OsCNGC13 promotes seed-setting rate by facilitating pollen tube growth in stylar tissues

OsIPK1 frameshift mutations disturb phosphorus homeostasis and impair starch synthesis during grain filling in rice

Inositol 1,3,4,5,6-pentakisphosphate 2-kinase (IPK1) catalyzes the final step in phytic acid (InsP6) synthesis. In this study, the effects of OsIPK1 mutations on InsP6 synthesis, grain filling and their underlying mechanisms were investigated. Seven gRNAs were designed to disrupt the OsIPK1 gene via CRISPR/CAS9 system. Only 4 of them generated 29 individual insertion or deletion T0 plants, in which nine biallelic or heterozygous genotypes were identified. Segregation analysis revealed that OsIPK1 frameshift mutants are homozygous lethality. The biallelic and heterozygous frameshift mutants exhibited significant reduction in yield-related traits, particularly in the seed-setting rate and yield per plant. Despite a notable decline in pollen viability, the male and female gametes had comparable transmission rates to their progenies in the mutants. A significant number of the filling-aborted (FA) grains was observed in mature grains of these heterozygous frameshift mutants. These grains exhibited a nearly complete blockage of InsP6 synthesis, resulting in a pronounced increase in Pi content. In contrast, a slight decline in InsP6 content was observed in the plump grains. During the filling stage, owing to the excessive accumulation of Pi, starch synthesis was significantly impaired, and the endosperm development-specific gene expression was nearly abolished. Consistently, the activity of whereas AGPase, a key enzyme in starch synthesis, was significantly decreased and Pi transporter gene expression was upregulated in the FA grains. Taken together, these results demonstrate that OsIPK1 frameshift mutations result in excessive Pi accumulation, decreased starch synthesis, and ultimately leading to lower yields in rice.

FLOURY ENDOSPERM24, a heat shock protein 101 (HSP101), is required for starch biosynthesis and endosperm development in rice

Endosperm is the main storage organ in cereal grain and determines grain yield and quality. The molecular mechanisms of heat shock proteins in regulating starch biosynthesis and endosperm development remain obscure. Here, we report a rice floury endosperm mutant flo24 that develops abnormal starch grains in the central starchy endosperm cells. Map-based cloning and complementation test showed that FLO24 encodes a heat shock protein HSP101, which is localized in plastids. The mutated protein FLO24T296I dramatically lost its ability to hydrolyze ATP and to rescue the thermotolerance defects of the yeast hsp104 mutant. The flo24 mutant develops more severe floury endosperm when grown under high-temperature conditions than normal conditions. And the FLO24 protein was dramatically induced at high temperature. FLO24 physically interacts with several key enzymes required for starch biosynthesis, including AGPL1, AGPL3 and PHO1. Combined biochemical and genetic evidence suggests that FLO24 acts cooperatively with HSP70cp-2 to regulate starch biosynthesis and endosperm development in rice. Our results reveal that FLO24 acts as an important regulator of endosperm development, which might function in maintaining the activities of enzymes involved in starch biosynthesis in rice.

Two ABCI family transporters, OsABCI15 and OsABCI16, are involved in grain-filling in rice

Seed development is critical for plant reproduction and crop yield, with panicle seed-setting rate, grain-filling, and grain weight being key seed characteristics for yield improvement. However, few genes are known to regulate grain filling. Here, we identify two adenosine triphosphate (ATP)-binding cassette (ABC)I-type transporter genes, OsABCI15 and OsABCI16, involved in rice grain-filling. Both genes are highly expressed in developing seeds, and their proteins are localized to the plasma membrane and cytosol. Interestingly, knockout of OsABCI15 and OsABCI16 results in a significant reduction in seed-setting rate, caused predominantly by the severe empty pericarp phenotype, which differs from the previously reported low seed-setting phenotype resulting from failed pollination. Further analysis indicates that OsABCI15 and OsABCI16 participate in ion homeostasis and likely export ions between filial tissues and maternal tissues during grain filling. Importantly, overexpression of OsABCI15 and OsABCI16 enhances the seed-setting rate and grain yield in transgenic plants and decreases ion accumulation in brown rice. Moreover, the OsABCI15/16 orthologues in maize exhibit a similar role in kernel development, as demonstrated by their disruption in transgenic maize. Therefore, our findings reveal the important roles of two ABC transporters in cereal grain filling, highlighting their value in crop yield improvement.

Redundant role of OsCNGC4 and OsCNGC5 encoding cyclic nucleotide-gated channels in rice pollen germination and tube growth

In staple crops, such as rice (Oryza sativa L.), pollen plays a crucial role in seed production. However, the molecular mechanisms underlying rice pollen germination and tube growth remain underexplored. Notably, we recently uncovered the redundant expression and mutual interaction of two rice genes encoding cyclic nucleotide-gated channels (CNGCs), OsCNGC4 and OsCNGC5, in mature pollen. Building on these findings, the current study focused on clarifying the functional roles of these two genes in pollen germination and tube growth. To overcome functional redundancy, we produced gene-edited rice plants with mutations in both genes using the CRISPR-Cas9 system. The resulting homozygous OsCNGC4 and OsCNGC5 gene-edited mutants (oscngc4/5) exhibited significantly lower pollen germination rates than the wild type (WT), along with severely reduced fertility. Transcriptome analysis of the double oscngc4/5 mutant revealed downregulation of genes related to receptor kinases, transporters, and cell wall metabolism. To identify the direct regulators of OsCNGC4, which form a heterodimer with OsCNGC5, we screened a yeast two-hybrid library containing rice cDNAs from mature anthers. Subsequently, we identified two calmodulin isoforms (CaM1-1 and CaM1-2), NETWORKED 2 A (NET2A), and proline-rich extension-like receptor kinase 13 (PERK13) proteins as interactors of OsCNGC4, suggesting its roles in regulating Ca2+ channel activity and F-actin organization. Overall, our results suggest that OsCNGC4 and OsCNGC5 may play critical roles in pollen germination and elongation by regulating the Ca2+ gradient in growing pollen tubes.

Knockout of a rice K5.2 gene increases Ca accumulation in the grain

Rice is a staple food for half of the world's population, but it is a poor dietary source of calcium (Ca) due to the low concentration. It is an important issue to boost Ca concentration in this grain to improve Ca deficiency risk, but the mechanisms underlying Ca accumulation are poorly understood. Here, we obtained a rice (Oryza sativa) mutant with high shoot Ca accumulation. The mutant exhibited 26%-53% higher Ca in shoots than did wild-type rice (WT) at different Ca supplies. Ca concentration in the xylem sap was 36% higher in the mutant than in the WT. There was no difference in agronomic traits between the WT and mutant, but the mutant showed 25% higher Ca in the polished grain compared with the WT. Map-based cloning combined with a complementation test revealed that the mutant phenotype was caused by an 18-bp deletion of a gene, OsK5.2, belonging to the Shaker-like K+ channel family. OsK5.2 was highly expressed in the mature region of the roots and its expression in the roots was not affected by Ca levels, but upregulated by low K. Immunostaining showed that OsK5.2 was mainly expressed in the pericycle of the roots. Taken together, our results revealed a novel role for OsK5.2 in Ca translocation in rice, and will be a good target for Ca biofortification in rice.

