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Summary Plant secondary growth drives stem thickening and biomass accumulation, but its regulation is not yet fully understood.We have identified a novel semi‐dominant mutant,rbe‐d, characterized by a significant reduction in cambium cells and a complete absence of secondary growth in interfascicular regions in the stem. Gene cloning experiments indicated that the activation of the C2H2 zinc finger transcription factor, AT5G06070/RABBIT EARS (RBE), is responsible for therbe‐dphenotype.Transgenic analysis confirmed that overexpression of RBE represses secondary growth, while therbe‐2mutant increased the width of the interfascicular cambium‐derived (ICD) region. TheRBEgene is expressed in the procambium and cambium regions. Transcriptomic analysis showed that genes of the tracheary element differentiation inhibitory factor‐phloem intercalated with xylem (TDIF‐PXY) central regulatory pathway are repressed in therbe‐dmutant plants. Biochemical analyses confirmed that RBE binds directly to the promoter of WUSCHEL‐related homeobox (WOX4), a TDIF‐PXY downstreamWOXgene that regulates cambium cell proliferation. Moreover, genetic analysis confirmed thatWOX4is epistatic toRBEin secondary growth.Our results indicate that RBE inhibits cambium proliferation and thereby impacts secondary growth by directly repressingWOX4. These findings offer valuable new insight into the regulation of secondary growth in the Arabidopsis stem.
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Water lily ( Nymphaea thermarum ) genome reveals variable genomic signatures of ancient vascular cambium losses
For more than 225 million y, all seed plants were woody trees, shrubs, or vines. Shortly after the origin of angiosperms ∼140 million y ago (MYA), the Nymphaeales (water lilies) became one of the first lineages to deviate from their ancestral, woody habit by losing the vascular cambium, the meristematic population of cells that produces secondary xylem (wood) and phloem. Many of the genes and gene families that regulate differentiation of secondary tissues also regulate the differentiation of primary xylem and phloem, which are produced by apical meristems and retained in nearly all seed plants. Here, we sequenced and assembled a draft genome of the water lily Nymphaea thermarum , an emerging system for the study of early flowering plant evolution, and compared it to genomes from other cambium-bearing and cambium-less lineages (e.g., monocots and Nelumbo ). This revealed lineage-specific patterns of gene loss and divergence. Nymphaea is characterized by a significant contraction of the HD-ZIP III transcription factors, specifically loss of REVOLUTA , which influences cambial activity in other angiosperms. We also found the Nymphaea and monocot copies of cambium-associated CLE signaling peptides display unique substitutions at otherwise highly conserved amino acids. Nelumbo displays no obvious divergence in cambium-associated genes. The divergent genomic signatures of convergent loss of vascular cambium reveals that even pleiotropic genes can exhibit unique divergence patterns in association with independent events of trait loss. Our results shed light on the evolution of herbaceousness—one of the key biological innovations associated with the earliest phases of angiosperm evolution.
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- Award ID(s):
- 1812116
- PAR ID:
- 10180914
- Date Published:
- Journal Name:
- Proceedings of the National Academy of Sciences
- Volume:
- 117
- Issue:
- 15
- ISSN:
- 0027-8424
- Page Range / eLocation ID:
- 8649 to 8656
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1073/pnas.1922873117
