*2.4. MiPEP165a*

MicroRNAs (miRNAs) play a significant role in the regulation of gene expression. Ormancey et al. investigated the role of a miRNA-encoded peptide, miPEP165a, in *A. thaliana* [6]. The authors found that passive di ffusion followed by an endocytosis process are two functions of entry of miPEP165a.

#### **3. Cell Signaling in Response to Biotic Stresses**

Ethylene is a gaseous phytohormone that is involved in response to biotic and abiotic stresses. Zhang et al. studied the e ffect of ethylene signaling on resistance to some aphids in *Medicago truncatula* using an ethylene-insensitive mutant called sickle mutant [7]. Their results revealed that the sickle mutant can cause a moderate resistance to some aphids from the independent pathway of R-genes such as *AKR* (*Acyrthosiphon kondoi* resistance), *APR* (*Acyrthosiphon pisum* resistance), and *TTR* (*Therioaphis trifolii* resistance).

*Botrytis cinerea* is a necrotrophic fungus causing grey mold disease in many plant species. Maqsood et al. examined the e ffect of iprodione, as a fungicide, on the molecular mechanisms of *B. cinerea* resistant mutant [8]. By analyzing the whole transcriptome sequencing, they pronounced that genes involved in metabolism, production of detoxification enzymes, mitogen-activated protein kinases (MAPK) signaling, transporter function, catalytic activity, and drug e fflux are linked with resistance to iprodione.

#### **4. Cell Signaling Related to Plant Acclimation**

### *4.1. Brassinosteroid Signaling*

Brassinosteroids (BRs) as steroid hormones play critical roles in regulating the plant growth and development stages. Mao and Li reviewed the regulatory mechanisms of brassinosteroid-insensitive 1 (BRI1), BRI1-associated receptor kinase (BAK1), and brassinosteroid-insensitive 2 (BIN2), as three key kinases, involved in the BR signaling cascade [9]. They stated that BIN2, as an interface kinase, plays an important role in BR signal transduction from receptor kinase, BRI1/BAK1, to nucleus through two transcription factors, BRI1-EMS-supressor 1 (BES1), and brassinazole-resistant 1 protein (BZR1).

BRs have many interactions with other phytohormones that regulate the downstream pathways related to plant growth or response to stresses. Bulgakov and Avramenko reviewed the links between BR and abscisic acid (ABA) signaling that a ffect the stress-acclimation processes [10]. They proposed three interconnected mechanisms that in the first mechanism, BIN2, as a kinase of BR signaling, is responsible for interaction with ABA signaling.

#### *4.2. Cyclic AMP Signaling*

The cyclic nucleotide, cAMP (3-,5--cyclic adenosine monophosphate) as a signaling molecule, is involved in molecular processes linked to response to environmental stresses. Blanco et al. reviewed the current knowledge of cAMP signaling [11]. They indicated that the main signaling mechanism of this cyclic nucleotide is exchange of cAMP into Ca2+ signals through cyclic nucleotides-gated channels.

### *4.3. Hydrogen Sulfide*

Hydrogen Sulfide (H2S) is a toxic gaseous molecule. Recent studies revealed that H2S plays a positive role in regulating plant growth. Xuan et al. considered the H2S roles in cellular processes and also indicated all possible crosstalk between H2S and phytohormones [12]. Interestingly, the authors suggested that H2S may affect the protein activities and subcellular localization by contributing to post-translational modification.

#### **5. E**ff**ects of Selenium and Sedimentary Calcite-Processed Particles on Cell Signaling**

Selenium (Se) is identified as a beneficial element that can be included in improving the plant's resistance facing adverse environmental conditions. Kamran et al. reviewed the protectant roles of Se in response to soil salinity [13]. They indicated that Se can improve salinity tolerance by decreasing Na<sup>+</sup> ion accumulation through the expression induction of the Na+/H<sup>+</sup> antiport and increasing the antioxidants.

Tran et al. investigated the early cellular responses of bright yellow2 tobacco cultured cells under the application of calcite processed particles (CaPPs) [14]. Their results revealed that CaPPs act such as nanoparticles and induce various signaling pathways. CaPPs firstly induced ROS and then, increased the cytosolic Ca2+ and activation of anion channels.

#### **6. Conclusions and Perspectives**

This Special Issue provides new and in-depth insights into molecular aspects of plant cell signaling in response to biotic, such as aphid- and grey mold disease-resistance, and abiotic stresses, such as soil salinity and drought stress, and additionally, functional analysis on signaling components involved in flowering, juvenility, GA signaling, and biosynthesis, and miRNA-regulated gene expression. Furthermore, plant acclimation was reported, with emphasis on mechanistic insights into the roles of brassinosteroids, cyclic AMP, and hydrogen sulfide, and the recent advances of transmembrane receptor-like kinases were refined. Clearly, plant cell signaling is an intensive topic and whether it is now or in the future, the emerging technology in functional analysis such as genome editing technologies, high-throughput technologies, integrative multiple-omics as well as bioinformatics can assist researchers to reveal novel aspects of the regulatory mechanisms of plant growth and development, and acclimation to environmental and biotic stresses. The achievement of such research will be useful in improving crop stress tolerances to increase agricultural productivity and sustainability for the food supply of the world.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The author declares no conflict of interest.
