The pillars of land plants: new insights into stem development

Abstract

In spite of its central importance in evolution, plant architecture and crop improvement, stem development remains poorly understood relative to other plant organs. Here, we summarise current knowledge of stem ontogenesis and its regulation, including insights from new image analysis and biophysical approaches. The stem initiates in the rib zone (RZ) of the shoot apical meristem, under transcriptional control by DELLA and BLH proteins. Links have emerged between these regulators and cell proliferation, patterning and oriented growth in the RZ. During subsequent internode elongation, cell wall properties and mechanics have been analysed in detail, revealing pectin modification as a prominent control point. Recent work has also highlighted signalling to coordinate growth of stem tissues with different mechanical properties.

Figure 1

Early stages of inflorescence stem growth. (a) Three-dimensional (3D) view based on a stack of confocal images, showing the Arabidopsis inflorescence apex with most floral buds removed; SAM: shoot apical meristem; fb: position of floral buds; red arrows indicate the proliferating internode epidermis. (b) Longitudinal section through the image in (a), with the central zone (CZ), peripheral zone (PZ) and rib zone (RZ) indicated; the RZ is subdivided into central (cRZ) and peripheral (pRZ) regions; different stem tissues are indicated in yellow. (c) Close up of a SAM section similar to (b), with different SAM regions indicated as in (b); planes of recent cell divisions are coloured according to their orientation in 3D (the colour bar above the image shows angles between the stem main axis and the vector normal to a plane fitted to the new cell wall; red indicates divisions perpendicular to the main axis, green marks divisions at a low angle to the main axis). (d) Clonal analysis showing growth patterns in the SAM; each set of coloured dots indicates the position of cells descended from a single cell that had been genetically marked three days before imaging; clones were imaged in different apices, which were aligned and overlapped based on the position of floral buds; grey dots correspond to the position of epidermal cells in each of the overlapped apices. Bars: 50 μm. For details of methods used in (c) and (d), see [14^••].

Figure 2

Internode elongation in Arabidopsis. The images show the same Arabidopsis inflorescence photographed at 1-day intervals; to facilitate growth tracking, inks marks were placed on part of the stem on day 1; yellow arrows on the right show the regions that continued to grow in subsequent time points; the bars on the left of each image track growth of different regions of the stem in day 1 (0–1 cm: white; 1–2 cm: pink; 2–3 cm: red). Note that the growth rapidly ceases below the proliferative region (white), which is responsible for most of the stem elongation seen in the following days.