The genetic and molecular basis of haploinsufficiency in flowering plants

In diploid organisms, haploinsufficiency can be defined as the requirement for more than one fully functional copy of a gene. In contrast to most genes, whose loss-of-function alleles are recessive, loss-of-function alleles of haploinsufficient genes are dominant. However, forward and reverse genetic screens are biased toward obtaining recessive, loss-of-function mutations, and therefore, dominant mutations of all types are underrepresented in mutant collections. Despite this underrepresentation, haploinsufficient loci have intriguing implications for studies of genome evolution, gene dosage, stability of protein complexes, genetic redundancy, and gene expression. Here we review examples of haploinsufficiency in flowering plants and describe the underlying molecular mechanisms and evolutionary forces driving haploinsufficiency. Finally, we discuss the masking of haploinsufficiency by genetic redundancy, a widespread phenomenon among angiosperms.

Identification of the CNGC Gene Family in Rice and Mining of Alleles for Application in Rice Improvement

Cyclic nucleotide-gated ion channel (CNGC) gene regulation plays important roles in plant immune and abiotic stress response. Here, we identified 16 CNGC genes in rice (Oryza sativa). Then, we analyzed their chromosomal location, physicochemical properties, subcellular localization, gene functional interaction network, cis-acting elements, phylogenetic relationships, collinearity, expression in tissues under normal conditions and abiotic stresses, and geng-cds-haplotype (gcHap) diversity in 3010 gcHaps. As a result, OsCNGC3 (Os06g0527300) was identified as a gene different from previous report, and OsCNGC genes were found to play important roles in rice population differentiation and rice improvement. Our results revealed their very strong differentiation between subspecies and populations, important roles in response to abiotic stresses, as well as strong genetic bottleneck effects and artificial selection of gcHap diversity in the modern breeding process of Xian (indica) and Geng (japonica) populations. The results also suggested that natural variations in most rice CNGC loci are potentially valuable for improving rice productivity and tolerance to abiotic stresses. The favorable alleles at the CNGC loci should be explored to facilitate their application in future rice improvement.

A small heat shock protein GmHSP18.5a improves the male fertility restorability of cytoplasmic male sterility-based restorer line under high temperature stress in soybean

Small heat shock protein (sHSP) is involved in high temperature (HT) stress response. However, the function of sHSPs in regulating male fertility of soybean under HT stress remains largely unknown. Here, we identified a sHSP gene, GmHSP18.5a, which was responded to HT stress during flowering in cytoplasmic male sterility (CMS)-based restorer line of soybean. Moreover, GmHSFA6b turned out to directly activated the expression of GmHSP18.5a by binding to the heat shock cis-element in its promoter. Overexpression of GmHSP18.5a increased male fertility in transgenic Arabidopsis, soybean CMS-based restorer line and its hybrid F1 with CMS line under HT stress. Reactive oxygen species (ROS) content detection revealed that GmHSP18.5a promoted the ROS scavenging ability of Arabidopsis inflorescence and soybean flower bud under HT stress. Enzyme activity assay and gene expression analysis indicated that GmHS18.5a mainly increased the activity of antioxidant enzymes and the expression level of ROS metabolism-related genes under HT stress. Our results indicated that GmHSP18.5a improved the male fertility restorability of CMS-based restorer line in soybean by regulating ROS metabolic pathway and reducing ROS accumulation. Our findings not only revealed the molecular mechanism of sHSP regulating the male fertility of soybean under HT stress, but also provided a theoretical basis for creating strong restorer line with thermotolerance.

A CYP78As-small grain4-coat protein complex Ⅱ pathway promotes grain size in rice

CYP78A, a cytochrome P450 subfamily that includes rice (Oryza sativa L.) BIG GRAIN2 (BG2, CYP78A13) and Arabidopsis thaliana KLUH (KLU, CYP78A5), generate an unknown mobile growth signal (referred to as a CYP78A-derived signal) that increases grain (seed) size. However, the mechanism by which the CYP78A pathway increases grain size remains elusive. Here, we characterized a rice small grain mutant, small grain4 (smg4), with smaller grains than its wild type due to restricted cell expansion and cell proliferation in spikelet hulls. SMG4 encodes a multidrug and toxic compound extrusion (MATE) transporter. Loss of function of SMG4 causes smaller grains while overexpressing SMG4 results in larger grains. SMG4 is mainly localized to endoplasmic reticulum (ER) exit sites (ERESs) and partially localized to the ER and Golgi. Biochemically, SMG4 interacts with coat protein complex Ⅱ (COPⅡ) components (Sar1, Sec23, and Sec24) and CYP78As (BG2, GRAIN LENGTH 3.2 [GL3.2], and BG2-LIKE 1 [BG2L1]). Genetically, SMG4 acts, at least in part, in a common pathway with Sar1 and CYP78As to regulate grain size. In summary, our findings reveal a CYP78As-SMG4-COPⅡ regulatory pathway for grain size in rice, thus providing new insights into the molecular and genetic regulatory mechanism of grain size.

Homeostasis of flavonoids and triterpenoids most likely modulates starch metabolism for pollen tube penetration in rice

In angiosperms, the timely delivery of sperm cell nuclei by pollen tube (PT) to the ovule is vital for double fertilization. Penetration of PT into maternal stigma tissue is a critical step for sperm cell nuclei delivery, yet little is known about the process. Here, a male-specific and sporophytic mutant xt6, where PTs are able to germinate but unable to penetrate the stigma tissue, is reported in Oryza sativa. Through genetic study, the causative gene was identified as Chalcone synthase (OsCHS1), encoding the first enzyme in flavonoid biosynthesis. Indeed, flavonols were undetected in mutant pollen grains and PTs, indicating that the mutation abolished flavonoid biosynthesis. Nevertheless, the phenotype cannot be rescued by exogenous application of quercetin and kaempferol as reported in maize and petunia, suggesting a different mechanism exists in rice. Further analysis showed that loss of OsCHS1 function disrupted the homeostasis of flavonoid and triterpenoid metabolism and led to the accumulation of triterpenoid, which inhibits significantly α-amylase activity, amyloplast hydrolysis and monosaccharide content in xt6, these ultimately impaired tricarboxylic acid (TCA) cycle, reduced ATP content and lowered the turgor pressure as well. Our findings reveal a new mechanism that OsCHS1 modulates starch hydrolysis and glycometabolism through modulating the metabolic homeostasis of flavonoids and triterpenoids which affects α-amylase activity to maintain PT penetration in rice, which contributes to a better understanding of the function of CHS1 in crop fertility and breeding.

