Genetic and molecular analysis of trichome development in Arabis alpina

Abstract

The genetic and molecular analysis of trichome development in Arabidopsis thaliana has generated a detailed knowledge about the underlying regulatory genes and networks. However, how rapidly these mechanisms diverge during evolution is unknown. To address this problem, we used an unbiased forward genetic approach to identify most genes involved in trichome development in the related crucifer species Arabisalpina In general, we found most trichome mutant classes known in A. thaliana We identified orthologous genes of the relevant A. thaliana genes by sequence similarity and synteny and sequenced candidate genes in the A. alpina mutants. While in most cases we found a highly similar gene-phenotype relationship as known from Arabidopsis, there were also striking differences in the regulation of trichome patterning, differentiation, and morphogenesis. Our analysis of trichome patterning suggests that the formation of two classes of trichomes is regulated differentially by the homeodomain transcription factor AaGL2 Moreover, we show that overexpression of the GL3 basic helix-loop-helix transcription factor in A. alpina leads to the opposite phenotype as described in A. thaliana Mathematical modeling helps to explain how this nonintuitive behavior can be explained by different ratios of GL3 and GL1 in the two species.

Keywords: Arabis alpina; genetic analysis; trichomes.

Fig. 1.

Trichome development on A. alpina rosette leaves. (A) Mature leaf; trichomes densely cover the whole surface. (B) In slightly older stages, incipient trichomes are found between older trichomes (arrowheads). (C) On very young leaves, incipient trichomes are found at the leaf base and advanced developmental stages in more distal regions. (D) Schematic representation of the trichome distribution along the proximal-distal axis on a very young leaf as shown in C, and an older leaf as shown in B. Blue-colored trichomes are intercalating between already existing ones. (E–J) Scanning electron micrographs and optical section of DAPI-stained trichomes at different developmental stages. (E) Incipient trichomes beginning to expand. (F) Unbranched trichomes. (G) Two-branched trichomes. (H) Four-branched trichomes. (I) Mature trichomes. (J) Top view of a mature trichome. Note that the immediately adjacent cells are shaped like pavement cells. n = 20. (Scale bars: A, B, and E–I: 10 μm; C and D: 20 μm; and J: 100 μm.)

Fig. 2.

Phenotypes of patterning mutants. (A) Scanning electron microscope (SEM) picture of a mature wild-type trichome. (B) SEM picture of a mature Aatry mutant trichome. (C) SEM picture of an Aagl2 mutant leaf showing the distribution of large wild-type shaped trichomes and small aborted trichomes. (D) SEM picture of an aborted Aagl2 mutant trichome. (E) Wild-type A. alpina (Pajares) leaf with large and small trichomes. (F) 35S:AaGL3 with large trichomes. (Scale bars: A and B: 100 μm; C: 500 μm; D: 50 μm; E and F: 1 mm.)

Fig. 3.

Modeling A. thaliana and A. alpina wild-type and 35S:GL3 phenotypes. (A) mRNA levels determined by qPCR of GL1 and TRY relative to GL3 for A. thaliana (gray bars) and A. alpina (white bars). Note that in Arabis we find relatively low levels of GL1 and relatively higher levels of TRY compared with Arabidopsis. (B) Schematic network of the model. The GL1 basal production parameter is decreased (red arrow) to reproduce A. alpina phenotypes. Simulation examples of (C) A. thaliana wild-type and (D) A. alpina wild-type on a 1D grid discretized into 100 cells (x-axis). Concentrations of activating complex (AC) and inactive complex (IC) are indicated by green and black lines, respectively. E and G show the development of 35S:GL3 patterns for A. thaliana conditions over time, where (E) is intermediate and (G) is the final state. F and H show the development of 35S:GL3 patterns for A. alpina conditions over time, where F is intermediate and (H) is the final state.