Plant Growth Regulators: True Managers of Plant Life (2024)

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Natural or synthetic compounds that can regulate developmental and metabolic processes in higher plants are designated plant growth regulators (PGRs) (Sabagh etal. 2021). The term ‘PGRs’ is more commonly used in reference to phytohormones and may also comprise inhibitors of phytohormone synthesis or translocation and blockers of their receptors (Rademacher 2015). Auxins, cytokinins (CKs), gibberellins, abscisic acid and ethylene constitute the ‘classical’ group of phytohormones, with salicylic acid, brassinosteroids (BRs) and jasmonates being the newly accepted members. The past decade has witnessed an increasing number of studies on karrikins (KARs)/yet-unknown endogenous KAR INSENSITIVE2 (KAI2) ligand and strigolactones (SLs) as two new promising classes of PGRs (Morffy etal. 2016, Antala etal. 2019). Melatonin is also considered an interesting candidate for use as a PGR due to its growth-promoting and anti-stress effects along with its ability to modulate the activities of various other phytohormones (Nawaz etal. 2020). The activities of PGRs are not only restricted to the longitudinal growth of plants, but they also affect a plethora of key processes including seed germination, defoliation, flowering, fruit formation and ripening and fruit drop, among others (Rademacher 2015). In addition, PGRs are frequently employed in agriculture, horticulture and viticulture for various benefits, such as improving morphological structure, increasing resistance and tolerance against biotic and abiotic stresses, and for qualitatively and quantitatively enhancing yields (Naeem and Aftab 2022).

Latest Research on PGRs

Plenty of research has been conducted on the complex network comprising signal transduction, perception and cross talk of phytohormones or their cross talk with other signaling molecules (Khan etal. 2020). Research on the interactions of plant growth–promoting rhizobacteria with endogenous phytohormones has also presented a new paradigm regarding hormonal interactions (Khan etal. 2020). Phytohormone priming is another expanding field of PGR application. Phytohormones are exogenously supplied to positively modulate several biological processes in plants, such as cell division, transpiration, photosynthesis, ascorbate–glutathione cycle, nitrogen metabolism and antioxidant activities in both normal and stressful environments (Sytar etal. 2019, Hossain et al. 2022, Malek etal. 2022, Tahaei etal. 2022). PGRs are being used as external supplements for medicinal plants to increase the production of secondary metabolites, thereby improving plant defense against stresses (Jamwal etal. 2018). Moreover, PGRs are currently employed to improve the efficacy of phytoremediation of contaminated soils (Rostami and Azhdarpoor 2019). The impact of exogenous PGRs in plant tissue cultures and in vitro proliferation is also a subject matter of interest (Cavallaro etal. 2022). To further shed light on the role of various PGRs in plant life and their associated mechanisms, this special issue presents nine original research papers and seven review articles, as described later.

Highlights of This Special Issue

Agronomic traits can be modified using the information related to hormonal pathways for designing novel crop varieties with enhanced yield. Gupta etal. (2022) review the knowledge available on the roles of auxin, BRs and CKs in improving rice (Oryza sativa) yield and discuss the molecular characterization of yield-associated quantitative trait loci governing cross talk among these phytohormones. Molecular genetics and genomic studies have already reported improvement in crop productivity through the induction of allelic variations in genes related to phytohormone-mediated regulation of plant growth (Singh etal. 2017, He etal. 2018). Hirayama and Mochida (2022) review the advancements in plant hormonomics (multiple phytohormone profiling), such as plant hormone profiling techniques based on mass spectrometry and nanosensors, and present hormonomics as a potential tool for physiological phenotyping and improvement of crop productivity.

Research on nanoparticles (NPs) has intensified in recent years due to their extensive applications in different fields, including agriculture (Lasso-Robledo etal. 2022, Vats etal. 2022), but the role of phytohormones in the plant–NP interaction is still a relatively unexplored area (Sonkar et al. 2021). Kandhol etal. (2022) review the scarce yet significant information available regarding the interactions between NPs and phytohormones in plants, as well as nanocarrier-mediated phytohormone delivery into plants, and suggest future research perspectives in this context.

Understanding the interaction between important genetic and phytohormonal factors regulating the root system architecture in crop plants can be significant for developing crop improvement strategies. H. Singh etal. (2022) review the role of several transcription factors, phytohormones, microRNAs and their interactions during various stages of adventitious root formation in two important cereal crops, maize (Zea mays) and rice. Auxin is a prerequisite for the initiation of crown root primordium and root organogenesis in rice (Meng etal. 2019). Z. Singh et al. (2022) report that an auxin stimulus induces changes in abundance of proteins and metabolites and phosphorylation of proteins during crown root development and demonstrate that auxin-dependent phosphorylation of cell wall proteins and the CYCLIN-DEPENDENT KINASE G-2 is crucial for root organogenesis.

