Microtubule-associated ROP interactors affect microtubule dynamics and modulate cell wall patterning and root hair growth.

Rho of plant (ROP) proteins and the interactor of constitutively active ROP (ICR) family member ICR5/MIDD1 have been implicated to function as signaling modules that regulate metaxylem secondary cell wall patterning. Yet, loss-of-function mutants of ICR5 and its closest homologs have not been studied and, hence, the functions of these ICR family members are not fully established. Here, we studied the functions of ICR2 and its homolog ICR5. We show that ICR2 is a microtubule-associated protein that affects microtubule dynamics. Secondary cell wall pits in the metaxylem of Arabidopsis icr2 and icr5 single mutants and icr2 icr5 double mutants are smaller than those in wild-type Col-0 seedlings; however, they are remarkably denser, implying a complex function of ICRs in secondary cell wall patterning. ICR5 has a unique function in protoxylem secondary cell wall patterning, whereas icr2, but not icr5, mutants develop split root hairs, demonstrating functional diversification. Taken together, our results show that ICR2 and ICR5 have unique and cooperative functions as microtubule-associated proteins and as ROP effectors.

Arabidopsis REM16 acts as a B3 domain transcription factor to promote flowering time via directly binding to the promoters of SOC1 and FT.

Actin depolymerizing factor (ADF) is a key modulator for dynamic organization of actin cytoskeleton. Interestingly, it was found that the ADF1 gene silencing delays flowering, but its mechanism remains unclear. In this study, ADF1 was used as a bait to screen its interacting proteins by the yeast two-hybrid (Y2H) system. One of them, the REM16 transcription factor was identified. As one of the AP2/B3-like transcriptional factor family members, the REM16 contains two B3 domains and its transcript levels kept increasing during the floral transition stage. Overexpression of REM16 accelerates flowering while silencing of REM16 delays flowering. Gene expression analysis indicated that the key flowering activation genes such as CONSTANS (CO), FLOWERING LOCUS T (FT), LEAFY (LFY) and SUPPRESSOR OF OVEREXPRESSION OF CONSTANS (SOC1) were upregulated in the REM16 overexpression lines, while the transcription of the flowering suppression gene FLOWERING LOCUS C (FLC) was decreased. In contrast, the REM16 gene silencing lines contained lower transcript levels of the CO, FT, LFY and SOC1 but higher transcript levels of the FLC compared with the wild-type plants. It was proved that REM16 could directly bind to the promoter regions of SOC1 and FT by in vitro and in vivo assays. Genetic analysis supported that REM16 acts upstream of SOC1 and FT in flowering pathways. All these studies provided strong evidence demonstrating that REM16 promotes flowering by directly activating SOC1 and FT.

Molecular association of Arabidopsis RTH with its homolog RTE1 in regulating ethylene signaling.

The plant hormone ethylene affects many biological processes during plant growth and development. Ethylene is perceived by ethylene receptors at the endoplasmic reticulum (ER) membrane. The ETR1 ethylene receptor is positively regulated by the transmembrane protein RTE1, which localizes to the ER and Golgi apparatus. The RTE1 gene family is conserved in animals, plants, and lower eukaryotes. In Arabidopsis, RTE1-HOMOLOG (RTH) is the only homolog of the Arabidopsis RTE1 gene family. The regulatory function of the Arabidopsis RTH in ethylene signaling and plant growth is largely unknown. The present study shows Arabidopsis RTH gene expression patterns, protein co-localization with the ER and Golgi apparatus, and the altered ethylene response phenotype when RTH is knocked out or overexpressed in Arabidopsis. Compared with rte1 mutants, rth mutants exhibit less sensitivity to exogenous ethylene, while RTH overexpression confers ethylene hypersensitivity. Genetic analyses indicate that Arabidopsis RTH might not directly regulate the ethylene receptors. RTH can physically interact with RTE1, and evidence supports that RTH might act via RTE1 in regulating ethylene responses and signaling. The present study advances our understanding of the regulatory function of the Arabidopsis RTE1 gene family members in ethylene signaling.

Protein post-translational modifications in auxin signaling.

Protein post-translational modifications (PTMs), such as ubiquitination, phosphorylation, and small ubiquitin-like modifier (SUMO)ylation, are crucial for regulating protein stability, activity, subcellular localization, and binding with cofactors. Such modifications remarkably increase the variety and complexity of proteomes, which are essential for regulating numerous cellular and physiological processes. The regulation of auxin signaling is finely tuned in time and space to guide various plant growth and development. Accumulating evidence indicates that PTMs play critical roles in auxin signaling regulations. Thus, a thorough and systematic review of the functions of PTMs in auxin signal transduction will improve our profound comprehension of the regulation mechanism of auxin signaling and auxin-mediated various processes. This review discusses the progress of protein ubiquitination, phosphorylation, histone acetylation and methylation, SUMOylation, and S-nitrosylation in the regulation of auxin signaling.

