In-depth Understanding of Camellia oleifera Self-incompatibility by Comparative Transcriptome, Proteome and Metabolome

Identification and analysis of MAPK cascade gene families of Camellia oleifera and their roles in response to cold stress

BACKGROUND

Low-temperature severely limits the growth and development of Camellia oleifera (C. oleifera). The mitogen-activated protein kinase (MAPK) cascade plays a key role in the response to cold stress.

METHODS AND RESULTS

Our study aims to identify MAPK cascade genes in C. oleifera and reveal their roles in response to cold stress. In our study, we systematically identified and analyzed the MAPK cascade gene families of C. oleifera, including their physical and chemical properties, conserved motifs, and multiple sequence alignments. In addition, we characterized the interacting networks of MAPKK kinase (MAPKKK)-MAPK kinase (MAPKK)-MAPK in C. oleifera. The molecular mechanism of cold stress resistance of MAPK cascade genes in wild C. oleifera was analyzed by differential gene expression and real-time quantitative reverse transcription-PCR (qRT-PCR).

CONCLUSION

In this study, 21 MAPKs, 4 MAPKKs and 55 MAPKKKs genes were identified in the leaf transcriptome of C. oleifera. According to the phylogenetic results, MAPKs were divided into 4 groups (A, B, C and D), MAPKKs were divided into 3 groups (A, B and D), and MAPKKKs were divided into 2 groups (MEKK and Raf). Motif analysis showed that the motifs in each subfamily were conserved, and most of the motifs in the same subfamily were basically the same. The protein interaction network based on Arabidopsis thaliana (A. thaliana) homologs revealed that MAPK, MAPKK, and MAPKKK genes were widely involved in C. oleifera growth and development and in responses to biotic and abiotic stresses. Gene expression analysis revealed that the CoMAPKKK5/CoMAPKKK43/CoMAPKKK49-CoMAPKK4-CoMAPK8 module may play a key role in the cold stress resistance of wild C. oleifera at a high-elevation site in Lu Mountain (LSG). This study can facilitate the mining and utilization of genetic resources of C. oleifera with low-temperature tolerance.

Transcriptomic Analysis of Self-Incompatibility in Alfalfa

Alfalfa (Medicago sativa L.) is an important forage crop worldwide, but molecular genetics and breeding research in this species are hindered by its self-incompatibility (SI). Although the mechanisms underlying SI have been extensively studied in other plant families, SI in legumes, including alfalfa, remains poorly understood. Here, we determined that self-pollinated pollen tubes could germinate on the stigma of alfalfa, grow through the style, and reach the ovarian cavity, but the ovules collapsed ~48 h after self-pollination. A transcriptomic analysis of dissected pistils 24 h after self-pollination identified 941 differently expressed genes (DEGs), including 784 upregulated and 157 downregulated genes. A gene ontology (GO) analysis showed that the DEGs were highly enriched in functions associated with the regulation of pollen tube growth and pollen germination. A Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis indicated that pentose and glucuronate interconversion, plant hormone signal transduction, the spliceosome, and ribosomes might play important roles in SI. Our co-expression analysis showed that F-box proteins, serine/threonine protein kinases, calcium-dependent protein kinases (CDPKs), bHLHs, bZIPs, and MYB-related family proteins were likely involved in the SI response. Our study provides a catalog of candidate genes for further study to understand SI in alfalfa and related legumes.

Comparative stigmatic transcriptomics reveals self and cross pollination responses to heteromorphic incompatibility in Plumbago auriculata Lam

"Heteromorphic self-incompatibility" (HetSI) in plants is a mechanism of defense to avoid self-pollination and promote outcrossing. However, the molecular mechanism underlying HetSI remains largely unknown. In this study, RNA-seq was conducted to explore the molecular mechanisms underlying self-compatible (SC, "T × P" and "P × T") and self-incompatible (SI, "T × T" and "P × P") pollination in the two types of flowers of Plumbago auriculata Lam. which is a representative HetSI plant. By comparing "T × P" vs. "T × T", 3773 (1407 upregulated and 2366 downregulated) differentially expressed genes (DEGs) were identified, 1261 DEGs between "P × T" and "P × P" (502 upregulated and 759 downregulated). The processes in which these DEGs were significantly enriched were "MAPK (Mitogen-Activated Protein Kinases-plant) signaling pathway", "plant-pathogen interaction","plant hormone signal transduction", and "pentose and glucuronate interconversion" pathways. Surprisingly, we discovered that under various pollination conditions, multiple notable genes that may be involved in HetSI exhibited distinct regulation. We can infer that the HetSI strategy might be unique in P. auriculata. It was similar to "sporophytic self-incompatibility" (SSI) but the HetSI mechanisms in pin and thrum flowers are diverse. In this study, new hypotheses and inferences were proposed, which can provide a reference for crop production and breeding.

