RHO OF PLANT proteins are essential for pollen germination in Arabidopsis

Genome-wide identification and analysis of Rop GTPase family members reveal their potential functions in biotic stress in potato (Solanum tuberosum L.)

BACKGROUND

Rop (RHO of plants) proteins are the plant-specific subfamily of RHO small GTP-binding proteins and act as a molecular switch to converge on a wide range of upstream signals and elicit downstream signaling cascades involving in modulating developmental processes and managing environmental stress. Although the function of Rops has been well studied in many plant species, the research conducting on Rops in potato is limited.

RESULTS

In this work, a total of 11 Rop members were identified in the potato (Solanum tuberosum) genome. A comprehensive analysis encompassing their phylogenetic relationships, chromosomal locations, collinearity, conserved motifs, gene structures, cis-regulatory elements, tissue-specific expression profiles, and responses to biotic stress were undertaken. Phylogenetic and collinearity analyses suggested that 11 StRops were categorized into four groups, and five StRop genes (StRop6, StRop7, StRop8, StRop9 and StRop10) were incorporated in segmental duplication events. Synteny analysis indicated that five and eight StRop genes were orthologous to Arabidopsis (Arabidopsis thaliana) and tomato (Solanum lycopersicum), respectively. Tissue-specific expression analysis confirmed that StRops were widely expressed in various potato tissues, with variety-specific expression, implicating their multiple roles in growth and development in potato. The cis-regulatory elements related to stress response and hormone response were found in the promoters of StRop genes. Most StRops, including StRop2, StRop3, StRop8, StRop9, StRop10 and StRop11, were shown to be significantly differentially expressed in three different cultivars after infection with various pathogens (Phytophthora infestans, Fusarium oxysporum and Verticillium dahliae). Knock-down each of StRop3, StRop7 and StRop8 by virus induced gene silencing (VIGS) resulted in increased susceptibility of potato to pathogens P. infestans and V. dahliae, and transient silencing of StRop6 led to enhanced potato root colonization by V. dahliae, indicating their distinct roles in response to different pathogen challenges.

CONCLUSIONS

The results unveil the structural characteristics of StRop genes, and provide the basic knowledge for further elucidating the gene functions of individual members in response to biotic stress.

Polarized subcellular activation of Rho proteins by specific ROPGEFs drives pollen germination in Arabidopsis thaliana

During plant fertilization, excess male gametes compete for a limited number of female gametes. The dormant male gametophyte, encapsulated in the pollen grain, consists of two sperm cells enclosed in a vegetative cell. After reaching the stigma of a compatible flower, quick and efficient germination of the vegetative cell to a tip-growing pollen tube is crucial to ensure fertilization success. Rho of Plants (ROP) signaling and their activating ROP Guanine Nucleotide Exchange Factors (ROPGEFs) are essential for initiating polar growth processes in multiple cell types. However, which ROPGEFs activate pollen germination is unknown. We investigated the role of ROPGEFs in initiating pollen germination and the required cell polarity establishment. Of the five pollen-expressed ROPGEFs, we found that GEF8, GEF9, and GEF12 are required for pollen germination and male fertilization success, as gef8;gef9;gef12 triple mutants showed almost complete loss of pollen germination in vitro and had a reduced allele transmission rate. Live-cell imaging and spatiotemporal analysis of subcellular protein distribution showed that GEF8, GEF9, and GEF11, but not GEF12, displayed transient polar protein accumulations at the future site of pollen germination minutes before pollen germination, demonstrating specific roles for GEF8 and GEF9 during the initiation of pollen germination. Furthermore, this novel GEF accumulation appears in a biphasic temporal manner and can shift its location laterally. We showed that the C-terminal domain of GEF8 and GEF9 confers their protein accumulation and demonstrated that GEFs locally activate ROPs and alter Ca2+ levels, which is required for pollen tube germination. We demonstrated that not all GEFs act redundantly during pollen germination, and we described for the first time a polar domain with spatiotemporal flexibility, which is crucial for the de novo establishment of a polar growth domain within a cell and, thus, for pollen function and fertilization success.

Three Rho of Plant (ROP)-GTPase regulatory proteins control ROP-mediated alkaloid biosynthesis in Catharanthus roseus

Rho of plant (ROP)-GTPase regulatory proteins (RGRPs) have been shown to control plant morphogenesis, development, and immunity; however, their role in specialized metabolism remains unknown. Here, we demonstrate that specific RGRPs control monoterpene indole alkaloid (MIA) biosynthesis by interacting with distinct ROPs in Madagascar periwinkle (Catharanthus roseus). Among the 5 guanine nucleotide exchange factors (GEFs), 4 GTPase-activating proteins (GAPs), and 2 GDP dissociation inhibitors (GDIs) identified in the C. roseus genome, only CrGEF1, CrGAP1, and CrGDI2 specifically interacted with CrROP3 and CrROP5. These RGRPs displayed distinct cytosolic and/or membrane localization patterns, with their transcripts predominantly expressed in aerial tissues. Functional studies revealed that CrGEF1 acts as a positive regulator of MIA biosynthesis, as silencing its gene led to a reduction in MIA production, while overexpression enhanced MIA levels. Conversely, CrGAP1 and CrGDI2 function as negative regulators, with silencing resulting in increased MIA production and overexpression causing reduced MIA levels. Notably, terminal truncated forms of these RGRPs showed interaction with CrROP3 or CrROP5 but failed to influence MIA biosynthesis, underscoring the importance of these domains in their regulatory functions. Overall, our findings uncover a mechanism by which distinct RGRPs coordinate with specific ROPs to regulate transcription factors and fine-tune MIA biosynthesis in C. roseus.