Endomembrane trafficking driven by microtubule growth regulates stomatal movement in Arabidopsis.

Microtubule-based vesicle trafficking usually relies upon kinesin and dynein motors and few reports describe microtubule polymerisation driving directional vesicle trafficking. Here we show that Arabidopsis END BINDING1b (EB1b), a microtubule plus-end binding protein, directly interacts with SYP121, a SNARE protein that mediates the trafficking of the K+ channel KAT1 and its distribution to the plasma membrane (PM) in Arabidopsis guard cells. Knockout of AtEB1b and its homologous proteins results in a modest but significant change in the distribution of KAT1 and SYP121 in guard cells and consequently delays light-induced stomatal opening. Live-cell imaging reveals that a portion of SYP121-associated endomembrane compartments co-localise with AtEB1b at the growing ends of microtubules, trafficking along with the growth of microtubules for targeting to the PM. Our study reveals a mechanism of vesicle trafficking driven by microtubule growth, which is involved in the redistribution of PM proteins to modulate guard cell movement.

LKS4-mediated SYP121 phosphorylation participates in light-induced stomatal opening in Arabidopsis.

By modulating stomatal opening and closure, plants control gas exchange, water loss, and photosynthesis in response to various environmental signals. During light-induced stomatal opening, the transport of ions and solutes across the plasma membrane (PM) of the surrounding guard cells results in an increase in turgor pressure, leading to cell swelling. Simultaneously, vesicles for exocytosis are delivered via membrane trafficking to compensate for the enlarged cell surface area and maintain an appropriate ion-channel density in the PM. In eukaryotic cells, soluble N-ethylmaleimide-sensitive factor adaptor protein receptors (SNAREs) mediate membrane fusion between vesicles and target compartments by pairing the cognate glutamine (Q)- and arginine (R)-SNAREs to form a core SNARE complex. Syntaxin of plants 121 (SYP121) is a known Q-SNARE involved in stomatal movement, which not only facilitates the recycling of K+ channels to the PM but also binds to the channels to regulate their activity. In this study, we found that the expression of a receptor-like cytoplasmic kinase, low-K+ sensitive 4/schengen 1 (LKS4/SGN1), was induced by light; it directly interacted with SYP121 and phosphorylated T270 within the SNARE motif. Further investigation revealed that LKS4-dependent phosphorylation of SYP121 facilitated the interaction between SYP121 and R-SNARE vesicle-associated membrane protein 722 (VAMP722), promoting the assembly of the SNARE complex. Our findings demonstrate that the phosphorylation of SNARE proteins is an important strategy adopted by plants to regulate the SNARE complex assembly as well as membrane fusion. Additionally, we discovered the function of LKS4/SGN1 in light-induced stomatal opening via the phosphorylation of SYP121.

Ethylene signaling and regulation in plant growth and stress responses.

Gaseous phytohormone ethylene affects many aspects of plant growth and development. The ethylene signaling pathway starts when ethylene binds to its receptors. Since the cloning of the first ethylene receptor ETR1 from Arabidopsis, a large number of studies have steadily improved our understanding of the receptors and downstream components in ethylene signal transduction pathway. This article reviews the regulation of ethylene receptors, signal transduction, and the posttranscriptional modulation of downstream components. Functional roles and importance of the ethylene signaling components in plant growth and stress responses are also discussed. Cross-reactions of ethylene with auxin and other phytohormones in plant organ growth will be analyzed. The studies of ethylene signaling in plant growth, development, and stress responses in the past decade greatly advanced our knowledge of how plants respond to endogenous signals and environmental factors.

Arabidopsis SYP121 acts as an ROP2 effector in the regulation of root hair tip growth.

Tip growth is an extreme form of polarized cell expansion that occurs in all eukaryotic kingdoms to generate highly elongated tubular cells with specialized functions, including fungal hyphae, animal neurons, plant pollen tubes, and root hairs (RHs). RHs are tubular structures that protrude from the root epidermis to facilitate water and nutrient uptake, microbial interactions, and plant anchorage. RH tip growth requires polarized vesicle targeting and active exocytosis at apical growth sites. However, how apical exocytosis is spatially and temporally controlled during tip growth remains elusive. Here, we report that the Qa-Soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) SYP121 acts as an effector of Rho of Plants 2 (ROP2), mediating the regulation of RH tip growth. We show that active ROP2 promotes SYP121 targeting to the apical plasma membrane. Moreover, ROP2 directly interacts with SYP121 and promotes the interaction between SYP121 and the R-SNARE VAMP722 to form a SNARE complex, probably by facilitating the release of the Sec1/Munc18 protein SEC11, which suppresses the function of SYP121. Thus, the ROP2-SYP121 pathway facilitates exocytic trafficking during RH tip growth. Our study uncovers a direct link between an ROP GTPase and vesicular trafficking and a new mechanism for the control of apical exocytosis, whereby ROP GTPase signaling spatially regulates SNARE complex assembly and the polar distribution of a Q-SNARE.