Single-cell RNA-seq reveals a link of ovule abortion and sugar transport in Camellia oleifera

Camellia oleifera is the most important woody oil crop in China. Seed number per fruit is an important yield trait in C. oleifera. Ovule abortion is generally observed in C. oleifera and significantly decreases the seed number per fruit. However, the mechanisms of ovule abortion remain poorly understood at present. Single-cell RNA sequencing (scRNA-seq) was performed using mature ovaries of two C. oleifera varieties with different ovule abortion rates (OARs). In total, 20,526 high-quality cells were obtained, and 18 putative cell clusters were identified. Six cell types including female gametophyte, protoxylem, protophloem, procambium, epidermis, and parenchyma cells were identified from three main tissue types of ovule, placenta, and pericarp inner layer. A comparative analysis on scRNA-seq data between high- and low-OAR varieties demonstrated that the overall expression of CoSWEET and CoCWINV in procambium cells, and CoSTP in the integument was significantly upregulated in the low-OAR variety. Both the infertile ovule before pollination and the abortion ovule producing after compatible pollination might be attributed to selective abortion caused by low sugar levels in the apoplast around procambium cells and a low capability of hexose uptake in the integument. Here, the first single-cell transcriptional landscape is reported in woody crop ovaries. Our investigation demonstrates that ovule abortion may be related to sugar transport in placenta and ovules and sheds light on further deciphering the mechanism of regulating sugar transport and the improvement of seed yield in C. oleifera.

Development of a protoplast isolation system for functional gene expression and characterization using petals of Camellia Oleifera

Protoplasts preparation and purification have been frequently used in plant genetics and breeding studies, whereas application of protoplasts in woody plants is still in its infancy. Although transient gene expression using purified protoplasts is well-documented and widely used in model plants and agriculture crops, no instance of either stable transformation or transient gene expression in the woody plant Camellia Oleifera has as of yet been reported. Here, we developed a protoplast preparation and purification method using C. oleifera petals by optimizing osmotic condition with D-mannitol and polysaccharide-degrading enzyme concentrations for petal cell wall digestion, to reach a high efficiency of protoplast productivity and viability. The achieved protoplasts yield was approximately 1.42 × 107 cells per gram of petal material and the viability of protoplasts was up to 89%. In addition, we explored influencing factors of protoplast transformation, including concentrations of PEG4000 and plasmid DNA. The transformation efficiency of 81% could be reached under the optimized condition. This protoplast isolation and transient expression system were deployed to further identify the functional regulation of C. oleifera related genes and the subcellular distribution of their encoded products. In summary, the protoplast isolation and transient expression system we established using oil-tea tree petals is an efficient, versatile and time-saving system, being suitable for gene function characterization and molecular mechanism analysis.

Transcriptome Analysis of the Late-Acting Self-Incompatibility Associated with RNase T2 Family in Camellia oleifera

The Camellia oil tree (Camellia oleifera Abel.) is an important nonwood forest species in China, and the majority of its cultivars are late-acting self-incompatibility (LSI) types. Although several studies have examined the mechanism of LSI, the process is quite complicated and unclear. In this study, pollen tube growth and fruit setting of two Camellia oil tree cultivars Huashuo (HS) and Huajin (HJ) were investigated after non and self-pollination, and transcriptomic analysis of the ovaries was performed 48 h after self-pollination to identify the potential genes implicated in the LSI of Camellia oil trees. The results showed that the fruit set of HS was significantly higher than that of HJ after self-pollination. Transcriptomic analysis revealed that plant hormone signal transduction, the phosphatidylinositol signaling system, ATP-binding cassette (ABC) transporters, reactive oxygen species (ROS) metabolism, and Ca2+ signaling were mainly contributed in the LSI of reaction of Camellia oil tree. Moreover, nine RNase T2 genes were identified from the transcriptome analysis, which also showed that CoRNase7 participated in the self-incompatibility reaction in HS. Based on phylogenetic analysis, CoRNase6 was closely related to S-RNase from coffee, and CoRNase7 and CoRNase8 were closely related to S-RNase from Camellia sinensis. The 9 RNase T2 genes successfully produced proteins in prokaryotes. Subcellular localization indicated that CoRNase1 and CoRNase5 were cytoplasmic proteins, while CoRNase7 was a plasma membrane protein. These results screened the main metabolic pathways closely related to LSI in Camellia oil tree, and SI signal transduction might be regulated by a large molecular regulatory network. The discovery of T2 RNases provided evidence that Camellia oil tree might be under RNase-based gametophytic self-incompatibility.

