Phytosulfokine Is Involved in Positive Regulation of Lotus japonicus Nodulation

The peptide GOLVEN10 alters root development and noduletaxis in Medicago truncatula

The conservation of GOLVEN (GLV)/ROOT MERISTEM GROWTH FACTOR (RGF) peptide encoding genes across plant genomes capable of forming roots or root-like structures underscores their potential significance in the terrestrial adaptation of plants. This study investigates the function and role of GOLVEN peptide-coding genes in Medicago truncatula. Five out of fifteen GLV/RGF genes were notably upregulated during nodule organogenesis and were differentially responsive to nitrogen deficiency and auxin treatment. Specifically, the expression of MtGLV9 and MtGLV10 at nodule initiation sites was contingent upon the NODULE INCEPTION transcription factor. Overexpression of these five nodule-induced GLV genes in hairy roots of M. truncatula and application of their synthetic peptide analogues led to a decrease in nodule count by 25-50%. Uniquely, the GOLVEN10 peptide altered the positioning of the first formed lateral root and nodule on the primary root axis, an observation we term 'noduletaxis'; this decreased the length of the lateral organ formation zone on roots. Histological section of roots treated with synthetic GOLVEN10 peptide revealed an increased cell number within the root cortical cell layers without a corresponding increase in cell length, leading to an elongation of the root likely introducing a spatiotemporal delay in organ formation. At the transcription level, the GOLVEN10 peptide suppressed expression of microtubule-related genes and exerted its effects by changing expression of a large subset of Auxin responsive genes. These findings advance our understanding of the molecular mechanisms by which GOLVEN peptides modulate root morphology, nodule ontogeny, and interactions with key transcriptional pathways.

Phytosulfokine peptides, their receptors, and functions

Phytosulfokines (PSKs) are a class of disulfated pentapeptides and are regarded as plant peptide hormones. PSK-α, -γ, -δ, and -ϵ are four bioactive PSKs that are reported to have roles in plant growth, development, and immunity. In this review, we summarize recent advances in PSK biosynthesis, signaling, and function. PSKs are encoded by precursor genes that are widespread in higher plants. PSKs maturation from these precursors requires a sulfation step, which is catalyzed by a tyrosylprotein sulfotransferase, as well as proteolytic cleavage by subtilisin serine proteases. PSK signaling is mediated by plasma membrane-localized receptors PSKRs that belong to the leucine-rich repeat receptor-like kinase family. Moreover, multiple biological functions can be attributed to PSKs, including promoting cell division and cell growth, regulating plant reproduction, inducing somatic embryogenesis, enhancing legume nodulation, and regulating plant resistance to biotic and abiotic stress. Finally, we propose several research directions in this field. This review provides important insights into PSKs that will facilitate biotechnological development and PSK application in agriculture.

Genome-Wide Identification and Comparative Analysis of RALF Gene Family in Legume and Non-Legume Species

Rapid alkalinization factor (RALF) are small secreted peptide hormones that can induce rapid alkalinization in a medium. They act as signaling molecules in plants, playing a critical role in plant development and growth, especially in plant immunity. Although the function of RALF peptides has been comprehensively analyzed, the evolutionary mechanism of RALFs in symbiosis has not been studied. In this study, 41, 24, 17 and 12 RALFs were identified in Arabidopsis, soybean, Lotus and Medicago, respectively. A comparative analysis including the molecular characteristics and conserved motifs suggested that the RALF pre-peptides in soybean represented a higher value of isoelectric point and more conservative motifs/residues composition than other species. All 94 RALFs were divided into two clades according to the phylogenetic analysis. Chromosome distribution and synteny analysis suggested that the expansion of the RALF gene family in Arabidopsis mainly depended on tandem duplication, while segment duplication played a dominant role in legume species. The expression levels of most RALFs in soybean were significantly affected by the treatment of rhizobia. Seven GmRALFs are potentially involved in the release of rhizobia in the cortex cells. Overall, our research provides novel insights into the understanding of the role of the RALF gene family in nodule symbiosis.

