Li-Qun Chen

The eukaryotic radiation sensitive 51 (RAD51) gene family comprises 7 ancient paralogs that have been remarkably conserved across both plants and animals. Among these paralogs, X-ray repair cross-complementing 2 (XRCC2) plays a pivotal role in mammalian embryonic development. However, in Arabidopsis (Arabidopsis thaliana), disruption of its function leads to seemingly normal meiotic processes, and intriguingly, even appears to increase meiotic recombination. Given these disparate observations, the precise role of XRCC2 during meiosis remains largely elusive. Here, we identified the XRCC2 gene in maize (Zea mays), a member of the RAD51 paralog family and the homolog of Arabidopsis XRCC2. Plants with mutated XRCC2 exhibited partial male and female sterility. Cytological investigations of xrcc2 mutants revealed that the meiocytes form univalent chromosomes and chromosome bridges and undergo chromosome fragmentation, while homologous pairing and synapsis occur normally. Notably, the xrcc2 mutant showed a significant reduction in the number of meiotic chiasmata and RAD51 foci. Furthermore, XRCC2 was essential for maintaining genome stability and affected the transcription of RAD51 paralogs. Within yeast and tobacco systems, we detected interactions between XRCC2 and disrupted meiotic cDNA1, radiation sensitive 51C, and radiation sensitive 51D. Collectively, XRCC2 demonstrates remarkable functional diversity across species and plays an important role in maize crossover formation.

Stomata are epidermal pores that control gas exchange between plants and the atmosphere. In Arabidopsis, the ERECTA family (ERECTAf) receptors, including ERECTA, ERECTA-LIKE 1 (ERL1) and ERL2, redundantly play pivotal roles in enforcing the ‘one-cell-spacing’ rule. Accumulating evidence has demonstrated that the functional specificities of receptors are likely associated with their differential subcellular dynamics. The endoplasmic reticulum (ER)-resident chaperone complex SDF2-ERdj3B-BiP functions in many aspects of plant development. We employed pharmacological treatments combined with cell biological and biochemical approaches to demonstrate that the abundance of ERECTA was reduced in the erdj3b-1 mutant, but the localization and dynamics of ERECTA were not noticeably affected. By contrast, the erdj3b mutation caused the retention of ERL1/ERL2 in the ER. Furthermore, we found that the function of SDF2-ERdj3B-BiP is implicated with the distinct roles of ERECTAf receptors. Our findings establish that the ERECTAf receptor-mediated signaling in stomatal development is ensured by the activities of the ER quality control system, which preferentially maintains the protein abundance of ERECTA and proper subcellular dynamics of ERL1/ERL2, prior to the receptors reaching their destination – the plasma membrane – to execute their functions.

The endoplasmic reticulum-localized DnaJ family 3B (ERdj3B), is a component of the stromal cell-derived factor 2 (SDF2)–ERdj3B–binding immunoglobulin protein (BiP) chaperone complex, which functions in protein folding, translocation, and quality control. We found that ERdj3B mutations affected integument development in the Ler ecotype but not in the Col-0 ecotype of Arabidopsis (Arabidopsis thaliana). Map-based cloning identified the ERECTA (ER) gene as a natural modifier of ERdj3B. The double mutation of ERdj3B and ER caused a major defect in the inner integument under heat stress. Additional mutation of the ER paralog ERECTA-LIKE 1 (ERL1) or ERL2 to the erdj3b er double mutant exacerbated the defective integument phenotype. The double mutation of ER and SDF2, the other component of the SDF2–ERdj3B–BiP complex, resulted in similar defects in the inner integument. Furthermore, both the protein abundance and plasma membrane partitioning of ER, ERL1, and ERL2 were markedly reduced in erdj3b plants, indicating that the SDF2–ERdj3B–BiP chaperone complex might control the translocation of ERECTA-family proteins from the endoplasmic reticulum to the plasma membrane. Our results suggest that the SDF2–ERdj3B–BiP complex functions in ovule development and the heat stress response in coordination with ERECTA-family receptor kinases.