Identification of Camellia oleifera WRKY transcription factor genes and functional characterization of CoWRKY78

Camellia oleifera Abel is a highly valued woody edible oil tree, which is endemic to China. It has great economic value because C. oleifera seed oil contains a high proportion of polyunsaturated fatty acids. C. oleifera anthracnose caused by Colletotrichum fructicola, poses a serious threat to C. oleifera growth and yield and causes the benefit of the C. oleifera industry to suffer directly. The WRKY transcription factor family members have been widely characterized as vital regulators in plant response to pathogen infection. Until now, the number, type and biological function of C. oleifera WRKY genes are remains unknown. Here, we identified 90 C. oleifera WRKY members, which were distributed across 15 chromosomes. C. oleifera WRKY gene expansion was mainly attributed to segmental duplication. We performed transcriptomic analyses to verify the expression patterns of CoWRKYs between anthracnose-resistant and -susceptible cultivars of C. oleifera. These results demonstrated that multiple candidate CoWRKYs can be induced by anthracnose and provide useful clues for their functional studies. CoWRKY78, an anthracnose-induced WRKY gene, was isolated from C. oleifera. It was significantly down-regulated in anthracnose-resistant cultivars. Overexpression of CoWRKY78 in tobacco markedly reduced resistance to anthracnose than WT plants, as evidenced by more cell death, higher malonaldehyde content and reactive oxygen species (ROS), but lower activities of superoxide dismutase (SOD), peroxidase (POD), as well as phenylalanine ammonia-lyase (PAL). Furthermore, the expression of multiple stress-related genes, which are associated with ROS-homeostasis (NtSOD and NtPOD), pathogen challenge (NtPAL), and pathogen defense (NtPR1, NtNPR1, and NtPDF1.2) were altered in the CoWRKY78-overexpressing plants. These findings increase our understanding of the CoWRKY genes and lay the foundation for the exploration of anthracnose resistance mechanisms and expedite the breeding of anthracnose-resistant C. oleifera cultivars.

Gene coexpression analysis reveals key pathways and hub genes related to late-acting self-incompatibility in Camellia oleifera

INTRODUCTION

Self-incompatibility (SI) is an important strategy for plants to maintain abundant variation to enhance their adaptability to the environment. Camellia oleifera is one of the most important woody oil plants and is widely cultivated in China. Late acting self-incompatibility (LSI) in C. oleifera results in a relatively poor fruit yield in the natural state, and understanding of the LSI mechanism remains limited.

METHODS

To better understand the molecular expression and gene coexpression network in the LSI reaction in C. oleifera, we conducted self- and cross-pollination experiments at two different flower bud developmental stages (3-4 d before flowering and 1 d before flowering), and cytological observation, fruit setting rate (FSR) investigation and RNA-Seq analysis were performed to investigate the mechanism of the male -female interaction and identify hub genes responsible for the LSI in C. oleifera.

RESULTS

Based on the 21 ovary transcriptomes, a total of 7669 DEGs were identified after filtering out low-expression genes. Weighted gene coexpression network analysis (WGCNA) divided the DEGs into 15 modules. Genes in the blue module (1163 genes) were positively correlated with FSR, and genes in the pink module (339 genes) were negatively correlated with FSR. KEGG analysis indicated that flavonoid biosynthesis, plant MAPK signaling pathways, ubiquitin-mediated proteolysis, and plant-pathogen interaction were the crucial pathways for the LSI reaction. Fifty four transcription factors (TFs) were obtained in the two key modules, and WRKY and MYB were potentially involved in the LSI reaction in C. oleifera. Network establishment indicated that genes encoding G-type lectin S-receptor-like serine (lecRLK), isoflavone 3'-hydroxylase-like (CYP81Q32), cytochrome P450 87A3-like (CYP87A3), and probable calcium-binding protein (CML41) were the hub genes that positively responded to the LSI reaction. The other DEGs inside the two modules, including protein RALF-like 10 (RALF), F-box and pectin acetylesterase (MTERF5), might also play vital roles in the LSI reaction in C. oleifera.

