Shoot apical meristem and plant body organization: a cross-species comparative study

Abstract

Background and aims: The shoot apical meristem (SAM) is the key organizing element in the plant body and is responsible for the core of plant body organization and shape. Surprisingly, there are almost no comparative data that would show links between parameters of the SAM and whole-plant traits as drivers of the plant's response to the environment.

Methods: Interspecific differences in SAM anatomy were examined in 104 perennial herbaceous angiosperms.

Key results: There were differences in SAM parameters among individual species, their phylogenetic patterns, and how their variation is linked to variation in plant above-ground organs and hence species' environmental niches. SAM parameters were correlated with the size-related traits of leaf area, seed mass and stem diameter. Of the two key SAM parameters (cell size and number), variation in all organ traits was linked more strongly to cell number, with cell size being important only for seed mass. Some of these correlations were due to shared phylogenetic history (e.g. SAM diameter versus stem diameter), whereas others were due to parallel evolution (e.g. SAM cell size and seed mass).

Conclusion: These findings show that SAM parameters provide a functional link among sizes and numbers of plant organs, constituting species' environmental responses.

Keywords: Corner’s rule; cell number; cell size; genome size; phylogenetic analysis.

Fig. 1.

Measured parameters of the SAM, i.e. width, thickness, inner (=lower) periphery and outer periphery, are shown on a longitudinal medial cross-section of Centaurea phrygia meristem stained with PI and viewed through the confocal microscope. For definitions of measured parameters see the Materials and methods section. Scale bar = 50 µm.

Fig. 2.

First three axes of the principal components analysis of meristem parameters based on species means. NC_Innerp, number of cells at the Inner Periphery; NC_Thickness, number of cells along the meristem thickness (longest vertical dimension); NC_periphery, number of cells along the (outer) periphery. For definitions of these morphological terms see Fig. 1.

Fig. 3.

Components of variance of meristem parameters (normalized to sum to 1, residual variation not included). For additional details see the Materials and methods section.

Fig. 4.

Mapping of meristem size on the phylogenetic tree from the Daphne phylogeny.

Fig. 5.

Longitudinal medial cross-sections of SAMs stained by PI and viewed through the confocal microscope. The image shows a bulgy SAM of Poa pratensis (A), a flat SAM of Epilobium hirsutum (B), a large SAM of Euphorbia lucida (C) and a small SAM of Hypericum maculatum (D). Scale bar (A–D) = 50 µm.

Fig. 6.

Relationship between stem diameter and overall meristem size (area of the cross-section) based on species means. The line is the ordinary least squares regression line. R² = 0.261.

Fig. 7.

Relationship between cell size at the outer periphery and nuclear genome size. The continuous line is the standard major axis regression (SMA) line, dashed lines show the 95 % confidence interval (CI) for this line, and the red line is the expected isometric relationship (slope 1/3). R² = 0.568 (ordinary least squares).

Fig. 8.

Path analysis of relationship between meristem summary variables, nuclear genome size and plant traits. χ² = 10.16, d.f. = 8, P = 0.253. R² for individual plant traits: leaf area, 0.088; stem diameter, 0.347; seed mass, 0.232; height, 0.014. For additional details see the Materials and methods section.