The triticale mature pollen and stigma proteomes - assembling the proteins for a productive encounter

Triticeae crops are major contributors to global food production and ensuring their capacity to reproduce and generate seeds is critical. However, despite their importance our knowledge of the proteins underlying Triticeae reproduction is severely lacking and this is not only true of pollen and stigma development, but also of their pivotal interaction. When the pollen grain and stigma are brought together they have each accumulated the proteins required for their intended meeting and accordingly studying their mature proteomes is bound to reveal proteins involved in their diverse and complex interactions. Using triticale as a Triticeae representative, gel-free shotgun proteomics was used to identify 11,533 and 2977 mature stigma and pollen proteins respectively. These datasets, by far the largest to date, provide unprecedented insights into the proteins participating in Triticeae pollen and stigma development and interactions. The study of the Triticeae stigma has been particularly neglected. To begin filling this knowledge gap, a developmental iTRAQ analysis was performed revealing 647 proteins displaying differential abundance as the stigma matures in preparation for pollination. An in-depth comparison to an equivalent Brassicaceae analysis divulged both conservation and diversification in the makeup and function of proteins involved in the pollen and stigma encounter. SIGNIFICANCE: Successful pollination brings together the mature pollen and stigma thus initiating an intricate series of molecular processes vital to crop reproduction. In the Triticeae crops (e.g. wheat, barley, rye, triticale) there persists a vast deficit in our knowledge of the proteins involved which needs to be addressed if we are to face the many upcoming challenges to crop production such as those associated with climate change. At maturity, both the pollen and stigma have acquired the protein complement necessary for their forthcoming encounter and investigating their proteomes will inevitably provide unprecedented insights into the proteins enabling their interactions. By combining the analysis of the most comprehensive Triticeae pollen and stigma global proteome datasets to date with developmental iTRAQ investigations, proteins implicated in the different phases of pollen-stigma interaction enabling pollen adhesion, recognition, hydration, germination and tube growth, as well as those underlying stigma development were revealed. Extensive comparisons between equivalent Triticeae and Brassiceae datasets highlighted both the conservation of biological processes in line with the shared goal of activating the pollen grain and promoting pollen tube invasion of the pistil to effect fertilization, as well as the significant distinctions in their proteomes consistent with the considerable differences in their biochemistry, physiology and morphology.

Modifying the Expression of Cysteine Protease Gene PCP Affects Pollen Development, Germination and Plant Drought Tolerance in Maize

Cysteine proteases (CPs) are vital proteolytic enzymes that play critical roles in various plant processes. However, the particular functions of CPs in maize remain largely unknown. We recently identified a pollen-specific CP (named PCP), which highly accumulated on the surface of maize pollen. Here, we reported that PCP played an important role in pollen germination and drought response in maize. Overexpression of PCP inhibited pollen germination, while mutation of PCP promoted pollen germination to some extent. Furthermore, we observed that germinal apertures of pollen grains in the PCP-overexpression transgenic lines were excessively covered, whereas this phenomenon was not observed in the wild type (WT), suggesting that PCP regulated pollen germination by affecting the germinal aperture structure. In addition, overexpression of PCP enhanced drought tolerance in maize plants, along with the increased activities of the antioxidant enzymes and the decreased numbers of the root cortical cells. Conversely, mutation of PCP significantly impaired drought tolerance. These results may aid in clarifying the precise functions of CPs in maize and contribute to the development of drought-tolerant maize materials.

RGA1 alleviates low-light-repressed pollen tube elongation by improving the metabolism and allocation of sugars and energy

Low-light stress compromises photosynthetic and energy efficiency and leads to spikelet sterility; however, the effect of low-light stress on pollen tube elongation in the pistil remains poorly understood. The gene RGA1, which encodes a Gα-subunit of the heterotrimeric G-protein, enhanced low-light tolerance at anthesis by preventing the cessation of pollen tube elongation in the pistil of rice plants. In this process, marked increases in the activities of acid invertase (INV), sucrose synthase (SUS) and mitochondrial respiratory electron transport chain complexes, as well as the relative expression levels of SUTs (sucrose transporter), SWEETs (sugars will eventually be exported transporters), SUSs, INVs, CINs (cell-wall INV 1), SnRK1A (sucrose-nonfermenting 1-related kinase 1) and SnRK1B, were observed in OE-1 plants. Accordingly, notable increases in contents of ATP and ATPase were presented in OE-1 plants under low-light conditions, while they were decreased in d1 plants. Importantly, INV and ATPase activators (sucrose and Na₂ SO₃ , respectively) increased spikelet fertility by improving the energy status in the pistil under low-light conditions, and the ATPase inhibitor Na₂ VO₄ induced spikelet sterility and decreased ATPase activity. These results suggest that RGA1 could alleviate the low-light stress-induced impairment of pollen tube elongation to increase spikelet fertility by promoting sucrose unloading in the pistil and improving the metabolism and allocation of energy.

Mutation of DEFECTIVE EMBRYO SAC1 results in a low seed-setting rate in rice by regulating embryo sac development

The seed-setting rate has a significant effect on grain yield in rice (Oryza sativa L.). Embryo sac development is essential for seed setting; however, the molecular mechanism underlying this process remains unclear. Here, we isolated defective embryo sac1 (des1), a rice mutant with a low seed-setting rate. Cytological examination showed degenerated embryo sacs and reduced fertilization capacity in des1. Map-based cloning revealed a nonsense mutation in OsDES1, a gene that encodes a putative nuclear envelope membrane protein (NEMP)-domain-containing protein that is preferentially expressed in pistils. The OsDES1 mutation disrupts the normal formation of functional megaspores, which ultimately results in a degenerated embryo sac in des1. Reciprocal crosses showed that fertilization is abnormal and that the female reproductive organ is defective in des1. OsDES1 interacts with LONELY GUY (LOG), a cytokinin-activating enzyme that acts in the final step of cytokinin synthesis; mutation of LOG led to defective female reproductive organ development. These results demonstrate that OsDES1 functions in determining the rice seed-setting rate by regulating embryo sac development and fertilization. Our study sheds light on the function of NEMP-type proteins in rice reproductive development.

Knockout of OsSWEET15 Impairs Rice Embryo Formation and Seed-Setting

We show that the knockout of a sugar transporter gene OsSWEET15 led to a significant drop in rice fertility with around half of the knockout mutant's spikelets bearing blighted or empty grains. The rest of the spikelets bore fertile grains with a slightly reduced weight. Notably, the ovaries in the blighted grains of the ossweet15 mutants expanded after flowering but terminated their development before the endosperm cellularization stage and subsequently aborted. β- glucuronidase (GUS) and Green Fluorescent Protein (GFP) reporter lines representing the OsSWEET15 expression showed that the gene was expressed in the endosperm tissues surrounding the embryo, which supposedly supplies nutrients to sustain embryo development. These results together with the protein's demonstrated sucrose transport capacity and plasma membrane localization suggest that OsSWEET15 plays a prominent role during the caryopsis formation stage, probably by releasing sucrose from the endosperm to support embryo development. By contrast, the empty grains were probably caused by the reduced pollen viability of the ossweet15 mutants. Investigation of ossweet11 mutant grains revealed similar phenotypes to those observed in the ossweet15 mutants. These results indicate that both OsSWEET15 and OsSWEET11 play important and similar roles in rice pollen development, caryopsis formation and seed-setting, in addition to their function in seed-filling that was demonstrated previously.