DISCUSSION

Overall, our study provides a meaningful resource for gene network studies of the LSI reaction process and subsequent analyses of pollen-pistil interactions and TF roles in the LSI reaction, and it also provides new insights for exploring the mechanisms of the LSI response.

The fasciclin-like arabinogalactan proteins of Camellia oil tree are involved in pollen tube growth

Fasciclin-like arabinogalactan proteins (FLAs) are a class of highly glycosylated glycoproteins that perform crucial functions in plant growth and development. This study was carried out to further explore their roles in pollen tube growth. The results showed that seven members (CoFLA1/2/3/4/7/8/17) of the CoFLAs family were identified by sequence characteristics, and they all possessed the fasciclin 1 (FAS1) domain and H1 and H2 conserved domains. They were all located on the plasma membranes of tobacco epidermal cells, and the GPI-anchor sequences of CoFLA1/2/3/4 determined the membrane localization. In flower tissues, CoFLA2 and CoFLA8 were not expressed in the pollen tube but were expressed in the unpollinated style and ovary; the others were all expressed in the pollen tube. In the pollination-compatible style and ovary, they exhibited different expression patterns. Furthermore, all CoFLAs promoted pollen germination in vitro, while only CoFLA7 significantly promoted pollen tube elongation, and the expression of CoFLA1/3/4/7/17 in pollen tubes was regulated by CoFLA proteins. The ABA and ABA synthetic inhibitor (sodium tungstate, ST) both inhibited pollen tube elongation; however, only ST downregulated the expression of CoFLA1/7/17 and upregulated the expression of CoFLA4. Taken together, these results demonstrate that CoFLAs may be significant in pollen tube growth in C. oleifera and that some CoFLAs may participate in the regulation of ABA signaling.

Integration of semi-in vivo assays and multi-omics data reveals the effect of galloylated catechins on self-pollen tube inhibition in Camellia oleifera

Camellia oil extracted from the seeds of Camellia oleifera Abel. is a popular and high-quality edible oil, but its yield is limited by seed setting, which is mainly caused by self-incompatibility (SI). One of the obvious biological features of SI plants is the inhibition of self-pollen tubes; however, the underlying mechanism of this inhibition in C. oleifera is poorly understood. In this study, we constructed a semi-in vivo pollen tube growth test (SIV-PGT) system that can screen for substances that inhibit self-pollen tubes without interference from the genetic background. Combined with multi-omics analysis, the results revealed the important role of galloylated catechins in self-pollen tube inhibition, and a possible molecular regulatory network mediated by UDP-glycosyltransferase (UGT) and serine carboxypeptidase-like (SCPL) was proposed. In summary, galloylation of catechins and high levels of galloylated catechins are specifically involved in pollen tube inhibition under self-pollination rather than cross-pollination, which provides a new understanding of SI in C. oleifera. These results will contribute to sexual reproduction research on C. oleifera and provide theoretical support for improving Camellia oil yield in production.

The Core Jasmonic Acid-Signalling Module CoCOI1/CoJAZ1/CoMYC2 Are Involved in Jas Mediated Growth of the Pollen Tube in Camellia oleifera

Camellia oleifera is a woody edible oil species with late self-incompatibility characteristics. Previous transcriptome analysis showed that genes involved in jasmonic acid signal transduction were significantly different in self-and cross-pollinated pistils of Camellia oleifera. To investigate the relationship between jasmonate signal and self-incompatibility by studying the core genes of jasmonate signal transduction. The results showed that exogenous JA and MeJA at 1.0 mM significantly inhibited pollen tube germination and pollen tube elongation. and JA up-regulated CoCOI1, CoJAZ1, and CoMYC, the core genes of jasmonate signal transduction. Subcellular localization indicated that CoCOI1 and CoJAZ1 were located in the nucleus and CoMYC2 in the endoplasmic reticulum. The three genes exhibited tissue-specific expression pattern. CoCOI1 was significantly expressed in pollen, CoJAZ1 was significantly expressed in ovary, CoMYC2 was significantly expressed in filaments, but not in pollen. Furthermore, CoJAZ1 and CoMYC2 were highly expressing at 24 h in self-pollinated styles. These results suggested that JA signal transduction of C. oleifera was involved in the process of self-pollination, and thus in the process of plant defense. When pollen tubes grew slowly in the style, ovary may receive JA signal, which initiates the molecular mechanism of inhibiting the growth of self-pollinating pollen tubes.