Phytosulfokine-δ: A Small Peptide, but a Big Player in Symbiosis Gene Regulation

Nitrogen availability is one of the critical determinants of agricultural yield. Biological nitrogen fixation, such as legume–rhizobia symbiotic association, might function as a solution to fix nitrogen. Using phytosulfokine (PSK)-α sequences as a query, Yu et al., 2022 performed a comprehensive genome-wide search of legume species to identify PSK-δ, a divergent pentapeptide differing in single amino acid. Furthermore, PSK-δ exhibited nodule-specific expression with lower expression in the root, substantiating the nodule-specific temporal expression and suggesting its role in nodule development and nitrogen fixation. Additionally, in planta functional characterization in Medicago truncatula using overexpression and Tnt1-insertion mutant analysis indicated the role of PSK-δ in symbiotic nodulation. Interestingly, a similar phenotype of MtPSKδ mutant (mtpskδ) with that of wild-type control led to the hypothesis of its functional redundancy with PSK-α in nodule organogenesis. Further investigation regarding its position in the Nod-factor signaling pathway revealed the downstream function of PSK-δ in association with MtENOD11 in regulating nodule formation.

A rulebook for peptide control of legume-microbe endosymbioses

Plants engage in mutually beneficial relationships with microbes, such as arbuscular mycorrhizal fungi or nitrogen-fixing rhizobia, for optimized nutrient acquisition. In return, the microbial symbionts receive photosynthetic carbon from the plant. Both symbioses are regulated by the plant nutrient status, indicating the existence of signaling pathways that allow the host to fine-tune its interactions with the beneficial microbes depending on its nutrient requirements. Peptide hormones coordinate a plethora of developmental and physiological processes and, recently, various peptide families have gained special attention as systemic and local regulators of plant-microbe interactions and nutrient homeostasis. In this review, we identify five 'rules' or guiding principles that govern peptide function during symbiotic plant-microbe interactions, and highlight possible points of integration with nutrient acquisition pathways.

A legume-specific novel type of phytosulfokine, PSK-δ, promotes nodulation by enhancing nodule organogenesis

Phytosulfokine-α (PSK-α), a tyrosine-sulfated pentapeptide with the sequence YSO3IYSO3TQ, is widely distributed across the plant kingdom and plays multiple roles in plant growth, development, and immune response. Here, we report a novel type of phytosulfokine, PSK-δ, and its precursor proteins (MtPSKδ, LjPSKδ, and GmPSKδ1), specifically from legume species. The sequence YSO3IYSO3TN of sulfated PSK-δ peptide is different from PSK-α at the last amino acid. Expression pattern analysis revealed PSK-δ-encoding precursor genes to be expressed primarily in legume root nodules. Specifically, in Medicago truncatula, MtPSKδ expression was detected in root cortical cells undergoing nodule organogenesis, in nodule primordia and young nodules, and in the apical region of mature nodules. Accumulation of sulfated PSK-δ peptide in M. truncatula nodules was detected by LC/MS. Application of synthetic PSK-δ peptide significantly increased nodule number in legumes. Similarly, overexpression of MtPSKδ in transgenic M. truncatula markedly promoted symbiotic nodulation. This increase in nodule number was attributed to enhanced nodule organogenesis induced by PSK-δ. Additional genetic evidence from the MtPSKδ mutant and RNA interference assays suggested that the PSK-δ and PSK-α peptides function redundantly in regulating nodule organogenesis. These results suggest that PSK-δ, a legume-specific novel type of phytosulfokine, promotes symbiotic nodulation by enhancing nodule organogenesis.

Dynamic transcriptome and co-expression analysis suggest the potential roles of small secreted peptides from Tartary buckwheat (Fagopyrum tataricum) in low nitrogen stress response

Small secreted peptides (SSPs) regulate nitrogen (N) response and signaling in plants. Although much progress has been made in understanding the functions of SSPs in N response, very little information is available regarding non-model plants. Tartary buckwheat (Fagopyrum tataricum), a dicotyledonous crop, has a good adaptability to low N (LN) stress; however, little is known regarding the associated mechanisms underlying this adaptation. In this study, 932 putative SSPs were genome-wide characterized in TB genome. Of these SSPs, 233 SSPs were annotated as established SSPs, such as CLE, RALF, PSK, and CEP peptides. The gene expression of 675 putative SSPs was detected in five tissues and 258 SSPs were tissue-specific expressed genes. To analyze the responses of TB SSPs to LN, the dynamic expression analysis of TB roots under LN stress was conducted by RNA-seq. The expression of 378 putative TB SSP genes was detected with diverse expression patterns under LN stress, and some important LN-responsive SSPs were identified. Co-expression analysis suggested SSPs may regulate the adaptability of TB under LN conditions by modulating the expression of the genes involved in N transport and assimilation and IAA signaling. Furthermore, 53 LN stress-responsive RLKs encoding genes were identified and they were predicted as potential SSP receptors. This study expands the repertoire of SSPs in plants and provides useful information for further investigation of the functions of Tartary buckwheat SSPs in LN stress responses.

