ROP Interactive Partners are Involved in the Control of Cell Division Patterns in Arabidopsis Leaves

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.

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.

Microtubule-associated phase separation of MIDD1 tunes cell wall spacing in xylem vessels in Arabidopsis thaliana

Properly patterned cell walls specify cellular functions in plants. Differentiating protoxylem and metaxylem vessel cells exhibit thick secondary cell walls in striped and pitted patterns, respectively. Cortical microtubules are arranged in distinct patterns to direct cell wall deposition. The scaffold protein MIDD1 promotes microtubule depletion by interacting with ROP GTPases and KINESIN-13A in metaxylem vessels. Here we show that the phase separation of MIDD1 fine-tunes cell wall spacing in protoxylem vessels in Arabidopsis thaliana. Compared with wild-type, midd1 mutants exhibited narrower gaps and smaller pits in the secondary cell walls of protoxylem and metaxylem vessel cells, respectively. Live imaging of ectopically induced protoxylem vessels revealed that MIDD1 forms condensations along the depolymerizing microtubules, which in turn caused massive catastrophe of microtubules. The MIDD1 condensates exhibited rapid turnover and were susceptible to 1,6-hexanediol. Loss of ROP abolished the condensation of MIDD1 and resulted in narrow cell wall gaps in protoxylem vessels. These results suggest that the microtubule-associated phase separation of MIDD1 facilitates microtubule arrangement to regulate the size of gaps in secondary cell walls. This study reveals a new biological role of phase separation in the fine-tuning of cell wall patterning.

Update: on selected ROP cell polarity mechanisms in plant cell morphogenesis

The unequal (asymmetric) distribution of cell structures and proteins within a cell is designated as cell polarity. Cell polarity is a crucial prerequisite for morphogenetic processes such as oriented cell division and directed cell expansion. Rho-related GTPase from plants (ROPs) are required for cellular morphogenesis through the reorganization of the cytoskeleton and vesicle transport in various tissues. Here, I review recent advances in ROP-dependent tip growth, vesicle transport, and tip architecture. I report on the regulatory mechanisms of ROP upstream regulators found in different cell types. It appears that these regulators assemble in nanodomains with specific lipid compositions and recruit ROPs for activation in a stimulus-dependent manner. Current models link mechanosensing/mechanotransduction to ROP polarity signaling involved in feedback mechanisms via the cytoskeleton. Finally, I discuss ROP signaling components that are upregulated by tissue-specific transcription factors and exhibit specific localization patterns during cell division, clearly suggesting ROP signaling in division plane alignment.

Redundant mechanisms in division plane positioning

Redundancies in plant cell division contribute to the maintenance of proper division plane orientation. Here we highlight three types of redundancy: 1) Temporal redundancy, or correction of earlier defects that results in proper final positioning, 2) Genetic redundancy, or functional compensation by homologous genes, and 3) Synthetic redundancy, or redundancy within or between pathways that contribute to proper division plane orientation. Understanding the types of redundant mechanisms involved provides insight into current models of division plane orientation and opens up new avenues for exploration.