Transcriptome analysis reveals the regulation of cyclic nucleotide-gated ion channels in response to exogenous abscisic acid and calcium treatment under drought stress in tomato

Background: Drought stress can limit the growth and development of tomato seedlings and cause considerable loss of tomato yield. Exogenous abscisic acid (ABA) and calcium (Ca²⁺) can effectively alleviate the damage of drought stress to plants in part because Ca²⁺ acts as a second messenger in the drought resistance pathway. Although cyclic nucleotide-gated ion channels (CNGCs) are common non-specific Ca²⁺ osmotic channels in cell membranes, a thorough understanding of the transcriptome characteristics of tomato treated with exogenous ABA and Ca²⁺ under drought stress is necessary to characterize the molecular mechanism of CNGC involved in tomato drought resistance. Results: There were 12,896 differentially expressed genes in tomato under drought stress, as well as 11,406 and 12,502 differentially expressed genes after exogenous ABA and Ca²⁺ application, respectively. According to functional annotations and reports, the 19 SlCNGC genes related to Ca²⁺ transport were initially screened, with 11 SlCNGC genes that were upregulated under drought stress and downregulated after exogenous ABA application. After exogenous Ca²⁺ application, the data showed that two of these genes were upregulated, while nine genes were downregulated. Based on these expression patterns, we predicted the role of SlCNGC genes in the drought resistance pathway and their regulation by exogenous ABA and Ca²⁺ in tomato. Conclusion: The results of this study provide foundational data for further study of the function of SlCNGC genes and a more comprehensive understanding of drought resistance mechanisms in tomato.

Genome-Wide Analysis of Cyclic Nucleotide-Gated Channel Genes Related to Pollen Development in Rice

In the angiosperm, pollen germinates and rapidly expands the pollen tube toward the ovule. This process is important for plant double fertilization and seed setting. It is well known that the tip-focused calcium gradient is essential for pollen germination and pollen tube growth. However, little is known about the Ca²⁺ channels that play a role in rice pollen germination and tube growth. Here, we divided the 16 cyclic nucleotide-gated channel (CNGC) genes from rice into five subgroups and found two subgroups (clades II and III) have pollen-preferential genes. Then, we performed a meta-expression analysis of all OsCNGC genes in anatomical samples and identified three pollen-preferred OsCNGCs (OsCNGC4, OsCNGC5, and OsCNGC8). The subcellular localization of these OsCNGC proteins is matched with their roles as ion channels on the plasma membrane. Unlike other OsCNGCs, these genes have a unique cis-acting element in the promoter. OsCNGC4 can act by forming a homomeric complex or a heteromeric complex with OsCNGC5 or OsCNGC8. In addition, it was suggested that they can form a multi-complex with Mildew Resistance Locus O (MLO) protein or other types of ion transporters, and that their expression can be modulated by Ruptured Pollen tube (RUPO) encoding receptor-like kinase. These results shed light on understanding the regulatory mechanisms of pollen germination and pollen tube growth through calcium channels in rice.

A retrotransposon insertion in MUTL-HOMOLOG 1 affects wild rice seed set and cultivated rice crossover rate

Wild rice (Oryza rufipogon) has a lower panicle seed setting rate (PSSR) and gamete fertility than domesticated rice (Oryza sativa), but the genetic mechanisms of this phenomenon remain unknown. Here, we cloned a null allele of OsMLH1, an ortholog of MutL-homolog 1 to yeast and mammals, from wild rice O. rufipogon W1943 and revealed a 5.4-kb retrotransposon insertion in OsMLH1 is responsible for the low PSSR in wild rice. In contrast to the wild-type, a near isogenic line NIL-mlh1 exhibits defective crossover (CO) formation during meiosis, resulting in reduced pollen viability, partial embryo lethality, and low PSSR. Except for the mutant of mismatch repair gene postmeiotic segregation 1 (Ospms1), all other MutL mutants from O. sativa indica subspecies displayed male and female semi-sterility similar to NIL-mlh1, but less severe than those from O. sativa japonica subspecies. MLH1 and MLH3 did not contribute in an additive fashion to fertility. Two types of MutL heterodimers, MLH1-PMS1 and MLH1-MLH3, were identified in rice, but only the latter functions in promoting meiotic CO formation. Compared to japonica varieties, indica cultivars had greater numbers of CO events per meiosis. Our results suggest that low fertility in wild rice may be caused by different gene defects, and indica and japonica subspecies have substantially different CO rates responsible for the discrepancy between the fertility of mlh1 and mlh3 mutants.

Pangenome-wide analysis of cyclic nucleotide-gated channel (CNGC) gene family in citrus Spp. Revealed their intraspecies diversity and potential roles in abiotic stress tolerance

Cyclic nucleotide-gated channels (CNGC) gene family has been found to be involved in physiological processes including signaling pathways, environmental stresses, plant growth, and development. This gene family of non-selective cation channels is known to regulate the uptake of calcium and is reported in several plant species. The pangenome-wide studies enable researchers to understand the genetic diversity comprehensively; as a comparative analysis of multiple plant species or member of a species at once helps to better understand the evolutionary relationships and diversity present among them. In the current study, pangenome-wide analysis of the CNGC gene family has been performed on five Citrus species. As a result, a total of 32 genes in Citrus sinensis, 27 genes in Citrus recticulata, 30 genes in Citrus grandis, 31 genes in Atalantia buxfolia, and 30 genes in Poncirus trifoliata were identified. In addition, two unique genes CNGC13 and CNGC14 were identified, which may have potential roles. All the identified CNGC genes were unevenly distributed on 9 chromosomes except P. trifoliata had genes distributed on 7 chromosomes and were classified into four major groups and two sub-groups namely I, II, III, IV-A, and IV-B. Cyclic nucleotide binding (CNB) motif, calmodulin-binding motif (CaMB), and motif for IQ-domain were conserved in Citrus Spp. Intron exon structures of citrus species were not exactly as same as the gene structures of Arabidopsis. The majority of cis-regulatory elements (CREs) were light responsive and others include growth, development, and stress-related indicating potential roles of the CNGC gene family in these functions. Both segmental and tandem duplication were involved in the expansion of the CNGC gene family in Citrus Spp. The miRNAs are involved in the response of CsCNGC genes towards drought stress along with having regulatory association in the expression of these genes. Protein- Protein interaction (PPI) analysis also showed the interaction of CNGC proteins with other CNGCs which suggested their potential role in pathways regulating different biological processes. GO enrichment revealed that CNGC genes were involved in the transport of ions across membranes. Furthermore, tissue-specific expression patterns of leaves sample of C. sinensis were studied under drought stress. Out of 32 genes of C. sinensis 3 genes i.e., CsCNGC1.4, CsCNGC2.1, and CsCNGC4.2 were highly up-regulated, and only CsCNGC4.6 was highly down-regulated. The qRT-PCR analysis also showed that CNGC genes were highly expressed after treatment with drought stress, while gene expression was lower under controlled conditions. This work includes findings based on multiple genomes instead of one, therefore, this will provide more genomic information rather than single genome-based studies. These findings will serve as a basis for further functional insights into the CNGC gene family.