The Gossypium stocksii genome as a novel resource for cotton improvement

Cotton is an important textile crop whose gains in production over the last century have been challenged by various diseases. Because many modern cultivars are susceptible to several pests and pathogens, breeding efforts have included attempts to introgress wild, naturally resistant germplasm into elite lines. Gossypium stocksii is a wild cotton species native to Africa, which is part of a clade of vastly understudied species. Most of what is known about this species comes from pest resistance surveys and/or breeding efforts, which suggests that G. stocksii could be a valuable reservoir of natural pest resistance. Here, we present a high-quality de novo genome sequence for G. stocksii. We compare the G. stocksii genome with resequencing data from a closely related, understudied species (Gossypium somalense) to generate insight into the relatedness of these cotton species. Finally, we discuss the utility of the G. stocksii genome for understanding pest resistance in cotton, particularly resistance to cotton leaf curl virus.

Interactions of gene expression, alternative splicing, and DNA methylation in determining nodule identity

Soybean nodulation is a highly controlled process that involves complex gene regulation at both transcriptional and post-transcriptional levels. In the present study, we profiled gene expression changes, alternative splicing events, and DNA methylation patterns during nodule formation, development, and senescence. The transcriptome data uncovered key transcription patterns of nodule development that included 9669 core genes and 7302 stage-specific genes. Alternative splicing analysis uncovered a total of 2323 genes that undergo alternative splicing events in at least one nodule developmental stage, with activation of exon skipping and repression of intron retention being the most common splicing events in nodules compared to roots. Approximately 40% of the differentially spliced genes were also differentially expressed at the same nodule developmental stage, implying a substantial association between gene expression and alternative splicing. Genome-wide-DNA methylation analysis revealed dynamic changes in nodule methylomes that were specific to each nodule stage, occurred in a sequence-specific manner, and impacted the expression of 1864 genes. An attractive hypothesis raised by our data is that increased DNA methylation may contribute to the efficiency of alternative splicing. Together, our results provide intriguing insights into the associations between gene expression, alternative splicing, and DNA methylation that may shape transcriptome complexity and proteome specificity in developing soybean nodules.

Time-course RNA-seq analysis provides an improved understanding of gene regulation during the formation of nodule-like structures in rice

Using a time-course RNA-seq analysis we identified transcriptomic changes during formation of nodule-like structures (NLS) in rice and compared rice RNA-seq dataset with a nodule transcriptome dataset in Medicago truncatula. Plant hormones can induce the formation of nodule-like structures (NLS) in plant roots even in the absence of bacteria. These structures can be induced in roots of both legumes and non-legumes. Moreover, nitrogen-fixing bacteria can recognize and colonize these root structures. Therefore, identifying the genetic switches controlling the NLS organogenesis program in crops, especially cereals, can have important agricultural implications. Our recent study evaluated the transcriptomic response occurring in rice roots during NLS formation, 7 days post-treatment (dpt) with auxin, 2,4-D. In this current study, we investigated the regulation of gene expression occurring in rice roots at different stages of NLS formation: early (1-dpt) and late (14-dpt). At 1-dpt and 14-dpt, we identified 1662 and 1986 differentially expressed genes (DEGs), respectively. Gene ontology enrichment analysis revealed that the dataset was enriched with genes involved in auxin response and signaling; and in anatomical structure development and morphogenesis. Next, we compared the gene expression profiles across the three time points (1-, 7-, and 14-dpt) and identified genes that were uniquely or commonly differentially expressed at all three time points. We compared our rice RNA-seq dataset with a nodule transcriptome dataset in Medicago truncatula. This analysis revealed there is some amount of overlap between the molecular mechanisms governing nodulation and NLS formation. We also identified that some key nodulation genes were not expressed in rice roots during NLS formation. We validated the expression pattern of several genes via reverse transcriptase polymerase chain reaction (RT-PCR). The DEGs identified in this dataset may serve as a useful resource for future studies to characterize the genetic pathways controlling NLS formation in cereals.

