Detecting early onset of intraspecific competition through root visualization
Plant compete intraspecifically above- and belowground. Detecting where and when this interaction first occurs is important for understanding the mechanisms through which plant growth may be limited during competition. How plants recognize the presence of other plants, including their own species, is still largely unknown; however, recent advances have revealed a variety of potential signaling molecules and pathways. Differences of gene expression in shoots indicate that Arabidopsis plants are responding to conspecific competitors at a molecular level prior to changes in shoot phenotype at 27 days after planting (DAP). We hypothesize that roots are the early sensors for the presence of conspecific plants between competing seedlings. We expect that competing plant roots will show asymmetry in root architecture in the presence of neighbors over time reflecting overexploitation (i.e., fight) versus avoidance (i.e., flight) behavior. Here we present results from experiments that create 3-dimensional (3D) visualizations and quantification of root growth among competing Arabidopsisseedlings. We grew seedlings of single or four competing Arabidopsis Columbia ecotypes and a transgenic line with green fluorescence proteins (GFP) in an optically clear gellan gum medium in a transparent container that allowed complete imaging of the entire root structure, including fine roots.
Digital images were processed to construct a 3D model of root architecture for each plant at multiple time points from seed germination to the time point when plant shoots start to show visible phenotypes. Root architecture was analyzed using custom-made pattern recognition software. Circular statistics quantified the distribution of roots. While significant circular asymmetry of roots had developed by 7 DAP, Rayleigh’s R was not greater in plants with neighbors through 25 DAP. However, when median direction of root growth between competing seedlings was categorized as head-to-head or not, departures from expected proportions of these interactions occurred from 21 DAP onwards suggesting the onset of competition. Among groups of 4 competing seedlings the median direction of root growth showed that there was usually a clear ‘winner’ and a clear ‘loser’ seedling. Our novel system for quantifying root competition identifies the onset of root competition before roots systems are intermingling belowground and before indications of aboveground competitive response.