Functional characterization of the EXO and EXO-LIKE proteins in Arabidopsis
Zusammenfassung der Projektergebnisse
The Arabidopsis genome encodes eight EXO and EXO-LIKE proteins. The characteristic feature of the EXO and EXO-LIKE proteins is the PHI1 conserved region (Pfam entry PF04674). Its putative N-terminal targeting sequence directs the protein into the apoplast. The remaining primary structure does not show similarity to other known protein domains. The EXO gene and further members of the gene family (i.e. EXL1, EXL3, and EXL5) were identified as brassinosteroid-regulated genes. This finding and in-depth expression analyses suggested a role in the control of growth under specific environmental conditions. Expression of EXO and EXL genes (including EXL1, EXL2, and EXL4) is under control of the carbon status. For example, EXL1 expression is induced by low light, extended night, hypoxia, and low sugar supply in synthetic growth medium. EXO is repressed by exogenous sucrose but induced by trehalose supply. The expression patterns triggered an in-depth analysis of mutants under different light regimes and in synthetic medium supplemented with different sugar levels. The EXL1 protein is required for adaptation to C- and energy-limiting growth conditions. EXL1 presumably is involved in the C-starvation response. Phenotypic changes of an exl1 loss of function mutant became evident only under corresponding experimental conditions. The mutant showed diminished biomass production in a short-day/low light growth regime, impaired survival during extended night, and impaired survival of anoxia stress. EXL1 reduced brassinosteroid-dependent growth under low C availability. The data suggest that EXL1 is part of a regulatory pathway that controls growth and development when C and energy supply is poor. The EXO protein connects growth with C responses. The exo mutant displayed altered responses to exogenous sucrose supplemented to the growth medium. Impaired growth of the mutant in synthetic medium was associated with the accumulation of starch and anthocyanins, altered expression of sugar-responsive genes, and increased abscisic acid levels. Thus, EXO modulates several responses related to the C availability. Trehalose feeding stimulated root growth and shoot biomass production of exo plants whereas it inhibited growth of the wild type. The phenotypic features of the exo mutant suggest that apoplastic processes coordinate growth and C responses. Analysis of single (exo, exl1, exl3, and exl5), double, and triple mutants revealed additional phenotypic changes under specific environmental conditions. Several approaches were followed to synthetize recombinant protein, but the achieved amount of protein was not sufficient for pull-down experiments. A yeast-two-hybrid did not reveal reliable candidate proteins. Transgenic Arabidopsis plants expressing HA-tagged proteins were established and are currently used for co-immunoprecipitation experiments. Additional physiological experiments included the analysis of the cell wall composition and levels of soluble heteroglycans. However, only minor or no differences between exo/exl mutants and the wild type were detected. In summary, the EXO and EXL1 proteins link the extracellular carbon status with intracellular sugar responses and brassinosteroid-dependent growth. Other members of the protein family presumably have similar, but not identical functions. Further characterization of the protein family in crops may hold the potential to control growth and biomass production in dependency of environmental conditions and the carbon status of plants.
Projektbezogene Publikationen (Auswahl)
- (2011) EXORDIUM-LIKE1 promotes growth during low carbon availability in Arabidopsis. Plant Physiol 156: 1620-1630
Schröder F, Lisso J, Müssig C
- (2012) Expression pattern and putative function of EXL1 and homologous genes in Arabidopsis. Plant Signal Behav 7: 22-27
Schröder F, Lisso J, Müssig C
- (2013) EXO modifies sucrose and trehalose responses and connects the extracellular carbon status to growth. Front Plant Sci 4:219
Lisso J, Schröder F, Müssic C
(Siehe online unter https://doi.org/10.3389/fpls.2013.00219)