BnaCRCs with domestication preference positively correlate with the seed-setting rate of canola

Canola (Brassica napus) is an important oil crop worldwide. The seed-setting rate (SS) is a critical factor in determining its yield, and the development of pistils affects pollination and seed sets. However, research on seed-setting defects has been limited owing to difficulties in the identification of phenotypes, mutations, and complex genetic mechanisms. In this study, we found a stigma defect (sd) mutant in B. napus, which had no nectary. The SS of sd mutants in the field was approximately 93.4% lower than that of the wild type. Scanning and transmission electron microscopy imaging of sd mutants showed a low density of stigma papillary cells and stigma papillary cell vacuoles that disappeared 16 h after flowering. Genetic analysis of segregated populations showed that two recessive nuclear genes are responsible for the mutant phenotype of sd. Based on re-sequencing and map-based cloning, we reduced the candidate sites on ChrA07 (BnaSSA07) and ChrC06 (BnaSSC06) to 30 and 67 kb, including six and eight predicted genes, respectively. Gene analyses showed that a pair of CRABS CLAW (CRC) homeologous genes at BnaSSA07 and BnaSSC06 were associated with the development of carpel and nectary. BnaSSA07.CRC and BnaSSC06.CRC candidate genes were found to be expressed in flower organs only, with significant differences in their expression in the pistils of the near-isogenic lines. DNA sequencing showed transposon insertions in the upstream region and intron of the candidate gene BnaSSA07.crc. We also found that BnaSSC06.crc exists widely in the natural population and we give possible reasons for its widespread existence.

Systematic analysis of CNGCs in cotton and the positive role of GhCNGC32 and GhCNGC35 in salt tolerance

BACKGROUND

Cyclic nucleotide-gated ion channels (CNGCs) are calcium-permeable channels that participate in a variety of biological functions, such as signaling pathways, plant development, and environmental stress and stimulus responses. Nevertheless, there have been few studies on CNGC gene family in cotton.

RESULTS

In this study, a total of 114 CNGC genes were identified from the genomes of 4 cotton species. These genes clustered into 5 main groups: I, II, III, IVa, and IVb. Gene structure and protein motif analysis showed that CNGCs on the same branch were highly conserved. In addition, collinearity analysis showed that the CNGC gene family had expanded mainly by whole-genome duplication (WGD). Promoter analysis of the GhCNGCs showed that there were a large number of cis-acting elements related to abscisic acid (ABA). Combination of transcriptome data and the results of quantitative RT-PCR (qRT-PCR) analysis revealed that some GhCNGC genes were induced in response to salt and drought stress and to exogenous ABA. Virus-induced gene silencing (VIGS) experiments showed that the silencing of the GhCNGC32 and GhCNGC35 genes decreased the salt tolerance of cotton plants (TRV:00). Specifically, physiological indexes showed that the malondialdehyde (MDA) content in gene-silenced plants (TRV:GhCNGC32 and TRV:GhCNGC35) increased significantly under salt stress but that the peroxidase (POD) activity decreased. After salt stress, the expression level of ABA-related genes increased significantly, indicating that salt stress can trigger the ABA signal regulatory mechanism.

CONCLUSIONS

we comprehensively analyzed CNGC genes in four cotton species, and found that GhCNGC32 and GhCNGC35 genes play an important role in cotton salt tolerance. These results laid a foundation for the subsequent study of the involvement of cotton CNGC genes in salt tolerance.

Identification of a unique allele in the quantitative trait locus for crown root number in japonica rice from Japan using genome-wide association studies

To explore the genetic resources that could be utilized to help improve root system architecture phenotypes in rice (Oryza sativa), we have conducted genome-wide association studies to investigate maximum root length and crown root number in 135 10-day-old Japanese rice accessions grown hydroponically. We identified a quantitative trait locus for crown root number at approximately 32.7 Mbp on chromosome 4 and designated it qNCR1 (quantitative trait locus for Number of Crown Root 1). A linkage disequilibrium map around qNCR1 suggested that three candidate genes are involved in crown root number: a cullin (LOC_Os04g55030), a gibberellin 20 oxidase 8 (LOC_Os04g55070), and a cyclic nucleotide-gated ion channel (LOC_Os04g55080). The combination of haplotypes for each gene was designated as a haploblock, and haploblocks 1, 2, and 3 were defined. Compared to haploblock 1, the accessions with haploblocks 2 and 3 had fewer crown roots; approximately 5% and 10% reductions in 10-day-old plants and 15% and 25% reductions in 42-day-old plants, respectively. A Japanese leading variety Koshihikari and its progenies harbored haploblock 3. Their crown root number could potentially be improved using haploblocks 1 and 2.

Dynamic Expression, Differential Regulation and Functional Diversity of the CNGC Family Genes in Cotton

Cyclic nucleotide-gated channels (CNGCs) constitute a family of non-selective cation channels that are primarily permeable to Ca2+ and activated by the direct binding of cyclic nucleotides (i.e., cAMP and cGMP) to mediate cellular signaling, both in animals and plants. Until now, our understanding of CNGCs in cotton (Gossypium spp.) remains poorly addressed. In the present study, we have identified 40, 41, 20, 20, and 20 CNGC genes in G. hirsutum, G. barbadense, G. herbaceum, G. arboreum, and G. raimondii, respectively, and demonstrated characteristics of the phylogenetic relationships, gene structures, chromosomal localization, gene duplication, and synteny. Further investigation of CNGC genes in G. hirsutum, named GhCNGC1-40, indicated that they are not only extensively expressed in various tissues and at different developmental stages, but also display diverse expression patterns in response to hormones (abscisic acid, salicylic acid, methyl jasmonate, ethylene), abiotic (salt stress) and biotic (Verticillium dahlia infection) stimuli, which conform with a variety of cis-acting regulatory elements residing in the promoter regions; moreover, a set of GhCNGCs are responsive to cAMP signaling during cotton fiber development. Protein–protein interactions supported the functional aspects of GhCNGCs in plant growth, development, and stress responses. Accordingly, the silencing of the homoeologous gene pair GhCNGC1&18 and GhCNGC12&31 impaired plant growth and development; however, GhCNGC1&18-silenced plants enhanced Verticillium wilt resistance and salt tolerance, whereas GhCNGC12&31-silenced plants had opposite effects. Together, these results unveiled the dynamic expression, differential regulation, and functional diversity of the CNGC family genes in cotton. The present work has laid the foundation for further studies and the utilization of CNGCs in cotton genetic improvement.

