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Tropisms, growth responses to environmental stimuli such as light or gravity, are spectacular examples of adaptive plant development. The plant hormone auxin serves as a major coordinative signal. The PIN auxin exporters, through their dynamic polar subcellular localizations, redirect auxin fluxes in response to environmental stimuli and the resulting auxin gradients across organs underlie differential cell elongation and bending. In this review, we discuss recent advances concerning regulations of PIN polarity during tropisms, focusing on PIN phosphorylation and trafficking. We also cover how environmental cues regulate PIN actions during tropisms, as well as the crucial role of auxin feedback on PIN polarity during bending termination. Finally, the interactions between different tropisms are reviewed to understand plant adaptive growth in the natural environment.

Main conclusionPeptide-receptor complexes activate distinct downstream regulatory networks to mediate plant adaptions to abiotic environmental stress.Plants are constantly exposed to various adverse environmental factors; thus they must adjust their growth accordingly. Plants recruit small secretory peptides to adapt to these detrimental environments. These small peptides, which are perceived by their corresponding receptors and/or co-receptors, act as local- or long-distance mobile signaling molecules to establish cell-to-cell regulatory networks, resulting in optimal cellular and physiological outputs. In this review, we highlight recent advances on the regulatory role of small peptides in plant abiotic responses and nutrients signaling.

1 Introduction The RAPID ALKALINIZATION FACTOR (RALF) peptides belong to the cysteine-rich small peptide family, they play essential roles in plant growth and development and biotic and abiotic stress responses (Blackburn et al., 2020;Zhang et al., 2020;Somoza et al., 2021;Cheung, 2024). LLPS triggers the condensation of scattered macromolecules such as proteins or nucleic acids into a more condensed phase to form a liquid droplet structure in a certain cellular environment (Figure 1A;Hyman et al., 2014). [...]it is plausible that LLPS could drive the formation of pectin-RALF1 condensates (Das et al., 2015;Emenecker et al., 2020;Alberti and Hyman, 2021;2021). GRP7 transcription is regulated by cold and heat (Kilian et al., 2007;Kim et al., 2007), and the grp7 mutant displays significantly shorter primary roots and lower survival rates under freezing (−20°C) or high temperature (42°C), respectively (Xu et al., 2024), suggesting the critical role of GRP7 in acclimation to temperature fluctuations.

The majority of crop plants can generate seeds classified as orthodox seeds, which possess the ability to withstand drying to low moisture content (below 7%) and harsh extreme environmental conditions such as freezing (-10°C) for a long time (Nadarajan et al., 2023;Waterworth et al., 2024). Collectively, a higher level of desiccation tolerance is crucial for maize mechanized harvesting, preventing grain breakage, mildew, and reducing the costs associated with harvest and storage (de Jager et al., 2004;Xiang et al., 2012;Kebebe et al., 2015;Wang et al., 2022;Xia et al., 2024). [...]a comprehensive understanding of the mechanisms governing desiccation tolerance of seeds is necessary and crucial. [...]enhancing KDR and minimizing kernel moisture content at the harvest stage are critical and has become a major aim of modern maize breeding (Sala et al., 2006;Qu et al., 2022). Furthermore, analysis of public chromatin immunoprecipitation sequencing (ChIP-seq) datasets has uncovered two MYB-related transcription factors, ZmMYBST1 and ZmMYBR43, that bind to the qKDR1 locus.