Self-Incompatibility of Camellia weiningensis Y.K. Li

This study compared the pollen tube growth, fruit setting, and seed setting characteristics of Camellia weiningensis Y.K. Li. under self- and cross-pollination to identify its self-incompatibility characteristics and types. C. weiningensis pollen tube growth was observed by fluorescence and scanning electron microscopy, and a field experiment with manual pollination verified fruit and seed setting characteristics. Both self- and cross-pollinated pollen germinated from the stigma. At 72 h after cross-pollination, the pollen tube reached the style base, with tube growth showing a slow-fast-slow pattern. The tube growth speed was maximal, 343.36 μm·h−1, at 12–24 h after pollination. For self-pollination, the pollen did not germinate on the stigma 4 h before pollination. At 12–24 h after pollination, the growth rate was maximal at 263.36 μm·h−1. At 96 h, a small amount of pollen reached the style base and stagnated. The pollen tube end showed callose reactions, such as abnormal swelling, distortion, and brightness. In the field experiment, the fruit setting rate under cross-pollination was 68.5%, while that under self-pollination was 15.3%. When the fruit grew to maturity, the growth dynamics of the transverse and longitudinal diameters showed a “slow-fast-slow”, S-shaped curve. The number of aborted selfed and outcrossed seeds was 13.9 and 4.7, respectively. Thus, C. weiningensis showed self-incompatibility. The self-incompatibility reaction occurred at the style base and represented prezygotic self-incompatibility. The self-incompatibility of C. weiningensis is one of the main reasons for its low seed setting rate, which should be fully considered in cross breeding.

Integrated Transcriptome and Metabolome Analysis Reveals Key Metabolites Involved in Camellia oleifera Defense against Anthracnose

Camellia oleifera (Ca. oleifera) is a woody tree species cultivated for the production of edible oil from its seed. The growth and yield of tea-oil trees are severely affected by anthracnose (caused by Colletotrichum gloeosporioides). In this study, the transcriptomic and metabolomic analyses were performed to detect the key transcripts and metabolites associated with differences in the susceptibility between anthracnose-resistant (ChangLin150) and susceptible (ChangLin102) varieties of Ca. oleifera. In total, 5001 differentially expressed genes (DEGs) were obtained, of which 479 DEGs were common between the susceptible and resistant varieties and further analyzed. KEGG enrichment analysis showed that these DEGs were significantly enriched in tyrosine metabolism, phenylpropanoid biosynthesis, flavonoid biosynthesis and isoquinoline alkaloid biosynthesis pathways. Furthermore, 68 differentially accumulated metabolites (DAMs) were detected, including flavonoids, such as epicatechin, phenethyl caffeate and procyanidin B2. Comparison of the DEGs and DAMs revealed that epicatechin, procyanidin B2 and arachidonic acid (peroxide free) are potentially important. The expression patterns of genes involved in flavonoid biosynthesis were confirmed by qRT-PCR. These results suggested that flavonoid biosynthesis might play an important role in the fight against anthracnose. This study provides valuable molecular information about the response of Ca. oleifera to Co. gloeosporioides infection and will aid the selection of resistant varieties using marker-assisted breeding.

Microbial Communities and Functions in the Rhizosphere of Disease-Resistant and Susceptible Camellia spp