Identification and Analysis of Genes Involved in Double Fertilization in Rice

Double fertilization is a key determinant of grain yield, and the failure of fertilization during hybridization is one important reason for reproductive isolation. Therefore, fertilization has a very important role in the production of high-yield and well-quality hybrid of rice. Here, we used RNA sequencing technology to study the change of the transcriptome during double fertilization with the help of the mutant fertilization barrier (feb) that failed to finish fertilization process and led to seed abortion. The results showed that 1669 genes were related to the guided growth of pollen tubes, 332 genes were involved in the recognition and fusion of the male–female gametes, and 430 genes were associated with zygote formation and early free endosperm nuclear division. Among them, the genes related to carbohydrate metabolism; signal transduction pathways were enriched in the guided growth of pollen tubes, the genes involved in the photosynthesis; fatty acid synthesis pathways were activated by the recognition and fusion of the male–female gametes; and the cell cycle-related genes might play an essential role in zygote formation and early endosperm nuclear division. Furthermore, among the 1669 pollen tube-related genes, it was found that 7 arabinogalactan proteins (AGPs), 1 cysteine-rich peptide (CRP), and 15 receptor-like kinases (RLKs) were specifically expressed in anther, while 2 AGPs, 7 CRPs, and 5 RLKs in pistil, showing obvious unequal distribution which implied they might play different roles in anther and pistil during fertilization. These studies laid a solid foundation for revealing double fertilization mechanism of rice and for the follow-up investigation.

Molecular mechanisms of early plant pattern-triggered immune signaling

All eukaryotic organisms have evolved sophisticated immune systems to appropriately respond to biotic stresses. In plants and animals, a key part of this immune system is pattern recognition receptors (PRRs). Plant PRRs are cell-surface-localized receptor kinases (RKs) or receptor proteins (RPs) that sense microbe- or self-derived molecular patterns to regulate pattern-triggered immunity (PTI), a robust form of antimicrobial immunity. Remarkable progress has been made in understanding how PRRs perceive their ligands, form active protein complexes, initiate cell signaling, and ultimately coordinate the cellular reprogramming that leads to PTI. Here, we discuss the critical roles of PRR complex formation and phosphorylation in activating PTI signaling, as well as the emerging paradigm in which receptor-like cytoplasmic kinases (RLCKs) act as executors of signaling downstream of PRR activation.

Rhizosheath microbes induce root immune response under soil drying

The rhizosheath is an important drought-adaptive trait in roots of many angiosperms and has been regarded as a potential trait for future agricultural sustainability. In recent studies, we found that rice roots could form a pronounced rhizosheath under moderate soil drying (MSD) but not under continuous flooding irrigation (CF). The formation of rhizosheaths substantially changes the microbial community structure in endosphere root tissues and the rhizosphere in rice, which may induce a plant immune response. However, the manner by which the formation of rhizosheaths regulates the immune system of roots remains largely unknown. Here, we have analyzed the root transcriptomes of drought-tolerant rice and drought-sensitive rice under both MSD (rhizosheath-root) and CF (root without rhizosheath) conditions. Our results suggest that rhizosheath-associated microbes may trigger plant immune pathways in root under MSD, including the first line of defense component pattern-triggered immunity and the second line of defense component effector-triggered immunity. These data expand our understanding of rhizosheath-associated microbes and plant interactions.

EMBRYO SAC DEVELOPMENT 1 affects seed setting rate in rice by controlling embryo sac development

Seed setting rate is one of the critical factors that determine rice yield. Grain formation is a complex biological process, whose molecular mechanism is yet to be improved. Here we investigated the function of an OVATE family protein, Embryo Sac Development 1 (ESD1), in the regulation of seed setting rate in rice (Oryza sativa) by examining its loss-of-function mutants generated via clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated9 (Cas9) technology. ESD1 was predominantly expressed at Stage 6 of panicle development, especially in the ovules. esd1 mutants displayed reduced seed setting rates with normal stamen development and pollen tube growth but abnormal pistil group. Investigation of embryo sacs revealed that during the mitosis of functional megaspores, some egg cells degraded during differentiation in esd1 mutants, thereby hindering subsequent fertilization process and reducing seed setting rate. In addition, the transcriptional level of O. sativa anaphase-promoting complex 6, a reported embryo sac developing gene, was significantly reduced in esd1 mutants. These results support that ESD1 is an important modulator of ESD and seed setting rate in rice. Together, this finding demonstrates that ESD1 positively regulates the seed setting rate by controlling ESD in rice and has implications for the improvement of rice yield.

The Role of Calcium/Calcium-Dependent Protein Kinases Signal Pathway in Pollen Tube Growth

Pollen tube (PT) growth as a key step for successful fertilization is essential for angiosperm survival and especially vital for grain yield in cereals. The process of PT growth is regulated by many complex and delicate signaling pathways. Among them, the calcium/calcium-dependent protein kinases (Ca2+/CPKs) signal pathway has become one research focus, as Ca2+ ion is a well-known essential signal molecule for PT growth, which can be instantly sensed and transduced by CPKs to control myriad biological processes. In this review, we summarize the recent progress in understanding the Ca2+/CPKs signal pathway governing PT growth. We also discuss how this pathway regulates PT growth and how reactive oxygen species (ROS) and cyclic nucleotide are integrated by Ca2+ signaling networks.

Transcriptional activation and phosphorylation of OsCNGC9 confer enhanced chilling tolerance in rice

Low temperature is a major environmental factor that limits plant growth and productivity. Although transient elevation of cytoplasmic calcium has long been recognized as a critical signal for plant cold tolerance, the calcium channels responsible for this process have remained largely elusive. Here we report that OsCNGC9, a cyclic nucleotide-gated channel, positively regulates chilling tolerance by mediating cytoplasmic calcium elevation in rice (Oryza sativa). We showed that the loss-of-function mutant of OsCNGC9 is defective in cold-induced calcium influx and more sensitive to prolonged cold treatment, whereas OsCNGC9 overexpression confers enhanced cold tolerance. Mechanistically, we demonstrated that in response to chilling stress, OsSAPK8, a homolog of Arabidopsis thaliana OST1, phosphorylates and activates OsCNGC9 to trigger Ca²⁺ influx. Moreover, we found that the transcription of OsCNGC9 is activated by a rice dehydration-responsive element-binding transcription factor, OsDREB1A. Taken together, our results suggest that OsCNGC9 enhances chilling tolerance in rice through regulating cold-induced calcium influx and cytoplasmic calcium elevation.

The Complex Story of Plant Cyclic Nucleotide-Gated Channels

Plant cyclic nucleotide-gated channels (CNGCs) are tetrameric cation channels which may be activated by the cyclic nucleotides (cNMPs) adenosine 3',5'-cyclic monophosphate (cAMP) and guanosine 3',5'-cyclic monophosphate (cGMP). The genome of Arabidopsis thaliana encodes 20 CNGC subunits associated with aspects of development, stress response and immunity. Recently, it has been demonstrated that CNGC subunits form heterotetrameric complexes which behave differently from the homotetramers produced by their constituent subunits. These findings have widespread implications for future signalling research and may help explain how specificity can be achieved by CNGCs that are known to act in disparate pathways. Regulation of complex formation may involve cyclic nucleotide-gated channel-like proteins.