Summary: Organ size is an important agronomic trait. Small peptides function in various stages of plant growth, but their regulatory mechanisms in organ growth remain poorly understood. Here, we characterize a novel small peptide, AtZSP1, which positively regulates organ size in Arabidopsis. Loss‐of‐function mutant atzsp1‐1 exhibited small organs, whereas AtZSP1 overexpression plants (p35S:AtZSP1#1) produced larger organs. Differentially expressed genes in the shoots of atzsp1‐1 and p35S:AtZSP1#1 were enriched in the cytokinin pathway. Further analysis on shoots of atzsp1‐1 showed that endogenous cytokinin levels were significantly reduced, consistent with reduced expression of the cytokinin response genes ARR5/6/7 and a decrease in pARR5:GUS activity. By contrast, cytokinin levels were elevated in p35S:AtZSP1#1. These results indicate that AtZSP1 affects shoot size via changes in cytokinin levels. AtZSP1 is ubiquitously expressed and encodes a 57‐amino acid endomembrane‐associated protein that is highly conserved among plant species. AtZSP1 interacts with ROCK1 at the endomembrane. Genetic analysis confirmed that the small organs and low cytokinin levels in atzsp1‐1 shoots are partially suppressed by the rock1‐4 mutation, suggesting that AtZSP1 may function in a common pathway with ROCK1 to antagonistically regulate organ growth. Our study identified an unknown small peptide, AtZSP1, and defined its function in regulating organ size in Arabidopsis.

Restriction of pollen germination before the pollen grain is pollinated to stigma is essential for successful fertilization in angiosperms. However, the mechanisms underlying the process remain poorly understood. Here, we report functional characterization of the MAPKKK kinases, MAP3Kε1 and MAP3Kε2, involve in control of pollen germination in Arabidopsis. The two genes were expressed in different tissues with higher expression levels in the tricellular pollen grains. The map3kε1 map3kε2 double mutation caused abnormal callose accumulation, increasing level of JA and precocious pollen germination, resulting in significantly reduced seed set. Furthermore, the map3kε1 map3kε2 double mutations obviously upregulated the expression levels of genes in JA biosynthesis and signaling. The MAP3Kε1/2 interacted with MOB1A/1B which shared homology with the core components of Hippo singling pathway in yeast. The Arabidopsis mob1a mob1b mutant also exhibited a similar phenotype of precocious pollen germination to that in map3kε1 map3kε2 mutants. Taken together, these results suggested that the MAP3Kεs interacted with MOB1s and played important role in restriction of the precocious pollen germination, possibly through crosstalk with JA signaling and influencing callose accumulation in Arabidopsis.

Background: Root hair, a special type of tubular-shaped cell, outgrows from root epidermal cell and plays important roles in the acquisition of nutrients and water, as well as interactions with biotic and abiotic stress. Although many genes involved in root hair development have been identified, genetic basis of natural variation in root hair growth has never been explored. Results: Here, we utilized a maize association panel including 281 inbred lines with tropical, subtropical, and temperate origins to decipher the phenotypic diversity and genetic basis of root hair length. We demonstrated significant associations of root hair length with many metabolic pathways and other agronomic traits. Combining root hair phenotypes with 1.25 million single nucleotide polymorphisms (SNPs) via genome-wide association study (GWAS) revealed several candidate genes implicated in cellular signaling, polar growth, disease resistance and various metabolic pathways. Conclusions: These results illustrate the genetic basis of root hair length in maize, offering a list of candidate genes predictably contributing to root hair growth, which are invaluable resource for the future functional investigation.

The plant-specific mildew resistance locus O (MLO) proteins, which contain seven transmembrane domains and a conserved calmodulin-binding domain, play important roles in many plant developmental processes. However, their mechanisms that regulate plant development remain unclear. Here, we report the functional characterization of the MLO4 protein in Arabidopsis roots. The MLO4 was identified as interacting with CML12 in a screening for the interaction between the proteins from Arabidopsis MLO and calmodulin/calmodulin-like (CaM/CML) families using yeast two hybrid (Y2H) assays. Then, the interaction between MLO4 and CML12 was further verified by Luciferase Complementation Imaging (LCI) and Bimolecular Fluorescence Complementation (BiFC) assays. Genetic analysis showed that the mlo4, cml12, and mlo4 cml12 mutants displayed similar defects in root gravity response. These results imply that the MLO4 might play an important role in root gravity response through interaction with CML12. Moreover, our results also demonstrated that the interaction between the MLO and CaM/CML families might be conservative.

