Root Cap to Soil Interface: A Driving Force Toward Plant Adaptation and Development

Cell-specific polymerization-driven biomolecular condensate formation fine-tunes root tissue morphogenesis

Formation of biomolecular condensates can be driven by weak multivalent interactions and emergent polymerization. However, the mechanism of polymerization-mediated condensate formation is less studied. We found lateral root cap cell (LRC)-specific SUPPRESSOR OF RPS4-RLD1 (SRFR1) condensates fine-tune primary root development. Polymerization of the SRFR1 N-terminal domain is required for both LRC condensate formation and optimal root growth. Surprisingly, the first intrinsically disordered region (IDR1) of SRFR1 can be functionally substituted by a specific group of intrinsically disordered proteins known as dehydrins. This finding facilitated the identification of functional segments in the IDR1 of SRFR1, a generalizable strategy to decode unknown IDRs. With this functional information we further improved root growth by modifying the SRFR1 condensation module, providing a strategy to improve plant growth and resilience.

The proteome landscape of the root cap reveals a role for the jacalin-associated lectin JAL10 in the salt-induced endoplasmic reticulum stress pathway

Rapid climate change has led to enhanced soil salinity, one of the major determinants of land degradation, resulting in low agricultural productivity. This has a strong negative impact on food security and environmental sustainability. Plants display various physiological, developmental, and cellular responses to deal with salt stress. Recent studies have highlighted the root cap as the primary stress sensor and revealed its crucial role in halotropism. The root cap covers the primary root meristem and is the first cell type to sense and respond to soil salinity, relaying the signal to neighboring cell types. However, it remains unclear how root-cap cells perceive salt stress and contribute to the salt-stress response. Here, we performed a root-cap cell-specific proteomics study to identify changes in the proteome caused by salt stress. The study revealed a very specific salt-stress response pattern in root-cap cells compared with non-root-cap cells and identified several novel proteins unique to the root cap. Root-cap-specific protein-protein interaction (PPI) networks derived by superimposing proteomics data onto known global PPI networks revealed that the endoplasmic reticulum (ER) stress pathway is specifically activated in root-cap cells upon salt stress. Importantly, we identified root-cap-specific jacalin-associated lectins (JALs) expressed in response to salt stress. A JAL10-GFP fusion protein was shown to be localized to the ER. Analysis of jal10 mutants indicated a role for JAL10 in regulating the ER stress pathway in response to salt. Taken together, our findings highlight the participation of specific root-cap proteins in salt-stress response pathways. Furthermore, root-cap-specific JAL proteins and their role in the salt-mediated ER stress pathway open a new avenue for exploring tolerance mechanisms and devising better strategies to increase plant salinity tolerance and enhance agricultural productivity.

Visualization of root extracellular traps in an ectomycorrhizal woody plant (Pinus densiflora) and their interactions with root-associated bacteria

MAIN CONCLUSION

Extracellular traps in the primary root of Pinus densiflora contribute to root-associated bacterial colonization. Trapped rhizobacteria induce the production of reactive oxygen species in root-associated, cap-derived cells. Ectomycorrhizal (ECM) woody plants, such as members of Pinaceae and Fagaceae, can acquire resistance to biotic and abiotic stresses through the formation of mycorrhiza with ECM fungi. However, germinated tree seedlings do not have mycorrhizae and it takes several weeks for ectomycorrhizae to form on their root tips. Therefore, to confer protection during the early growth stage, bare primary roots require defense mechanisms other than mycorrhization. Here, we attempted to visualize root extracellular traps (RETs), an innate root defense mechanism, in the primary root of Pinus densiflora and investigate the interactions with root-associated bacteria isolated from ECM and fine non-mycorrhizal roots. Histological and histochemical imaging and colony-forming unit assays demonstrated that RETs in P. densiflora, mainly consisting of root-associated, cap-derived cells (AC-DCs) and large amounts of root mucilage, promote bacterial colonization in the rhizosphere, despite also having bactericidal activity via extracellular DNA. Four rhizobacterial strains retarded the mycelial growth of a pathogenic strain belonging to the Fusarium oxysporum species complex in dual culture assay. They also induced the production of reactive oxygen species (ROS) from host tree AC-DCs without being excluded from the rhizosphere of P. densiflora. Applying three Paraburkholderia strains, especially PM O-EM8 and PF T-NM22, showed significant differences in the ROS levels from the control group. These results reveal the indirect contributions of rhizobacteria to host root defense and suggest that root-associated bacteria could be a component of RETs as a first line of defense against root pathogens in the early growth stage of ECM woody plants.