ROP GTPases with a geranylgeranylation motif modulate alkaloid biosynthesis in Catharanthus roseus

Rho of Plant (ROP) GTPases function as molecular switches that control signaling processes essential for growth, development, and defense. However, their role in specialized metabolism is poorly understood. Previously, we demonstrated that inhibition of protein geranylgeranyl transferase (PGGT-I) negatively impacts the biosynthesis of monoterpene indole alkaloids (MIA) in Madagascar periwinkle (Catharanthus roseus), indicating the involvement of prenylated proteins in signaling. Here, we show through biochemical, molecular, and in planta approaches that specific geranylgeranylated ROPs modulate C. roseus MIA biosynthesis. Among the six C. roseus ROP GTPases (CrROPs), only CrROP3 and CrROP5, having a C-terminal CSIL motif, were specifically prenylated by PGGT-I. Additionally, their transcripts showed higher expression in most parts than other CrROPs. Protein-protein interaction studies revealed that CrROP3 and CrROP5, but not ΔCrROP3, ΔCrROP5, and CrROP2 lacking the CSIL motif, interacted with CrPGGT-I. Further, CrROP3 and CrROP5 exhibited nuclear localization, whereas CrROP2 was localized to the plasma membrane. In planta functional studies revealed that silencing of CrROP3 and CrROP5 negatively affected MIA biosynthesis, while their overexpression upregulated MIA formation. In contrast, silencing and overexpression of CrROP2 had no effect on MIA biosynthesis. Moreover, overexpression of ΔCrROP3 and ΔCrROP5 mutants devoid of sequence coding for the CSIL motif failed to enhance MIA biosynthesis. These results implicate that CrROP3 and CrROP5 have a positive regulatory role on MIA biosynthesis and thus shed light on how geranylgeranylated ROP GTPases mediate the modulation of specialized metabolism in C. roseus.

RHO OF PLANTS signalling and the activating ROP GUANINE NUCLEOTIDE EXCHANGE FACTORS: specificity in cellular signal transduction in plants

Every cell constantly receives signals from its neighbours or the environment. In plants, most signals are perceived by RECEPTOR-LIKE KINASEs (RLKs) and then transmitted into the cell. The molecular switches RHO OF PLANTS (ROP) are critical proteins for polar signal transduction and regulate multiple cell polarity processes downstream of RLKs. Many ROP-regulating proteins and scaffold proteins of the ROP complex are known. However, the spatiotemporal ROP signalling complex composition is not yet understood. Moreover, how specificity is achieved in different ROP signalling pathways within one cell still needs to be determined. This review gives an overview of recent advances in ROP signalling and how specificity by downstream scaffold proteins can be achieved. The composition of the ROP signalling complexes is discussed, focusing on the possibility of the simultaneous presence of ROP activators and inactivators within the same complex to balance ROP activity. Furthermore, this review highlights the function of plant-specific ROP GUANINE NUCLEOTIDE EXCHANGE FACTORS polarizing ROP signalling and defining the specificity of the initiated ROP signalling pathway.

Calcium imaging: a technique to monitor calcium dynamics in biological systems

Calcium ion (Ca2+) is a multifaceted signaling molecule that acts as an important second messenger. During the course of evolution, plants and animals have developed Ca2+ signaling in order to respond against diverse stimuli, to regulate a large number of physiological and developmental pathways. Our understanding of Ca2+ signaling and its components in physiological phenomena ranging from lower to higher organisms, and from single cell to multiple tissues has grown exponentially. The generation of Ca2+ transients or signatures for various stress factor is a well-known mechanism adopted in plant and animal systems. However, the decoding of such remarkable signatures is an uphill task and is always an interesting goal for the scientific community. In the past few decades, studies on the concentration and dynamics of intracellular Ca2+ are significantly increasing and have become a trend in modern biology. The advancement in approaches from Ca2+ binding dyes to in vivo Ca2+ imaging through the use of Ca2+ biosensors to achieve spatio-temporal resolution in micro and milliseconds range, provide us phenomenal opportunities to study live cell Ca2+ imaging or dynamics. Here, we describe the usage, improvement and advancement of Ca2+ based dyes, genetically encoded probes and sensors to achieve extraordinary Ca2+ imaging in plants and animals.

Graphical abstract