A complex network of phytohormones, reactive oxygen species (ROS) and reactive nitrogen species has been known in plants to regulate their growth and responses to unfavorable environments (Devireddy etal. 2021). Parveen etal. (2022) review the intricate interactions among auxin, ROS and nitric oxide (NO) in regulating plant growth and development and abiotic stress–induced responses along with an evolutionary insight into these key signaling molecules. Aghdam et al. (2022) review the biochemistry and signaling of melatonin and its interaction with other phytohormones and signaling molecules to highlight its potential as a biotechnological tool to boost stress tolerance, to delay senescence and to maintain the nutritional quality of post-harvest horticultural products.

CKs are key phytohormones for both plant development and stress responses. L. Li et al. (2022) review the molecular mechanisms underlying the pathways of production, metabolism, transport and signaling transduction of CKs and suggest that these pathways are evolutionarily conserved in land plants. They also present a summary of the functions of CKs in providing tolerance against mineral toxicity and indicate that further research regarding the mechanisms of CK-mediated sodium exclusion under salt stress is needed. Heavy metal contamination of soil, water and air is known to impose a toxic impact on plants (Vaculík etal. 2020). Sustainable resources to boost plant tolerance against such abiotic stresses are the dire need at present times. Tripathi etal. (2022) have explored and confirmed the potential of silicon to mitigate chromium toxicity through the regulation of glutathione and indole-3-acetic acid–mediated formation of root hairs in rice.

SUPPRESSOR of MORE AXILLARY GROWTH 2 1 (SMAX1) and SMAX1-LIKE2 (SMXL2) are known to suppress KAR signaling and regulate the growth and development of plants (Stanga etal. 2016). Feng etal. (2022) evaluated the transcriptome profiles of drought resistance phenotypes and of smax1 smxl2 double mutants in Arabidopsis thaliana grown under drought stress and observed negative regulation of drought resistance by SMAX1 and SMXL2. An independent study showed that SMAX1 and SMXL2 participate in the SL signaling pathway through D14 to regulate Arabidopsis response to osmotic stress, suggesting the involvement of these two genes in SL signaling as well, at least under this specific osmotic stress condition (Q. Li etal. 2022). These results together with other previously published results suggest that the alteration of KAR and/or SL signaling pathway–related genes can provide an effective way to enhance crop tolerance to drought and osmotic stresses (Ha etal. 2014, Li etal. 2017, 2020a, 2020b, Q. Li et al. 2022). In addition, SLs are reported to aid plants to retain environmental memory by regulating the after-effects of drought (Visentin etal. 2020). In this issue, Krukowski et al. (2022) have identified BRI-EMS SUPPRESSOR1 as a transcription factor assisting SLs in regulating the transcriptional memory of Arabidopsis upon drought.

Genetic studies conducted in Arabidopsis plants related to KAI2 and KARs implicate KAI2 as a probable receptor for KARs (Guo etal. 2013). Through the evaluation of phenotypic and physiological traits, and transcriptome analysis, Abdelrahman etal. (2022) demonstrate that KAI2 regulates heat stress tolerance of Arabidopsis seedlings. They show that kai2 mutants depict a more severe heat-sensitive phenotype than wild-type plants. Mostofa etal. (2022) also project the ability of KAI2 to regulate tolerance mechanisms against salt stress in Arabidopsis and show that defects in KAI2 function cause delay or inhibition of cotyledon opening in seeds, several phenotypic aberrations, severe ion toxicity and reduced expression of other phytohormone-related genes in salt-stressed plants.

Finally, sinapic acid and sinapate esters (derivatives of sinapic acid) are end products of an important secondary metabolite pathway—the phenylpropanoid pathway—and act as PGRs to promote plant growth and development, as well as stress responses (Bi etal. 2017). W. Li et al. (2022) show that sinapic acid may reduce stomatal closure in A. thaliana leaves in response to ultraviolet-B (UV-B) stress possibly by interrupting an ROS–NO–cytosolic Ca²⁺–dependent pathway that promotes closure of stomata and an increase in malate accumulation that negatively regulates stomatal closure. They also report that sinapoylglucose or other sinapate esters may enhance malate accumulation and slow closure of stomata in plants exposed to UV-B, perhaps by adjusting the equilibrium between the two antagonistic regulatory pathways, i.e. malate- and ROS-based pathways (W. Li et al. 2022). Jiang etal. (2022) show that 14-3-3 proteins can also act as key regulators of stomata, in this case, in their response toward drought in barley (Hordeum vulgare), and they also reveal an early and highly conserved evolution of 14-3-3 proteins in green plants.

Promoting efforts toward the improvement of crop productivity and food quality depends on the available information related to PGRs. Thus, we need to further expand our research to identify novel substances that act as potential PGRs and understand their mechanisms of action in plants. To this aim, we hope that this special issue will inspire readers to engage in future research on PGRs.

Acknowledgements

Dr. D.K.T. is highly thankful to the Science and Engineering Research Board, New Delhi, for financial support (EMR/2017/000518).

Disclosures

The authors have no conflicts of interest to declare.

References

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