The Contribution of PGPR in Salt Stress Tolerance in Crops: Unravelling the Molecular Mechanisms of Cross-Talk between Plant and Bacteria

Soil salinity is a major abiotic stress in global agricultural productivity with an estimated 50% of arable land predicted to become salinized by 2050. Since most domesticated crops are glycophytes, they cannot be cultivated on salt soils. The use of beneficial microorganisms inhabiting the rhizosphere (PGPR) is a promising tool to alleviate salt stress in various crops and represents a strategy to increase agricultural productivity in salt soils. Increasing evidence underlines that PGPR affect plant physiological, biochemical, and molecular responses to salt stress. The mechanisms behind these phenomena include osmotic adjustment, modulation of the plant antioxidant system, ion homeostasis, modulation of the phytohormonal balance, increase in nutrient uptake, and the formation of biofilms. This review focuses on the recent literature regarding the molecular mechanisms that PGPR use to improve plant growth under salinity. In addition, very recent -OMICs approaches were reported, dissecting the role of PGPR in modulating plant genomes and epigenomes, opening up the possibility of combining the high genetic variations of plants with the action of PGPR for the selection of useful plant traits to cope with salt stress conditions.

Repressive ZINC FINGER OF ARABIDOPSIS THALIANA proteins promote programmed cell death in the Arabidopsis columella root cap

Developmental programmed cell death (dPCD) controls a plethora of functions in plant growth and reproduction. In the root cap of Arabidopsis (Arabidopsis thaliana), dPCD functions to control organ size in balance with the continuous stem cell activity in the root meristem. Key regulators of root cap dPCD including SOMBRERO/ANAC033 (SMB) belong to the NAC family of transcription factors. Here, we identify the C2H2 zinc finger protein ZINC FINGER OF ARABIDOPSIS THALIANA 14 ZAT14 as part of the gene regulatory network of root cap dPCD acting downstream of SMB. Similar to SMB, ZAT14-inducible misexpression leads to extensive ectopic cell death. Both the canonical EAR motif and a conserved L-box motif of ZAT14 act as transcriptional repression motifs and are required to trigger cell death. While a single zat14 mutant does not show a cell death-related phenotype, a quintuple mutant knocking out 5 related ZAT paralogs shows a delayed onset of dPCD execution in the columella and the adjacent lateral root cap. While ZAT14 is co-expressed with established dPCD-associated genes, it does not activate their expression. Our results suggest that ZAT14 acts as a transcriptional repressor controlling a so far uncharacterized subsection of the dPCD gene regulatory network active in specific root cap tissues.

The NIN-LIKE PROTEIN 7 transcription factor modulates auxin pathways to regulate root cap development in Arabidopsis

The root cap is a small tissue located at the tip of the root with critical functions for root growth. Present in nearly all vascular plants, the root cap protects the root meristem, influences soil penetration, and perceives and transmits environmental signals that are critical for root branching patterns. To perform these functions, the root cap must remain relatively stable in size and must integrate endogenous developmental pathways with environmental signals, yet the mechanism is not clear. We previously showed that low pH conditions altered root cap development, and these changes are mediated by the NIN LIKE PROTEIN 7 (NLP7) transcription factor, a master regulator of nitrate signaling. Here we show that in Arabidopsis NLP7 integrates nitrate signaling with auxin pathways to regulate root cap development. We found that low nitrate conditions promote aberrant release of root cap cells. Nitrate deficiency impacts auxin pathways in the last layer of the root cap, and this is mediated in part by NLP7. Mutations in NLP7 abolish the auxin minimum in the last layer of the root cap and alter root cap expression of the auxin carriers PIN-LIKES 3 (PILS3) and PIN-FORMED 7 (PIN7) as well as transcription factors that regulate PIN expression. Together, our data reveal NLP7 as a link between endogenous auxin pathways and nitrate signaling in the root cap.

DoMYB5 and DobHLH24, Transcription Factors Involved in Regulating Anthocyanin Accumulation in Dendrobium officinale

As a kind of orchid plant with both medicinal and ornamental value, Dendrobium officinale has garnered increasing research attention in recent years. The MYB and bHLH transcription factors play important roles in the synthesis and accumulation of anthocyanin. However, how MYB and bHLH transcription factors work in the synthesis and accumulation of anthocyanin in D. officinale is still unclear. In this study, we cloned and characterized one MYB and one bHLH transcription factor, namely, D. officinale MYB5 (DoMYB5) and D. officinaleb bHLH24 (DobHLH24), respectively. Their expression levels were positively correlated with the anthocyanin content in the flowers, stems, and leaves of D. officinale varieties with different colors. The transient expression of DoMYB5 and DobHLH24 in D. officinale leaf and their stable expression in tobacco significantly promoted the accumulation of anthocyanin. Both DoMYB5 and DobHLH24 could directly bind to the promoters of D. officinale CHS (DoCHS) and D. officinale DFR (DoDFR) and regulate DoCHS and DoDFR expression. The co-transformation of the two transcription factors significantly enhanced the expression levels of DoCHS and DoDFR. DoMYB5 and DobHLH24 may enhance the regulatory effect by forming heterodimers. Drawing on the results of our experiments, we propose that DobHLH24 may function as a regulatory partner by interacting directly with DoMYB5 to stimulate anthocyanin accumulation in D. officinale.