Early events in phloem formation: Exploring the molecular network of SMXL3/4/5

Growth and body shape of complex multicellular organisms is largely determined by a functional long-distance transport of energy metabolites that fuels stem cell activity. In vascular plants, sugars are photosynthetically produced in source tissues and delivered via the phloem to sink tissues for allocation into storage organs or to sustain distinct stem cell niches, called meristems. In the root apical meristem (RAM), which drives longitudinal root growth, sugar supply is ensured by a tight interplay between proto- and metaphloem. Formation of proto- and metaphloem starts with a single stem cell whose daughter cells divide and differentiate in a controlled spatio-temporal manner. Protophloem differentiates first within the RAM to enable sugar unloading close to the stem cell niche. Impaired or delayed protophloem formation has detrimental consequences for plant growth and vitality. Understanding the regulatory mechanisms behind (proto-)phloem formation is an important hub to enhance sink strength and thereby crop yield in the near or further future. In this study I report novel key-components in phloem regulation called SUPPRESSOR OF MAX2 1-LIKE3 (SMXL3), SMXL4 and SMXL5. Unlike most SMXL family members, SMXL3/4/5 act independently from strigolactone (SL) or karrikin (KAR) signaling as positive regulators of phloem formation. They are the first described phloem-specific genes that show promoter activity already in provascular tissues of the embryo, the first phloem stem cell in the RAM and along the whole phloem tissue in adult plants. SMXL3/4/5 promote protophloem initiation and differentiation in a dose-dependent manner. Deficiency of all three gene functions results in complete absence of phloem tissue and seedling lethality. In comparison, double mu-tants show reduced phloem-dependent transport and sugar accumulation in leaves. Moreover, SMXL3/5 play an additional and SMXL4-independent role in radial root growth by promoting procambial cell divisions. Interestingly, SMXL5 activity is sufficient to induce secondary phloem formation at the stem base, but acts redundantly with SMXL4 in suppressing radial stem thickening. This functional specialisation suggests that SMXL3/4/5 play distinct roles in molecular networks of phloem and/or (pro-)cambium formation. To integrate SMXL3/4/5 into such molecular networks, I characterized protein-protein interaction partners of SMXL5. The plant homeodomain (PHD)-finger protein OBERON 3 (OBE3) is the first interaction partner that genetically interacts with SMXL3/4/5 in protophloem formation. Previous studies reported that OBEs are important components in meristem maintenance and, potentially, chromatin remodelling. SMXL3/4/5 are nuclear localized, chap-eron-like proteins with conserved AAA ATPase and ETHYLENE-RESPONSE FACTOR Amphiphilic Repression (EAR) domain, which makes them perfect candidates to act in transcriptional regulation of downstream targets. This study and the characterization of SMXL3/4/5 and OBE3 as novel and fundamental phloem regulators enabled a deeper understanding of phloem development and sugar allocation in plants

Strigolactone- and Karrikin-Independent SMXL Proteins Are Central Regulators of Phloem Formation

Plant stem cell niches, the meristems, require long-distance transport of energy metabolites and signaling molecules along the phloem tissue. However, currently it is unclear how specification of phloem cells is controlled. Here we show that the genes SUPPRESSOR OF MAX2 1-LIKE3 (SMXL3), SMXL4, and SMXL5 act as cell-autonomous key regulators of phloem formation in Arabidopsis thaliana. The three genes form an uncharacterized subclade of the SMXL gene family that mediates hormonal strigolactone and karrikin signaling. Strigolactones are endogenous signaling molecules regulating shoot and root branching [1] whereas exogenous karrikin molecules induce germination after wildfires [2]. Both activities depend on the F-box protein and SCF (Skp, Cullin, F-box) complex component MORE AXILLARY GROWTH2 (MAX2) [3-5]. Strigolactone and karrikin perception leads to MAX2-dependent degradation of distinct SMXL protein family members, which is key for mediating hormonal effects [6-12]. However, the nature of events immediately downstream of SMXL protein degradation and whether all SMXL proteins mediate strigolactone or karrikin signaling is unknown. In this study we demonstrate that, within the SMXL gene family, specifically SMXL3/4/5 deficiency results in strong defects in phloem formation, alteredsugar accumulation, and seedling lethality. By comparing protein stabilities, we show that SMXL3/4/5 proteins function differently to canonical strigolactone and karrikin signaling mediators, although being functionally interchangeable with those under low strigolactone/karrikin signaling conditions. Our observations reveal a fundamental mechanism of phloem formation and indicate that diversity of SMXL protein functions is essential for a steady fuelling of plant meristems.Peer reviewe

Mobile PEAR transcription factors integrate positional cues to prime cambial growth.

