A Decrease in Ambient Temperature Induces Post-Mitotic Enlargement of Palisade Cells in North American Lake Cress

Abstract

In order to maintain organs and structures at their appropriate sizes, multicellular organisms orchestrate cell proliferation and post-mitotic cell expansion during morphogenesis. Recent studies using Arabidopsis leaves have shown that compensation, which is defined as post-mitotic cell expansion induced by a decrease in the number of cells during lateral organ development, is one example of such orchestration. Some of the basic molecular mechanisms underlying compensation have been revealed by genetic and chimeric analyses. However, to date, compensation had been observed only in mutants, transgenics, and γ-ray-treated plants, and it was unclear whether it occurs in plants under natural conditions. Here, we illustrate that a shift in ambient temperature could induce compensation in Rorippa aquatica (Brassicaceae), a semi-aquatic plant found in North America. The results suggest that compensation is a universal phenomenon among angiosperms and that the mechanism underlying compensation is shared, in part, between Arabidopsis and R. aquatica.

Fig 1. Gross morphology of Rorippa aquatica grown at different ambient temperatures.

Top views of R. aquatica plants grown at 30°C (A) and 20°C (B) for 50 days. Leaves (LN6) of R. aquatica grown at 30°C (C) and 20°C (D). Comparison of leaf forms of R. aquatica grown at 30°C (E) and 20°C (F). The oldest leaf is depicted at the left and the youngest at the right. Scale bars = 3 cm (A) and (B); 1 cm (C) and (D); 2 cm (E) and (F).

Fig 2. Cellular phenotypes of leaves from plants grown at 30°C and 20°C.

(A) Palisade cells in LN6 of Rorippa aquatica grown at 30°C (left) and 20°C (right). The upper panels show differential interference microscopy images, and the lower panels show the silhouettes of randomly selected cells. Scale bars = 100 μm. (B–D) Leaf area, number of cells per 100 mm², and palisade cell area, respectively. Error bars represent the standard error (SE); * = p < 0.05; ** = p < 0.01 by Student’s t-test (n = 6).

Fig 3. Observation of epidermal cells from plants grown at 30°C and 20°C.

(A) Epidermal cells in LN6 of Rorippa aquatica grown at 30°C (left) and 20°C (right). The upper panels show images of epidermal cells, and the lower panels show the silhouettes of randomly selected cells. Scale bars = 50 μm. (B) Epidermal cell area. (C) Dissection index (DI) of epidermal cells. Error bars represent the standard error (SE); * = p < 0.05 by Student’s t-test (n = 6).

Fig 4. Observation of inner structure of leaves of Rorippa aquatica grown at 30°C and 20°C.

(A) Inner cells in LN6 leaf blade of R. aquatica grown at 30°C (left) and 20°C (right). Cross-sections in the upper panels show images of inner leaf tissue cells, and the lower panels show the silhouettes of randomly selected cells. Scale bars = 100 μm. (B) Area of inner cells. (C) Thickness of leaves. Error bars represent the standard error (SE) (n = 6).

Fig 5. Expression analyses of orthologous genes related to compensation.

(A) Schematic presentation of three classes of compensation. (B) Expression levels of Ra AN3, Ra ERECTA, Ra FUGU2, Ra FUGU5, and Ra KRP2 in leaf primordia of Rorippa aquatica grown at 30°C and 20°C. Error bars represent the standard error (SE). * = p < 0.05 by Welch’s t-test (n = 4).