Identification and Functional Investigation of Genome-Encoded, Small, Secreted Peptides in Plants

Hundreds to thousands of small secreted peptides (SSPs) are encoded in plant genomes but have been overlooked, and most remain unannotated and unstudied. Despite their low profile, they have been found to confer dramatic effects on growth and development of plants. With the growing appreciation of their significance, the development of appropriate methods to identify and functionally assess the myriad SSPs encoded in plant genomes has become critical. Here, we provide protocols for the computational and physiological analysis of SSPs in plant genomes. We first describe our methodology successfully used for genome-wide identification and annotation of SSP-coding genes in the model legume Medicago truncatula, which can be readily adapted for other plant species. We then provide protocols for the functional analysis of SSPs using various synthetic peptide screens. Considerations for the design and handling of peptides are included. © 2019 by John Wiley & Sons, Inc.

The peptide-encoding MtRGF3 gene negatively regulates nodulation of Medicago truncatula

Leguminous root nodules specifically induced by rhizobium species fix nitrogen gas to gain nitrogen sources, which is important in sustainable agriculture and ecological balance. Several peptide signals are reported to be involved in regulation of legume nodule number and development. There are fifteen genes coding Root Meristem Growth Factor (RGF) peptide in Medicago truncatula, herein we find the expression of MtRGF3 is significantly induced by Sinorhizobium meliloti with production of Nod factors. The gene promoter is active in nodule primordia, young nodules and the meristem region of mature nodules. Knock-down (RNAi) roots of the gene (MtRGF3-RNAi) formed more root nodules than the empty vector control, and the nodule number decreased in MtRGF3-overexpressing (MtRGF3-OX) roots. Exogenous addition of the synthesized peptide significantly promoted primary root growth and inhibited lateral root emergence, in addition, the peptide application reduced the number of infection threads, nodule primordia and root nodules of M. truncatula. We also found that tyrosine sulfation determines the biological activity of MtRGF3 functioning in nodulation process, and MtRGF3 peptide negatively regulates nodulation in a dosage manner. These results demonstrate that the MtRGF3 peptide is a novel regulator during nodulation of Medicago trucatula.

Overexpression of PSK-γ in Arabidopsis promotes growth without influencing pattern-triggered immunity

Phytosulfokine-α (PSK-α) is a disulfated pentapeptide with the sequence YIYTQ. As a new peptide hormone, PSK-α promotes plant growth and development but represses pattern-triggered immunity (PTI) against bacterial pathogens. Our recent study identified a novel phytosulfokine, PSK-γ, from soybean. The sequence of PSK-γ is YVYTQ in which the tyrosines are sulfated. Expression of PSK-γ significantly increased seed size and weight in transgenic plants by inducing embryo cell expansion. In this study, we further found that the constitutive expression of PSK-γ in Arabidopsis promotes the growth of vegetative organs as well as seeds. Moreover, overexpressed PSK-γ does not influence plant PTI against bacterial pathogens. These findings demonstrate a potential use of PSK-γ in genetic improvement of crop growth and yield by molecular breeding.

Expression of a novel PSK-encoding gene from soybean improves seed growth and yield in transgenic plants

MAIN CONCLUSION

Expression of GmPSKγ1 , a novel PSK-encoding gene from soybean, increases seed size and yield in transgenic plants by promoting cell expansion. Phytosulfokine-α (PSK-α), a sulfated pentapeptide hormone with the sequence YIYTQ, plays important roles in many aspects of plant growth and development. In this study, we identified a pair of putative precursor genes in soybean, GmPSKγ1 and -2, encoding a PSK-like peptide: PSK-γ. Similar to PSK-α in amino acid composition, the sequence of PSK-γ is YVYTQ, and the tyrosines undergo sulfonylation. Treatment of Arabidopsis seedlings with synthetic sulfated PSK-γ significantly enhanced root elongation, indicating that PSK-γ might be a functional analog of PSK-α. Expression pattern analysis revealed that the two GmPSKγ genes, especially GmPSKγ1, are primarily expressed in developing soybean seeds. Heterologous expression of GmPSKγ1 under the control of a seed-specific promoter markedly increased seed size and weight in Arabidopsis, and this promoting effect of PSK-γ on seed growth was further confirmed in transgenic tobacco constitutively expressing GmPSKγ1. Cytological analysis of transgenic Arabidopsis seeds revealed that PSK-γ promotes seed growth by inducing embryo cell expansion. In addition, expression analysis of downstream candidate genes suggested that PSK-γ signaling might regulate cell wall loosening to promote cell expansion in Arabidopsis seeds. Overall, our results shed light on the mechanism by which PSK-γ promotes seed growth, paving the way for the use of this new peptide for biotechnological improvement of crop seed/grain size and yield.

