Molecular analysis of plant growth promoted with low molecular weight compounds in relation to genetically altered photosynthetic carbohydrate partitioning in higher plants

[摘要] ENGLISH ABSTRACT: Despite the impressive advances that have been made over the years in improving both plantgrowth and yields, there is little reason to become complacent about these developments,especially with regard to food supply for the world's increasing population. Other thanconventional and marker assisted breeding, the use of crude extracts from plant growthpromotingrhizobacteria (PGPR) bacteria that possess biostimulatory properties has beenconsidered as an alternative practice for enhancing plant development and crop productivity.One such substance is lumichrome (7,8 dimethylalatloxazine), a novel plant growthpromoting multitrophic signal molecule produced by the bacterium Sinorhizobium meliloti.Lumichrome has been shown to elicit growth promotion and trigger a compensatory increasein whole-plant net carbon assimilation through enhanced starch accumulation and alteredethylene metabolism. Despite physiological experimental advances in exploring the growthstimulatory mechanisms of lumichrome, a comprehensive molecular analysis in various plantspecies still remains elusive.This study focused on understanding the genetic, molecular and biochemical regulatorynetworks as key determinants for crop growth and productivity in relation to lumichrome inArabidopsis thaliana. Our specific objectives focused on unravelling i) global changes ingene expression, ii) protein expression and iii) key metabolites or biochemical pathwayswhich were seemingly affected by lumichrome treatments in Arabidopsis thaliana. The studyfurther made use of Arabidopsis mutant lines deficient in the primary gene encoding theADP-GLUCOSE PYROPHOSPHORYLASE small subunit (APS1) to study the regulatorymechanisms of carbon metabolism. Subsequently, the findings were used to infer functionalinteractions among genes, proteins and metabolites in order to speculate on the possiblecontrol mechanism(s) involved.Application of biologically-active levels of lumichrome has been demonstrated to enhanceplant development through changes in photosynthetic rates, leaf stomatal conductance andtranspiration in several plant species. Enhanced growth is reported to be attributed to xylemtransport and in situ accumulation of lumichrome in leaves, which subsequently triggersevents that promote cell division and leaf expansion in both monocots and dicots. However,previous studies have also demonstrated mixed physiological responses between plants species to lumichrome, hence molecular processes responsible for growth promotion in otherspecies remain somewhat elusive. Consistent with previous studies, our study demonstratedthat the addition of 5 nM lumichrome to Arabidopsis thaliana plants elicited a growthpromoting effect to increase overall plant size and biomass. The increased overall plant sizewas attributed to enhanced photosynthesis and the use of higher levels of photoassimilates forcell division and cell enlargement. This was further supported by Next GenerationSequencing (Transcriptomic profiling-True-Seq) which revealed that the growth stimulatoryeffect was effected predominantly through genes associated to cell wall modification, celldivision and expansion. Our proteomics study results further suggested that lumichrometreatment enhances and stabilizes photosynthesis, providing increased photoassimilates forgrowth in wild type Arabidopsis. Although starch levels were increased in lumichrometreatedwild type plants, levels of APS1 were unexpectedly decreased.To demonstrate that silencing APS1 enhances Arabidopsis growth, we profiled both proteinsand metabolites in Arabidopsis T-DNA knockout lines which were deficient in APS1. Thereverse genetics approach revealed that enhanced growth of aps1 mutant plants relative to thewild type can be ascribed to enhanced photosynthetic efficiency, which ensured the provisionof energy and carbon supply for Arabidopsis growth. The results further indicated that similarlevels of enhanced growth and photosynthesis following lumichrome-treatment of wild typeplants could be achieved in the knockout plants even in the absence of lumichrome. Therewas no further effect on the growth of these mutant lines following lumichrome, stronglysuggesting that APS1 is responsible for mediating the lumichrome-associated growthresponse in Arabidopsis. Unlike wild type plants, starch levels in the aps1 lines wereextremely low and were not affected by lumichrome treatment. We therefore conclude thatlumichrome enhances growth in Arabidopsis plants via enhanced photosynthesis in a processmediated via APS1, and that the enhanced levels of starch seen in lumichrome treated wildtype plants are merely an artifact of this enhanced photosynthesis. It will be interesting toinvestigate further the means by which APS1 regulates this mechanism. In summary,proteomic and metabolomic analyses all suggest that down-regulation of APS in lumichrometreatedplants enhanced photosynthesis, leading to increased availability of C for enhancedplant growth.

[发布机构] Stellenbosch University