To date, a plethora of methods including physical preservation (e.g., cold storage, modified atmosphere storage, edible coatings, heat treatment, and irradiation), chemical preservation (e.g., 1-methylcyclopropene (1-MCP), calcium chloride, hydrogen sulfide, diphenylamine, and synthetic compounds), and biological preservation (e.g., antibiotics and bacterial inhibitors) have been shown to effectively prolong the supply period and maintain the nutritional quality of fruits and vegetables (Sridhar et al., 2021;You et al., 2022;Gomes et al., 2023). [...]the development of more efficient, stable, cost-effective, and environment-friendly strategies are imperative to mitigate potential these adverse impacts on human health and the environment. Exogenous application of another peptide hormone, PHYTOSULFOKINE (PSK), can effectively retard yellowing and preserve the nutritional integrity of broccoli (Brassica oleracea L.var. italica Plenck) florets under cold storage through elevated levels of guanosine 3′, 5′-cyclic monophosphate (cGMP), phenols, flavonoids, and ascorbic acid (Aghdama et al., 2020). Furthermore, peptide hormones are typically degraded into non-toxic metabolites (amino acids). [...]since peptide hormones derived from plant sources offer an environmentally safe way for regulating fruit and vegetable production without causing environmental pollution.

Alteration of the C-terminal anchorage site of CLE peptides can significantly diminish their receptor binding efficacy (Li et al., 2017). [...]exchanging one or more N-terminal residues between INFLORESCENCE DEFICIENT IN ABSCISSION (IDA) and CLE9 peptides has been shown to reduce their cognate receptor affinity (Roman et al., 2022). [...]elucidating the intracellular dynamics and binding specificity of CLE peptides in vivo remains a challenge. [...]the peptide is released from the resin, typically under acidic conditions, which also removes any protecting groups on the side chains (Wellings and Atherton, 1997;Narasimhan et al., 2024). [...]the fluorescence labeled CLV3-TAMRA retain bioactivity, can be used to inspect the subcellular dynamics of CLV3 peptides in the cells and its binding specificity in vivo. 4 Photoactivation of CLV3 peptide Caged compounds are usually chemically or biologically active molecules that are deactivated by coupling to a photocleavable protecting group (caging group), and can be rapidly released upon UV illumination (Ellis-Davies, 2007;Li et al., 2023).

Background The CLE ( CLAVATA3/Endosperm Surrounding Region-related ) gene family encodes small signaling peptides that are primarily involved in coordinating stem cell fate in different types of plant meristems. Their roles in vascular cambium have highlighted their potential function in wood formation. Apart from recent advances on identification and characterization of CLE genes, little is known about this gene family in a tree species. Results Fifty PtCLE genes were identified from the Populus trichocarpa genome and were classified into four major groups based on sequence similarity. Analysis of the genomic organization of PtCLE genes indicates that genome duplication, as well as the diversity in the CLE motif, have contributed to the expansion of CLE gene family in poplar. A comparison with functionally characterized Arabidopsis CLE protein sequences showed that many PtCLE proteins are closely related to their predicted Arabidopsis counterparts. Particularly, PtCLE3, PtCLE12, PtCLE14 and PtCLE38 comprised an identical CLE motif to AtCLE41/TDIF, which is known as a regulator of vascular cambium homeostasis, strongly supporting the idea that similar signaling pathways exist in both species to regulate wood formation and secondary growth. Transcriptome profiling revealed that PtCLE genes generally were differentially expressed while some PtCLE genes exhibited tissue-specific expression patterns. Moreover, compared to their Arabidopsis counterparts, PtCLE genes showed either similar or distinct expression patterns, implying functional conservation in some cases and functional divergence in others. Conclusions Our study provides a genome-wide analysis of the CLE gene family in poplar, and highlights the potential roles of key PtCLE genes in the regulation of secondary growth and wood formation. The comparative analysis revealed that functional conservation may exist between PtCLEs and their AtCLE orthologues, which was further supported by transcriptomic analysis. Transcriptional profiling provided further insights into possible functional divergence, evidenced by differential expression patterns of various PtCLE genes.