Impact of Plant Peptides on Symbiotic Nodule Development and Functioning

Ribosomally synthesized peptides have wide ranges of functions in plants being, for example, signal molecules, transporters, alkaloids, or antimicrobial agents. Legumes are an unprecedented rich source of peptides, which are used to control the symbiosis of these plants with the nitrogen-fixing Rhizobium bacteria. Here, we discuss the function and the evolution of these peptides playing an important role in the formation or functioning of the symbiotic organs, the root nodules. We distinguish peptides that can be either cell-autonomous or secreted short-range or long-range signals, carrying messages in or between plant cells or that can act as effectors interacting with the symbiotic bacteria. Peptides are further classified according to the stage of the symbiotic process where they act. Several peptide classes, including RALF, DLV, ENOD40, and others, control Rhizobium infection and the initiation of cell divisions and the formation of nodule primordia. CLE and CEP peptides are implicated in systemic and local control of nodule initiation during autoregulation of nodulation and in response to the nutritional demands of the plant. Still other peptides act at later stages of the symbiosis. The PSK peptide is thought to be involved in the suppression of immunity in nodules and the nodule-specific cysteine-rich, GRP, and SNARP (LEED..PEED) peptide families are essential in the functioning of the nitrogen fixing root nodules. The NCRs and possibly also the GRP and SNARPs are targeted to the endosymbionts and play essential roles in the terminal differentiation of these bacteria.

Comparative transcriptome analysis provides key insights into gene expression pattern during the formation of nodule-like structures in Brachypodium

Auxins can induce the formation of nodule-like structures (NLS) in plant roots even in the absence of rhizobia and nitrogen-fixing bacteria can colonize these structures. Interestingly, NLS can be induced in roots of both legumes and non-legumes. However, our understanding of NLS formation in non-legumes at a molecular level is limited. This study aims to investigate NLS formation at a developmental and molecular level in Brachypodium distachyon. We treated Brachypodium roots with the synthetic auxin, 2,4-D, to induce NLS at a high frequency (> 80%) under controlled conditions. A broad base and a diffuse meristem characterized these structures. Next, we performed a comprehensive RNA-sequencing experiment to identify differentially expressed genes (DEGs) in Brachypodium roots during NLS formation. We identified 618 DEGs; several of which are promising candidates for control of NLS based on their biological and molecular functions. We validated the expression pattern of several genes via RT-PCR. Next, we compared the expression profile of Brachypodium roots with rice roots during NLS formation. We identified 76 single-copy ortholog pairs in rice and Brachypodium that are both differentially expressed during this process. Some of these genes are involved in auxin signaling, root development, and legume-rhizobia symbiosis. We established an experimental system to study NLS formation in Brachypodium at a developmental and genetic level, and used RNA-sequencing analysis to understand the molecular mechanisms controlling this root organogenesis program. Furthermore, our comparative transcriptome analysis in Brachypodium and rice identified a key set of genes for further investigating this genetic pathway in grasses.

Moonlighting Proteins and Their Role in the Control of Signaling Microenvironments, as Exemplified by cGMP and Phytosulfokine Receptor 1 (PSKR1)

Signal generating and processing complexes and changes in concentrations of messenger molecules such as calcium ions and cyclic nucleotides develop gradients that have critical roles in relaying messages within cells. Cytoplasmic contents are densely packed, and in plant cells this is compounded by the restricted cytoplasmic space. To function in such crowded spaces, scaffold proteins have evolved to keep key enzymes in the correct place to ensure ordered spatial and temporal and stimulus-specific message generation. Hence, throughout the cytoplasm there are gradients of messenger molecules that influence signaling processes. However, it is only recently becoming apparent that specific complexes involving receptor molecules can generate multiple signal gradients and enriched microenvironments around the cytoplasmic domains of the receptor that regulate downstream signaling. Such gradients or signal circuits can involve moonlighting proteins, so called because they can enable fine-tune signal cascades via cryptic additional functions that are just being defined. This perspective focuses on how enigmatic activity of moonlighting proteins potentially contributes to regional intracellular microenvironments. For instance, the proteins associated with moonlighting proteins that generate cyclic nucleotides may be regulated by cyclic nucleotide binding directly or indirectly. In this perspective, we discuss how generation of cyclic nucleotide-enriched microenvironments can promote and regulate signaling events. As an example, we use the phytosulfokine receptor (PSKR1), discuss the function of its domains and their mutual interactions and argue that this complex architecture and function enhances tuning of signals in microenvironments.

