During primary growth, plant tissues increase their length, and as these tissues mature, they initiate secondary growth to increase thickness.1 It is not known what activates this transition to ...secondary growth. Cytokinins are key plant hormones regulating vascular development during both primary and secondary growth. During primary growth of Arabidopsis roots, cytokinins promote procambial cell proliferation2,3 and vascular patterning together with the hormone auxin.4–7 In the absence of cytokinins, secondary growth fails to initiate.8 Enhanced cytokinin levels, in turn, promote secondary growth.8,9 Despite the importance of cytokinins, little is known about the downstream signaling events in this process. Here, we show that cytokinins and a few downstream LATERAL ORGAN BOUNDARIES DOMAIN (LBD) family of transcription factors are rate-limiting components in activating and further promoting secondary growth in Arabidopsis roots. Cytokinins directly activate transcription of two homologous LBD genes, LBD3 and LBD4. Two other homologous LBDs, LBD1 and LBD11, are induced only after prolonged cytokinin treatment. Our genetic studies revealed a two-stage mechanism downstream of cytokinin signaling: while LBD3 and LBD4 regulate activation of secondary growth, LBD1, LBD3, LBD4, and LBD11 together promote further radial growth and maintenance of cambial stem cells. LBD overexpression promoted rapid cell growth followed by accelerated cell divisions, thus leading to enhanced secondary growth. Finally, we show that LBDs rapidly inhibit cytokinin signaling. Together, our data suggest that the cambium-promoting LBDs negatively feed back into cytokinin signaling to keep root secondary growth in balance.
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•Transition from primary to secondary growth occurs gradually in Arabidopsis root•Cytokinins activate secondary growth through a set of LBD genes•LBDs are required for both cell division and cell growth during secondary growth•LBDs rapidly inhibit cytokinin signaling
Ye et al. demonstrate that phytohormone cytokinin and four downstream LATERAL ORGAN BOUNDARIES DOMAIN (LBD) family of transcription factors promote the transition from primary to secondary growth in Arabidopsis root. LBDs negatively feed back to cytokinin signaling to keep secondary growth in balance.
Vascular cambium, a lateral plant meristem, is a central producer of woody biomass. Although a few transcription factors have been shown to regulate cambial activity
, the phenotypes of the ...corresponding loss-of-function mutants are relatively modest, highlighting our limited understanding of the underlying transcriptional regulation. Here, we use cambium cell-specific transcript profiling followed by a combination of transcription factor network and genetic analyses to identify 62 new transcription factor genotypes displaying an array of cambial phenotypes. This approach culminated in virtual loss of cambial activity when both WUSCHEL-RELATED HOMEOBOX 4 (WOX4) and KNOTTED-like from Arabidopsis thaliana 1 (KNAT1; also known as BREVIPEDICELLUS) were mutated, thereby unlocking the genetic redundancy in the regulation of cambium development. We also identified transcription factors with dual functions in cambial cell proliferation and xylem differentiation, including WOX4, SHORT VEGETATIVE PHASE (SVP) and PETAL LOSS (PTL). Using the transcription factor network information, we combined overexpression of the cambial activator WOX4 and removal of the putative inhibitor PTL to engineer Arabidopsis for enhanced radial growth. This line also showed ectopic cambial activity, thus further highlighting the central roles of WOX4 and PTL in cambium development.
Oriented cell divisions are crucial throughout plant development to define the final size and shape of organs and tissues. As most of the tissues in mature roots and stems are derived from vascular ...tissues, studying cell proliferation in the vascular cell lineage is of great importance. Although perturbations of vascular development are often visible already at the whole plant macroscopic phenotype level, a more detailed characterization of the vascular anatomy, cellular organization, and differentiation status of specific vascular cell types can provide insights into which pathway or developmental program is affected. In particular, defects in the frequency or orientation of cell divisions can be reliably identified from the number of vascular cell files. Here, we provide a detailed description of the different clearing, staining, and imaging techniques that allow precise phenotypic analysis of vascular tissues in different organs of the model plant Arabidopsis thaliana throughout development, including the quantification of cell file numbers, differentiation status of vascular cell types, and expression of reporter genes.