• The phenylpropanoid pathway serves a central role in plant metabolism, providing numerous compounds involved in diverse physiological processes. Most carbon entering the pathway is incorporated into lignin. Although several phenylpropanoid pathway mutants show seedling growth arrest, the role for lignin in seedling growth and development is unexplored. • We use complementary pharmacological and genetic approaches to block CINNAMATE-4-HYDROXYLASE (C4H) functionality in Arabidopsis seedlings and a set of molecular and biochemical techniques to investigate the underlying phenotypes. • Blocking C4H resulted in reduced lateral rooting and increased adventitious rooting apically in the hypocotyl. These phenotypes coincided with an inhibition in AUX transport. The upstream accumulation in cis-cinnamic acid was found to be likely to cause polar AUX transport inhibition. Conversely, a downstream depletion in lignin perturbed phloem-mediated AUX transport. Restoring lignin deposition effectively reestablished phloem transport and, accordingly, AUX homeostasis. • Our results show that the accumulation of bioactive intermediates and depletion in lignin jointly cause the aberrant phenotypes upon blocking C4H, and demonstrate that proper deposition of lignin is essential for the establishment of AUX distribution in seedlings. Our data position the phenylpropanoid pathway and lignin in a new physiological framework, consolidating their importance in plant growth and development.

Notably, several small signaling peptide-encoding genes, including CLAVATA3/EMBRYO SURROUNDING REGION-RELATED (CLE), C-TERMINALLY ENCODED PEP TIDE (CEP), phytosulfokine (PSK), and GOLVEN/ROOT MERISTEM GROWTH FACTOR/CLE-LIKE (GLV/RGF/CLEL) are differentially activated at specific stages of DNSR, as revealed by single-cell transcriptomic analysis, suggesting their potential roles in the plant shoot regeneration process (Zhai and Xu, 2021;Wang et al., 2022). The wind1 tomato mutant exhibits severely compromised callus formation and shoot regeneration capacities, whereas WIND1 overexpression plants show enhanced regenerative capabilities. [...]the regenerative deficiency in wind1 mutants cannot be ameliorated by REF1 peptides. [...]the REF1-PORK1-WIND1 module constitutes a regulatory loop that finely tunes the plant regeneration potential (Figure 1C;Yang et al., 2024). Compared to WT plants, ralf33 mutant exhibits reduced regeneration rates when cuts are made at the transition zone. [...]exogenous application of synthetic RALF33 peptide enhances regeneration rates.

[...]the precise molecular mechanism underlying leaf senescence is still largely unexplored. Besides these well-established roles of phytohormones, small signaling peptides have emerged as indispensable regulators in various aspects of plant developmental and adaptive processes (Xie et al., 2022;Ji et al., 2025;Xiao et al., 2025;Zhang et al., 2025). Research has demonstrated that small signaling peptides from Arabidopsis thaliana such as CLAVATA3/EMBRYO-SURROUNDING REGION-RELATED (CLE) (Han et al., 2022;Zhang et al., 2022a,2022b), SERINE-RICH ENDOGENOUS PEPTIDE (SCOOPs) (Zhang et al., 2024a), PHYTOSULFOKINE (PSK) (Yamakawa et al., 1999;Matsubayashi et al., 2006;Komori et al., 2009), and INFLORESCENCE DEFICIENT IN ABSCISSION-LIKE6 (IDL6) (Guo et al., 2022) are integral in managing leaf senescence by modulating distinct signaling pathways, thereby providing novel mechanistic insights into the regulation of leaf senescence. 2 CLE peptides delay leaf senescence via ethylene and ROS pathways CLE proteins generally possess an N-terminal signal sequence that guides them into the secretory pathway, a central variable domain, and one or multiple conserved CLE motifs at the C-terminus, which are typically post-translationally modified to produce functional polypeptides (Fletcher, 2020;Xie et al., 2022). [...]a loss-of-function mutation in TPST precipitates premature leaf senescence, mirroring the effects observed with PSK peptide application (Yamakawa et al., 1999;Matsubayashi et al., 2006;Komori et al., 2009). The idl6 loss-of-function mutant exhibits a pronounced delay in leaf senescence, and this delayed senescence phenotype can be reversed by reintroducing the IDL6 gene into idl6 mutant plants.