Background In flowering plants, pollen formation is a very complex process. It is strictly regulated by various genetic factors. Some of these factors have been identified, but the regulatory mechanism concerning the process of late pollen maturing remains unknown.Results Previously we identified and reported an ARABIDOPSIS POLLEN ABORTION MUTANT 2 (APAM2) gene which was required for pollen formation in Arabidopsis thaliana. In this study, we further analysed the phenotype of atapam2 and the expression pattern of AtAPAM2. Mutation in AtAPAM2 drastically affected male gametophytic function, impacted the late pollen mature process, and resulted in mature pollen grains from atapam2/+ plants being shrivelled and dead. It was expressed among seedlings, roots, stems, leaves, siliques, flowers, and pollen grains. However, its expression levels in flowers and pollen grains were much higher than in other tissues. AtAPAM2 encodes an unknown protein with three WD40-repeats with localisation in plasma membrane and nucleus, and homologous proteins exist in many species.Conclusions We determined that AtAPAM2 may play a role in pollen formation and other development processes by interacting with other proteins, but we did not identify the protein that interacts with it.

Summary: Root hair, a special type of tubular‐shaped cell, outgrows from the root epidermal cell and plays important roles in the acquisition of nutrients and water, as well as interactions with biotic and abiotic stresses. Studies in the model plant Arabidopsis have revealed that root‐hair initiation and elongation are hierarchically regulated by a group of basic helix−loop−helix (bHLH) transcription factors (TFs). However, knowledge regarding the regulatory pathways of these bHLH TFs in controlling root hair growth remains limited. In this study, RNA‐seq analysis was conducted to profile the transcriptome in the elongating maize root hair and >1000 genes with preferential expression in root hair were identified. A consensus cis‐element previously featured as the potential bHLH‐TF binding sites was present in the regulatory regions for the majority of the root hair‐preferentially expressed genes. In addition, an individual change in ZmLRL5, the highest‐expressed bHLH‐TF in maize root hair resulted in a dramatic reduction in the elongation of root hair, and rendered the growth of root hair hypersensitive to translational inhibition. Moreover, RNA‐seq, yeast‐one‐hybrid and ribosome profile analysis suggested that ZmLRL5 may function as a key player in orchestrating the translational process by directly regulating the expression of translational processes/ribosomal genes during maize root hair growth.

Summary: In flowering plants, the interaction of pollen tubes with female tissues is important for the accomplishment of double fertilization. Little information is known about the mechanisms that underlie signalling between pollen tubes and female tissues. In this study, two Arabidopsis pollen tube‐expressed CrRLK1L protein kinases, Buddha's Paper Seal 1 (BUPS1) and BUPS2, were identified as being required for normal tip growth of pollen tubes in the pistil. They are expressed prolifically in pollen and pollen tubes and are localized on the plasma membrane of the pollen tube tip region. Mutations in BUPS1 drastically reduced seed set. Most of the bups1 mutant pollen tubes growing in the pistil exhibited a swollen pollen tube tip, leading to failure of fertilization. The bups2 pollen tubes had a slightly abnormal morphology but could still accomplish double fertilization. The bups1 bups2 double mutant exhibited a slightly enhanced phenotype compared to the single bups1 mutants. The BUPS1 proteins could form homomers and heteromers with BUPS2, whereas BUPS2 could only form heteromers with BUPS1. The BUPS proteins could interact with the Arabidopsis pollen‐expressed RopGEFs in the yeast two‐hybrid (Y2H) and bimolecular fluorescence complementation (BiFC) assays. The results indicated that the BUPSs may mediate normal polar growth of pollen tubes in the pistil.