Field Guide to Plant Model Systems

Abstract

For the past several decades, advances in plant development, physiology, cell biology, and genetics have relied heavily on the model (or reference) plant Arabidopsis thaliana. Arabidopsis resembles other plants, including crop plants, in many but by no means all respects. Study of Arabidopsis alone provides little information on the evolutionary history of plants, evolutionary differences between species, plants that survive in different environments, or plants that access nutrients and photosynthesize differently. Empowered by the availability of large-scale sequencing and new technologies for investigating gene function, many new plant models are being proposed and studied.

Figure 1. Plant Diversity

Depicted are relationships among the major lineages of plants: glaucophytes (freshwater algae), rhodophytes (red algae), and the Viridiplantae (chlorophytes, charophytes, and land plants [vascular plants and bryophytes]). Estimated dates for some nodes are listed in millions of years before present. The primary endosymbiotic event is estimated to have occurred at least 1.6 billion years ago. A deep split occurred within the green lineage creating the chlorophytes and the charophytes plus land plants. Note that both the charophytes and the bryophytes are grades and are not monophyletic. Key evolutionary innovations are indicated at some nodes: (1) chlorophyll a and b, highly developed chloroplast grana; (2) lateral flagella; (3) phragmoplast, plasmodesmata, apical cell growth; (4) alternation of generations (embryo), sporopollenin, three-dimensional organization, archegonia and antheridia; (5) stomata; (6) vasculature (xylem and phloem), sporophyte dominant; (7) seeds, loss of archegonia and antheridia, pollen; and (8) flowers, carpels, outer integument. Evolution of simple multicellularity (differentiation largely limited to soma and gametes) and complex multicellularity (3-dimensional tissue differentiation) are indicated by *. Taxa in red represent those whose genome sequencing are either in progress or proposed or reside at critical evolutionary positions (Mesostigma, Coleochaete). Secondary endo-symbiotic events have occurred from both the red and green algal lineages. The relative species richness of the major clades is depicted in the pie chart. The vast majority of species within the Plantae are angiosperms (350,000 species), with other groups having substantially fewer described species (numbers approximated): glaucophytes, 21; rhodophytes, 7095; chlorophytes, 6263, charophytes, 4644; bryophytes, 18,200 (liverworts, 7,270; mosses, 11,000; hornworts, 215); lycophytes, 1225; ferns, 12,000; and gymnosperms, 1000. Photos from top: Pinus radiata, Selaginella kraussiana, Physcomitrella patens, Marchantia berteroana, Spirogyra sp., Coleochaete orbicularis, Chara sp., Klebsormidium flaccidum, Mesostigma viride, Chlamydomomas reinhardtii, Volvox sp., and Ostreococcus tauri. Thanks to individuals who generously provided photos of poplar (Gayle Dupper, Institute of Forest Genetics, Placerville), Physcomitrella (Keiko Sakakibara), Spirogyra (Bram Van de Poel, University of Maryland), Coleochaete, Chara (Charles Delwiche, University of Maryland), Chlamydomonas (James Umen, Salk Institute), Volvox (Frank Fox, www.mikro-foto.de), and Ostreococcus (Hervé Moreau, Université Pierre et Marie Curie-Paris). Other photos by J. L. B. Mesostigma image from (Lauterborn, 1898) and Klebsormidium image from (Klebs, 1896). Figure adapted from Bowman et al. (2007).