Small peptide signaling pathways modulating macronutrient utilization in plants

Root system architecture (RSA) and physiological functions define macronutrient uptake efficiency. Small signaling peptides (SSPs), that act in manners similar to hormones, and their cognate receptors transmit signals both locally and systemically. Several SSPs controlling morphological and physiological traits of roots have been identified to be associated with macronutrient uptake. Recent development in plant genome research has provided an avenue toward systems-based identification and prediction of additional SSPs. This review highlights recent studies on SSP pathways important for optimization of macronutrient uptake and provides new insights into the diversity of SSPs regulated in response to changes in macronutrient availabilities.

The TOR Pathway Is Involved in Adventitious Root Formation in Arabidopsis and Potato

In the agriculture industry, adventitious root formation is a core issue of plants asexual propagation. However, the underlying molecular mechanism of adventitious root formation is far beyond understanding. In present study we found that target of rapamycin (TOR) signaling plays a key role in adventitious root formation in potato and Arabidopsis. The core components of TOR complex including TOR, RAPTOR, and LST8 are highly conserved in potato, but the seedlings of potato are insensitive to rapamycin, implying FK506 Binding Protein 12 KD (FKBP12) lost the function to bridge the interaction of rapamycin and TOR in potato. To dissect TOR signaling in potato, the rapamycin hypersensitive potato plants (BP12-OE) were engineered by introducing yeast FKBP12 (ScFKBP12) into potato. We found that rapamycin can significantly attenuate the capability of adventitious root formation in BP12-OE potatoes. KU63794 (KU, an active-site TOR inhibitor) combined with rapamycin can more significantly suppress adventitious root formation of BP12-OE potato than the single treatments, such as KU63794 or rapamycin, indicating its synergistic inhibitory effects on potato adventitious root formation. Furthermore, RNA-seq data showed that many genes associated with auxin signaling pathway were altered when BP12-OE potato seedlings were treated with rapamycin + KU, suggesting that TOR may play a major role in adventitious root formation via auxin signaling. The auxin receptor mutant tir1 was sensitive to TOR inhibitors and the double and quadruple mutants including tir1afb2, tir1afb3, and tir1afb1afb2afb3 displayed more sensitive to asTORis than single mutant tir1. Consistently, overexpression of AtTIR1 in Arabidopsis and potato can partially overcome the inhibitory effect of asTORis and promote adventitious root formation under asTORis treatments. These observations suggest that TOR signaling regulates adventitious root formation by mediating auxin signaling in Arabidopsis and potato.

Overexpression of phytosulfokine-α induces male sterility and cell growth by regulating cell wall development in Arabidopsis

Over-production of functional PSK-α in Arabidopsis caused increases in both plant cell growth and biomass and induced male sterility by regulating cell wall development. Phytosulfokine-α (PSK-α) is a novel disulfated pentapeptide hormone that is involved in promoting plant cell growth. Although a role for PSK-α in stimulating protoplast expansion has been suggested, how PSK-α regulates cell growth in planta remains poorly understood. In this study, we found that overexpression of the normal PSK-α precursor gene AtPSK4, which resulted in high levels of PSK-α, caused longer roots and larger leaves with enlarged cells. As expected, these changes were not observed in transgenic plants overexpressing mutated AtPSK4, which generated unsulfated PSK-α. These findings confirmed the role of PSK-α in promoting plant cell growth. Furthermore, we found that overexpressing AtPSK4, but not mutated AtPSK4, induced a phenotype of male sterility that resulted from the failure of fibrous cell wall development in the endothecium. In addition, overexpressing AtPSK4 enhanced expression of a number of genes encoding expansins, which are involved in cell wall loosening. Accordingly, in addition to its role in cell growth, we propose a novel function for PSK-α signaling in the modulation of plant male sterility via regulation of cell wall development.