Editorial overview: Plant morphogenesis — new understanding of its organization and evolution
Introduction
We live in a golden age when the mechanisms of plant growth and development are being revealed. Growth — increase in size — depends on cell division through the cell cycle coupled with cell expansion, both now better understood. Development — the processes of organogenesis and differentiation — not only includes growth, but also the progressive specialization of cells and tissues. The triumvirate of genetics, molecular biology and cell biology underlie our new understanding of growth and development [1].
In this issue, we focus broadly on plant morphogenesis rather than the individual mechanisms involved in growth and development. Thus tools such as imaging and omics, molecular processes such as hormone signalling and small RNA regulation, and approaches including modelling and epigenetic analysis, are included but only where relevant within the morphogenetic process under discussion. Individually, they have been the focus of many recent reviews, and here we have aimed to integrate them into analyses of development at a higher level. To this end, we have invited 21 groups to outline and interpret recent significant progress in morphogenesis, and their contributions fall within three main themes. The first involves meristems in which pluripotent stem cells reside. Coverage includes how meristems are organized, and their differing and changing nature. The second focus is organs, the products of meristems that include roots, leaves, wood, reproductive tissues and seeds. The third theme is evo-devo. We can now establish how and when developmental events arose and changed over time by comparative study of close relatives, or across wide phylogenetic distances back to the aquatic algae and beyond [2, 3].
Section snippets
Meristems and growth
In shoot apical meristems (SAMs), it has now been 13 years since interplay between the CLAVATA3 peptide and the WUSCHEL transcription factor was revealed as core signalling molecules in a pathway that maintains meristem dynamics. Holt et al. now survey recent studies that reveal new twists and intricacies in this important pathway. These include the fact that the WUSCHEL protein itself moves and could be the elusive mobile signal impacting on overlying stem cells. Also newly understood is that
Organ systems
The organized growth of the root system depends upon the maintenance of its apical stem cell niche and the balance between cell division and differentiation. As reviewed by Sozzani and Iyer-Pascuzzi, this is a complex process with many inputs, including hormones (auxin and cytokinin in particular), transcription factors (such as PLETHORA and SCARECROW) and even ROS pathways. While many of the inputs were discovered some time ago, recent studies are beginning to reveal how these different inputs
Evolution and development
Gene duplication provides much of the raw material of evolution, especially in plants, and Rensing analyses its role in developmental evolution. Gene duplication occurs as a consequence of whole genome doubling (polyploidy), or small scale events including aneuploidy, mis-replication, and transposition. In many cases, one of the duplicated genes dies leaving the other still working. However, the two copies may subdivide the original function (subfunctionalization) or take on new functions
Conclusion
Thus recent discoveries are giving us new understanding of the origin and generation of plant form. As usual in burgeoning fields, discoveries in plant biology have shown unexpected relevance in other fields. In plant development, new principles and mechanisms could well be applied in unforeseen ways not only in agriculture, but also more broadly in understanding the human condition [4].
David R Smyth's group aims to understand genetic mechanisms that underlie floral architecture and floral organ development. Using Arabidopsis, they have helped uncover a network of bHLH proteins that define the carpel margin and fruit dehiscence zone. They have also discovered a novel boundary gene that inhibits growth between developing sepals, maintaining their separation and allowing auxin to accumulate in adjacent petal initiation zones.
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50 years of Arabidopsis research: Highlights and future directions
2016, New Phytologist
David R Smyth's group aims to understand genetic mechanisms that underlie floral architecture and floral organ development. Using Arabidopsis, they have helped uncover a network of bHLH proteins that define the carpel margin and fruit dehiscence zone. They have also discovered a novel boundary gene that inhibits growth between developing sepals, maintaining their separation and allowing auxin to accumulate in adjacent petal initiation zones.
Jo Ann Banks's group studies sex determination and development of male and hermaphroditic gametophytes in the homosporous fern Ceratopteris. Using a genetics approach, they have defined a sex-determination pathway in this species. They have also used a comparative genomics approach to discover genes that are associated with major morphological transitions that have occurred during the evolution of land plants.