Apical growth in plants initiates upon seed germination, whereas radial growth is primed only during early ontogenesis in procambium cells and activated later by the vascular cambium1. Although it is not known how radial growth is organized and regulated in plants, this system resembles the developmental competence observed in some animal systems, in which pre-existing patterns of developmental potential are established early on2,3. Here we show that in Arabidopsis the initiation of radial growth occurs around early protophloem-sieve-element cell files of the root procambial tissue. In this domain, cytokinin signalling promotes the expression of a pair of mobile transcription factors-PHLOEM EARLY DOF 1 (PEAR1) and PHLOEM EARLY DOF 2 (PEAR2)-and their four homologues (DOF6, TMO6, OBP2 and HCA2), which we collectively name PEAR proteins. The PEAR proteins form a short-range concentration gradient that peaks at protophloem sieve elements, and activates gene expression that promotes radial growth. The expression and function of PEAR proteins are antagonized by the HD-ZIP III proteins, well-known polarity transcription factors4-the expression of which is concentrated in the more-internal domain of radially non-dividing procambial cells by the function of auxin, and mobile miR165 and miR166 microRNAs. The PEAR proteins locally promote transcription of their inhibitory HD-ZIP III genes, and thereby establish a negative-feedback loop that forms a robust boundary that demarks the zone of cell division. Taken together, our data establish that during root procambial development there exists a network in which a module that links PEAR and HD-ZIP III transcription factors integrates spatial information of the hormonal domains and miRNA gradients to provide adjacent zones of dividing and more-quiescent cells, which forms a foundation for further radial growth.Gatsby Foundation [GAT3395/PR3)] University of Helsinki [award 799992091] ERC Grant SYMDEV [No. 323052] NSF-BBSRC MCSB 1517058 etc

Spatial specificity of auxin responses coordinates wood formation

Spatial organization of signalling events of the phytohormone auxin is fundamental for maintaining a dynamic transition from plant stem cells to differentiated descendants. The cambium, the stem cell niche mediating wood formation, fundamentally depends on auxin signalling but its exact role and spatial organization is obscure. Here we show that, while auxin signalling levels increase in differentiating cambium descendants, a moderate level of signalling in cambial stem cells is essential for cambium activity. We identify the auxin-dependent transcription factor ARF5/MONOPTEROS to cell-autonomously restrict the number of stem cells by directly attenuating the activity of the stem cell-promoting WOX4 gene. In contrast, ARF3 and ARF4 function as cambium activators in a redundant fashion from outside of WOX4-expressing cells. Our results reveal an influence of auxin signalling on distinct cambium features by specific signalling components and allow the conceptual integration of plant stem cell systems with distinct anatomies

Optimization of adsorptive removal of α-toluic acid by CaO2 nanoparticles using response surface methodology

The present work addresses the optimization of process parameters for adsorptive removal of α-toluic acid by calcium peroxide (CaO2) nanoparticles using response surface methodology (RSM). CaO2 nanoparticles were synthesized by chemical precipitation method and confirmed by Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) analysis which shows the CaO2 nanoparticles size range of 5–15 nm. A series of batch adsorption experiments were performed using CaO2 nanoparticles to remove α-toluic acid from the aqueous solution. Further, an experimental based central composite design (CCD) was developed to study the interactive effect of CaO2 adsorbent dosage, initial concentration of α-toluic acid, and contact time on α-toluic acid removal efficiency (response) and optimization of the process. Analysis of variance (ANOVA) was performed to determine the significance of the individual and the interactive effects of variables on the response. The model predicted response showed a good agreement with the experimental response, and the coefficient of determination, (R2) was 0.92. Among the variables, the interactive effect of adsorbent dosage and the initial α-toluic acid concentration was found to have more influence on the response than the contact time. Numerical optimization of process by RSM showed the optimal adsorbent dosage, initial concentration of α-toluic acid, and contact time as 0.03 g, 7.06 g/L, and 34 min respectively. The predicted removal efficiency was 99.50%. The experiments performed under these conditions showed α-toluic acid removal efficiency up to 98.05%, which confirmed the adequacy of the model prediction