Automatic and Accurate Calculation of Rice Seed Setting Rate Based on Image Segmentation and Deep Learning

The rice seed setting rate (RSSR) is an important component in calculating rice yields and a key phenotype for its genetic analysis. Automatic calculations of RSSR through computer vision technology have great significance for rice yield predictions. The basic premise for calculating RSSR is having an accurate and high throughput identification of rice grains. In this study, we propose a method based on image segmentation and deep learning to automatically identify rice grains and calculate RSSR. By collecting information on the rice panicle, our proposed image automatic segmentation method can detect the full grain and empty grain, after which the RSSR can be calculated by our proposed rice seed setting rate optimization algorithm (RSSROA). Finally, the proposed method was used to predict the RSSR during which process, the average identification accuracy reached 99.43%. This method has therefore been proven as an effective, non-invasive method for high throughput identification and calculation of RSSR. It is also applicable to soybean yields, as well as wheat and other crops with similar characteristics.

Plant Cyclic Nucleotide-Gated Channels: New Insights on Their Functions and Regulation

Recent advances of plant cyclic nucleotide-gated channels give new insight into their molecular functions focusing on regulation, subunit assembly, and phosphorylation.

CYCLIC NUCLEOTIDE-GATED ION CHANNELs 14 and 16 Promote Tolerance to Heat and Chilling in Rice

Calcium signaling has been postulated to be critical for both heat and chilling tolerance in plants, but its molecular mechanisms are not fully understood. Here, we investigated the function of two closely related cyclic nucleotide-gated ion channel (CNGC) proteins, OsCNGC14 and OsCNGC16, in temperature-stress tolerance in rice (Oryza sativa) by examining their loss-of-function mutants generated by genome editing. Under both heat and chilling stress, both the cngc14 and cngc16 mutants displayed reduced survival rates, higher accumulation levels of hydrogen peroxide, and increased cell death. In the cngc16 mutant, the extent to which some genes were induced and repressed in response to heat stress was altered and some Heat Shock factor (HSF) and Heat Shock Protein (HSP) genes were slightly more induced compared to the wild type. Furthermore, the loss of either OsCNGC14 or OsCNGC16 reduced or abolished cytosolic calcium signals induced by either heat or chilling stress. Therefore, OsCNGC14 and OsCNGC16 are required for heat and chilling tolerance and are modulators of calcium signals in response to temperature stress. In addition, loss of their homologs AtCNGC2 and AtCNGC4 in Arabidopsis (Arabidopsis thaliana) also led to compromised tolerance of low temperature. Thus, this study indicates a critical role of CNGC genes in both chilling and heat tolerance in plants, suggesting a potential overlap in calcium signaling in response to high- and low-temperature stress.

A cyclic nucleotide-gated channel mediates cytoplasmic calcium elevation and disease resistance in rice

The transient elevation of cytoplasmic calcium is essential for pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI). However, the calcium channels responsible for this process have remained unknown. Here, we show that rice CDS1 (CELL DEATH and SUSCEPTIBLE to BLAST 1) encoding OsCNGC9, a cyclic nucleotide-gated channel protein, positively regulates the resistance to rice blast disease. We show that OsCNGC9 mediates PAMP-induced Ca²⁺ influx and that this event is critical for PAMPs-triggered ROS burst and induction of PTI-related defense gene expression. We further show that a PTI-related receptor-like cytoplasmic kinase OsRLCK185 physically interacts with and phosphorylates OsCNGC9 to activate its channel activity. Our results suggest a signaling cascade linking pattern recognition to calcium channel activation, which is required for initiation of PTI and disease resistance in rice.

GmHsp90A2 is involved in soybean heat stress as a positive regulator

Heat shock protein 90 s (Hsp90s), one of the most conserved and abundant molecular chaperones, is an essential component of the protective stress response. A previous study reported at least 12 genes in the GmHsp90s family in soybean and that GmHsp90A2 overexpression enhanced thermotolerance in Arabidopsis thaliana. Here, we investigate the roles of GmHsp90A2 in soybean by utilizing stable transgenic soybean lines overexpressing GmHsp90A2 and mutant lines generated by the CRISPR/Cas9 system. The results showed that compared with wild-type plants (WT) and empty vector control plants (VC), T3 transgenic soybean plants overexpressing GmHsp90A2 exhibited increased tolerance to heat stress through higher chlorophyll and lower malondialdehyde (MDA) contents in plants. Conversely, reduced chlorophyll and increased MDA contents in T2 homozygous GmHsp90A2-knockout mutants indicated decreased tolerance to heat stress. GmHsp90A2 was found to interact with GmHsp90A1 in yeast two-hybrid assays. Furthermore, subcellular localization analyses revealed that GmHsp90A2 was localized to the cytoplasm and cell membrane; as shown by bimolecular fluorescence complementation (BiFC) assays, GmHsp90A2 interacted with GmHsp90A1 in the nucleus and cytoplasm and cell membrane. Hence, we conclude that GmHsp90A1 is able to bind to GmHsp90A2 to form a complex and that this complex enters the nucleus. In summary, GmHsp90A2 might respond to heat stress and positively regulate thermotolerance by interacting with GmHsp90A1.

LSSR1 facilitates seed setting rate by promoting fertilization in rice

Seed setting rate is one of the major components that determine rice (Oryza sativa L.) yield. Successful fertilization is necessary for normal seed setting. However, little is known about the molecular mechanisms governing this process. In this study, we report a novel rice gene, LOW SEED SETTING RATE1 (LSSR1), which regulates the seed setting rate by facilitating rice fertilization. LSSR1 encodes a putative GH5 cellulase, which is highly conserved in plants. LSSR1 is predominantly expressed in anthers during the microsporogenesis stage, and its encoded protein contains a signal peptide at the N-terminal, which may be a secretory protein that stores in pollen grains and functions during rice fertilization. To explore the physiological function of LSSR1 in rice, loss-of-function mutants of LSSR1 were created through the CRISPR-Cas9 system, which showed a significant decrease in rice seed setting rate. However, the morphology of the vegetative and reproductive organs appears normal in lssr1 mutant lines. In addition, lssr1 pollen grains could be normally stained by I₂-KI solution. Cytological results demonstrate that the blockage of fertilization mostly accounted for the low seed setting rate in lssr1 mutant lines, which was most likely caused by abnormal pollen grain germination, failed pollen tube penetration, and retarded pollen tube elongation. Together, our results suggest that LSSR1 plays an important role in rice fertilization, which in turn is vital for maintaining rice seed setting rate.

