Divykriti Chopra, Mona Mapar, Lisa Stephan, Maria C. Albani, Anna Deneer, George Coupland, Eva-Maria Willing, Swen Schellmann, Korbinian Schneeberger, Christian Fleck, Andrea Schrader, and Martin Hülskamp

PNAS June 11, 2019 116 (24) 12078-12083

Significance

Typically, comparative developmental studies focus on the question whether key genes are conserved over large evolutionary distances. How the gene regulatory network of a whole developmental process evolves is virtually unknown. This requires a functional comparison of homologous processes in species closely related enough to find similarities and distant enough to see functional changes. We approach this question by genetically and molecularly comparing trichome development in Arabidopsis thaliana with that in Arabis alpina. We show which steps are regulated similarly and which are different. A closer analysis of trichome patterning revealed fascinating changes in the gene regulatory network; in particular, the finding that overexpression of one key regulator leads to opposite phenotypes in the two species suggested fundamental changes.

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 Arabis alpina. 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.

See https://www.pnas.org/content/116/24/12078

Figure 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.)