Due to their long lifespan, trees and bushes develop higher order of branches in a perennial manner. In contrast to a tall tree, with a clearly defined main stem and branching order, a bush is ...shorter and has a less apparent main stem and branching pattern. To address the developmental basis of these two forms, we studied several naturally occurring architectural variants in silver birch (
Betula pendula
). Using a candidate gene approach, we identified a bushy
kanttarelli
variant with a loss-of-function mutation in the
BpMAX1
gene required for strigolactone (SL) biosynthesis. While
kanttarelli
is shorter than the wild type (WT), it has the same number of primary branches, whereas the number of secondary branches is increased, contributing to its bush-like phenotype. To confirm that the identified mutation was responsible for the phenotype, we phenocopied
kanttarelli
in transgenic
BpMAX1::RNAi
birch lines. SL profiling confirmed that both
kanttarelli
and the transgenic lines produced very limited amounts of SL. Interestingly, the auxin (IAA) distribution along the main stem differed between WT and
BpMAX1::RNAi
. In the WT, the auxin concentration formed a gradient, being higher in the uppermost internodes and decreasing toward the basal part of the stem, whereas in the transgenic line, this gradient was not observed. Through modeling, we showed that the different IAA distribution patterns may result from the difference in the number of higher-order branches and plant height. Future studies will determine whether the IAA gradient itself regulates aspects of plant architecture.
Tree architecture has evolved to support a top-heavy above-ground biomass, but this integral feature poses a weight-induced challenge to trunk stability. Maintaining an upright stem is expected to ...require vertical proprioception through feedback between sensing stem weight and responding with radial growth. Despite its apparent importance, the principle by which plant stems respond to vertical loading forces remains largely unknown. Here, by manipulating the stem weight of downy birch (Betula pubescens) trees, we show that cambial development is modulated systemically along the stem. We carried out a genetic study on the underlying regulation by combining an accelerated birch flowering program with a recessive mutation at the ELIMÄKI locus (EKI), which causes a mechanically defective response to weight stimulus resulting in stem collapse after just 3 months. We observed delayed wood morphogenesis in eki compared with WT, along with a more mechanically elastic cambial zone and radial compression of xylem cell size, indicating that rapid tissue differentiation is critical for cambial growth under mechanical stress. Furthermore, the touch-induced mechanosensory pathway was transcriptionally misregulated in eki, indicating that the ELIMÄKI locus is required to integrate the weight-growth feedback regulation. By studying this birch mutant, we were able to dissect vertical proprioception from the gravitropic response associated with reaction wood formation. Our study provides evidence for both local and systemic responses to mechanical stimuli during secondary plant development.
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•Trees stems adjust their radial growth allometry in response to a weight increase•Fast-forward genetics by accelerated flowering reveals a Mendelian trait in birch•Vertical proprioception response in radial growth is linked to the ELIMÄKI locus•Timely tissue differentiation is critical for cambial growth under mechanical stress
Alonso-Serra et al. use birch trees to explore vertical proprioception, the growth response of stems to changes in aerial body weight. They identify a genetic locus, which is required for this developmental response, and study the consequences of cambial growth under mechanical stress.
Plant biomass plays an increasingly important role in the circular bioeconomy, replacing non-renewable fossil resources. Genetic engineering of this lignocellulosic biomass could benefit biorefinery ...transformation chains by lowering economic and technological barriers to industrial processing. However, previous efforts have mostly targeted the major constituents of woody biomass: cellulose, hemicellulose and lignin. Here we report the engineering of wood structure through the introduction of callose, a polysaccharide novel to most secondary cell walls. Our multiscale analysis of genetically engineered poplar trees shows that callose deposition modulates cell wall porosity, water and lignin contents and increases the lignin-cellulose distance, ultimately resulting in substantially decreased biomass recalcitrance. We provide a model of the wood cell wall nano-architecture engineered to accommodate the hydrated callose inclusions. Ectopic polymer introduction into biomass manifests in new physico-chemical properties and offers new avenues when considering lignocellulose engineering.
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