ESA1 Is Involved in Embryo Sac Abortion in Interspecific Hybrid Progeny of Rice

The emergence of sterile individuals in the hybrid backcross progeny of wild and cultivated rice limits the use of wild rice alleles for improving cultivated rice, but the molecular mechanisms underlying this sterility remain unclear. Here, we identified the semisterile introgression line YIL42, derived from a cross between the indica rice variety Teqing (Oryza sativa) and Oryza rufipogon accession YJCWR (Yuanjiang common wild rice), which exhibits semisterility. Using positional cloning, we isolated EMBRYO SAC ABORTION 1 (ESA1), which encodes a nuclear-membrane localized protein containing an armadillo repeat domain. A mutation in ESA1 at position 1819 (^T1819^C) converts a stop codon into an Arg (R) codon, causing delayed termination of protein translation. Analysis of transgenic lines indicated that the difference in ESA1 protein structure between O. rufipogon-derived ESA1 and Teqing-derived esa1 affects female gamete abortion during early mitosis. Fertility investigation and expression analysis indicated that the interaction between ESA1 ^T1819 and unknown gene(s) of Teqing affects spikelet fertility of the hybrid backcross progeny. The ESA1 ^T1819 allele is present in O. rufipogon but absent in O. sativa, suggesting that variation in ESA1 may be associated with interspecific hybrid incompatibility between wild and cultivated rice. Our findings provide insight into the molecular mechanism underlying female sterility, which is useful for improving the panicle seed setting rate of rice and for developing a strategy to overcome interspecific hybrid sterility between cultivated rice and wild rice.

Ca2+ to the rescue - Ca2+channels and signaling in plant immunity

Ca²⁺ is a universal second messenger in many signaling pathways in all eukaryotes including plants. Transient changes in [Ca²⁺]cyt are rapidly generated upon a diverse range of stimuli such as drought, heat, wounding, and biotic stresses (infection by pathogenic and symbiotic microorganisms), as well as developmental cues. It has been known for a while that [Ca²⁺]cyt transient signals play crucial roles to activate plant immunity and recently significant progresses have been made in this research field. However the identity and regulation of ion channels that are involved in defense related Ca²⁺ signals are still enigmatic. Members of two ligand gated ion channel families, glutamate receptor-like channels (GLRs) and cyclic nucleotide-gated channels (CNGCs) have been implicated in immune responses; nevertheless more precise data to understand their direct involvement in the creation of Ca²⁺ signals during immune responses is necessary. Furthermore, the study of other ion channel groups is also required to understand the whole picture of the intra- and inter-cellular Ca²⁺ signalling network. In this review we summarize Ca²⁺ signals in plant immunity from an ion channel point of view and discuss future challenges in this exciting research field.

Getting physical: invasive growth events during plant development

Plant cells are enclosed in cell walls that weld them together, meaning that cells rarely change neighbours. Nonetheless, invasive growth events play critical roles in plant development and are often key hubs for the integration of environmental and/or developmental signalling. Here we review cellular processes involved in three such events: lateral root emergence, pollen tube growth through stigma and style tissues, and embryo expansion through the endosperm (Figures 1-3). We consider processes such as regulation of water fluxes and cell turgor (driving growth), cell wall modifications (e.g. cell separation) and cell death (for creating space) within these three contexts with the aim of identifying key mechanisms implicated in providing a chemical and biophysical environments permitting invasive growth events.

Genome-wide identification and analysis of the CNGC gene family in maize

As one of the non-selective cation channel gene families, the cyclic nucleotide-gated channel (CNGC) gene family plays a vital role in plant physiological processes that are related to signal pathways, plant development, and environmental stresses. However, genome-wide identification and analysis of the CNGC gene family in maize has not yet been undertaken. In the present study, twelve ZmCNGC genes were identified in the maize genome, which were unevenly distributed on chromosomes 1, 2, 4, 5, 6, 7, and 8. They were classified into five major groups: Groups I, II, III, IVa, and IVb. Phylogenetic analysis showed that gramineous plant CNGC genes expanded unequally during evolution. Group IV CNGC genes emerged first, whereas Groups I and II appeared later. Prediction analysis of cis-acting regulatory elements showed that 137 putative cis-elements were related to hormone-response, abiotic stress, and organ development. Furthermore, 120 protein pairs were predicted to interact with the 12 ZmCNGC proteins and other maize proteins. The expression profiles of the ZmCNGC genes were expressed in tissue-specific patterns. These results provide important information that will increase our understanding of the CNGC gene family in maize and other plants.

A Cyclic Nucleotide-Gated Channel, HvCNGC2-3, Is Activated by the Co-Presence of Na⁺ and K⁺ and Permeable to Na⁺ and K⁺ Non-Selectively

Cyclic nucleotide-gated channels (CNGCs) have been postulated to contribute significantly in plant development and stress resistance. However, their electrophysiological properties remain poorly understood. Here, we characterized barley CNGC2-3 (HvCNGC2-3) by the two-electrode voltage-clamp technique in the Xenopus laevis oocyte heterologous expression system. Current was not observed in X. laevis oocytes injected with HvCNGC2-3 complementary RNA (cRNA) in a bathing solution containing either Na⁺ or K⁺ solely, even in the presence of 8-bromoadenosine 3′,5′-cyclic monophosphate (8Br-cAMP) or 8-bromoguanosine 3′,5′-cyclic monophosphate (8Br-cGMP). A weakly voltage-dependent slow hyperpolarization-activated ion current was observed in the co-presence of Na⁺ and K⁺ in the bathing solution and in the presence of 10 µM 8Br-cAMP, but not 8Br-cGMP. Permeability ratios of HvCNGC2-3 to K⁺, Na⁺ and Cl⁻ were determined as 1:0.63:0.03 according to reversal-potential analyses. Amino-acid replacement of the unique ion-selective motif of HvCNGC2-3, AQGL, with the canonical motif, GQGL, resulted in the abolition of the current. This study reports a unique two-ion-dependent activation characteristic of the barley CNGC, HvCNGC2-3.

Multilayered signaling pathways for pollen tube growth and guidance

Sexual reproductive success is essential for the survival of all higher organisms. As the most prosperous and diverse group of land plants on earth, flowering plants evolved highly sophisticated fertilization mechanisms. To adapt to the terrestrial environment, a tubular structure pollen tube has been evolved to deliver the immobile sperm cells to the egg and central cell enclosed within the ovule. The pollen tube is generated from the vegetative cell of the pollen (male gametophyte), where two sperm cells are hosted. Pollen tube elongation in the maternal tissue and navigation to the ovule require intimate cell-cell interactions between the tube and female tissues. Questions on how the single-celled pollen tube accomplishes such task and how the female tissues accommodate the tube have attracted many plant biologists. Here, we review recent progresses and concepts in understanding the molecular mechanisms governing pollen tube growth and its interactions with the female tissues. We will also discuss the future perspective in this field.

Cell-cell communications and molecular mechanisms in plant sexual reproduction

Sexual reproduction is achieved by precise interactions between male and female reproductive organs. In plant fertilization, sperm cells are carried to ovules by pollen tubes. Signals from the pistil are involved in elongation and control of the direction of the pollen tube. Genetic, reverse genetic, and cell biological analyses using model plants have identified various factors related to the regulation of pollen tube growth and guidance. In this review, I summarize the mechanisms and molecules controlling pollen tube growth to the ovule, micropylar guidance, reception of the guidance signal in the pollen tube, rupture of the pollen tube to release sperm cells, and cessation of the tube guidance signal. I also briefly introduce various techniques used to analyze pollen tube guidance in vitro.