isoprenoid metabolism and the rate of leaf senescence. The current view sees volatiles as both means and ends of abiotic stress response, and LEAF-OF-LIFE provides a major reappraisal of volatile hydrocarbons as regulators of plant development, a progress beyond the state-of-the art.
(b) LEAF-OF-LIFE successfully combines annual and perennial plant model systems to investigate the constraints on leaf and plant senescence. State-of-the art in plant senescence research is dominated by functional characterisation of select genes, transcriptional factors and enzymes. LEAF-OF-LIFE combines leaf age-specific genomic analysis, photosynthesis and metabolite characterisation to show how the process (senescence) impacts isoprenoid metabolism and in turn how presence of some isoprenoids shape phenotype to determine the course of senescence. This will be seen as a significant break-away from conventional template to understand senescence.
These fundamental advances in our knowledge of isoprene-cytokinin relationships in leaf development and senescence have significant implications for volatile hydrocarbon and hormone interactions overall in altering plant development, phenotype, reproductive senescence and productivity. The sequencing results (in preparation) and analyses will provide further insights into the genomic regions and transcription factors responsible for isoprene-CK mediated changes in leaf and organismal life history. We have reached an evolutionary dead-end in our quest to enhance food grain productivity both due to spatial (plant body architecture and land availability) and temporal (minimum ~70 to 90-day life cycle) limitations. New insights into volatile and hormone interactions regulating leaf senescence obtained during LEAF-OF-LIFE have informed experiments involving crop plants (commenced at CNR), where novel sustainable avenues to maintain productivity are being addressed using volatile and hormone interactions. Results of LEAF-OF-LIFE prompted the MSCA candidate and his host to take a lead role in several new proposals and projects, which take the legacy of LEAF-OF-LIFE forward.