Oil tea (Camellia spp.) is endemic to the hilly regions in the subtropics. Camellia yuhsienensis is resistant to diseases such as anthracnose and root rot, while Camellia oleifera is a high-yield species but susceptible to these diseases. We hypothesize that differences in the rhizosphere microbial communities and functions will elucidate the resistance mechanisms of these species. We used high-throughput sequencing over four seasons to characterize the rhizosphere microbiome of C. oleifera (Rhizo-Sus) and C. yuhsienensis (Rhizo-Res) and of the bulk soil control (BulkS). In Rhizo-Res, bacterial richness and diversity (Shannon index) in autumn and winter were both higher than that in Rhizo-Sus. In Rhizo-Res, fungal richness in autumn and winter and diversity in summer, autumn, and winter were higher than that in Rhizo-Sus. The seasonal variations in bacterial community structure were different, while that of fungal community structure were similar between Rhizo-Res and Rhizo-Sus. Gram-positive, facultatively anaerobic, and stress-tolerant bacteria were the dominant groups in Rhizo-Sus, while Gram-negative bacteria were the dominant group in Rhizo-Res. The significant differences in bacterial and fungal functions between Rhizo-Sus and Rhizo-Res were as follows: (1) in Rhizo-Sus, there were three bacterial and four fungal groups with plant growth promoting potentials, such as Brevibacterium epidermidis and Oidiodendron maius, and one bacterium and three fungi with pathogenic potentials, such as Gryllotalpicola sp. and Cyphellophora sessilis; (2) in Rhizo-Res, there were also three bacteria and four fungal groups with plant-growth-promoting potentials (e.g., Acinetobacter lwoffii and Cenococcum geophilum) but only one phytopathogen (Schizophyllum commune). In summary, the rhizosphere microbiome of disease-resistant C. yuhsienensis is characterized by a higher richness and diversity of microbial communities, more symbiotic fungal communities, and fewer pathogens compared to the rhizosphere of high-yield but disease-susceptible C. oleifera.

CoNPR1 and CoNPR3.1 are involved in SA- and MeSA- mediated growth of the pollen tube in Camellia oleifera

Salicylic acid (SA) is a plant hormone involved in a series of growth, development, and stress responses in plants. Nonexpressor of pathogenesis-related genes 1 (NPR1) is the core regulatory gene in the process of SA-mediated systemic acquired resistance (SAR). Whether NPR1 is involved in pollen tube growth mediated by SA and its derivative MeSA (methyl salicylate) remains to be explored. Here, we found that the contents of endogenous SA and MeSA in self- or cross-pollinated pistils changed significantly, and exogenous SA and MeSA significantly promoted pollen germination and pollen tube elongation of Camellia oleifera at lower concentrations. CoNPR1, CoNPR3.1, CoNPR3.2, and CoNPR5 were identified, and they were all located in the nucleus. A high level of consistency was observed across the phylogenetic relationships, gene structures, and functional domains, indicating a clear division of function, as observed in other species. The expression levels of CoNPR1, CoNPR3.1, CoNPR3.2, and CoNPR5 in self- and cross-pollinated pistils had certain regularity. Furthermore, they exhibited tissue-specific expression pattern. CoNPR1 and CoNPR3.1 were expressed in pollen tubes, whose expression was regulated by SA or MeSA, and their expression patterns were basically consistent with the trend of pollen germination. These results indicate that SA and MeSA are involved in the pollen tube growth of C. oleifera, and CoNPRs may play an important role therein.

Comparative study on fruit development and oil synthesis in two cultivars of Camellia oleifera

BACKGROUND

The oil-tea tree (Camellia oleifera Abel.) is a woody tree species that produces edible oil in the seed. C. oleifera oil has high nutritional value and is also an important raw material for medicine and cosmetics. In China, due to the uncertainty on maturity period and oil synthesis mechanism of many C. oleifera cultivars, growers may harvest fruits prematurely, which could not maximize fruit and oil yields. In this study, our objective was to explore the mechanism and differences of oil synthesis between two Camellia oleifera cultivars for a precise definition of the fruit ripening period and the selection of appropriate cultivars.

RESULTS

The results showed that 'Huashuo' had smaller fruits and seeds, lower dry seed weight and lower expression levels of fatty acid biosynthesis genes in July. We could not detect the presence of oil and oil bodies in 'Huashuo' seeds until August, and oil and oil bodies were detected in 'Huajin' seeds in July. Moreover, 'Huashuo' seeds were not completely blackened in October with up to 60.38% of water and approximately 37.98% of oil in seed kernels whose oil content was much lower than normal mature seed kernels. The oil bodies in seed endosperm cells of 'Huajin' were always higher than those of 'Huashuo' from July to October.

CONCLUSION

Our results confirmed that C. oleifera 'Huashuo' fruits matured at a lower rate compared to 'Huajin' fruits and that 'Huajin' seeds entered the oil synthesis period earlier than 'Huashuo' seeds. Moreover, 'Huashuo' fruits did not mature during the Frost's Descent period (October 23-24 each year).

Caspase-like proteases and the phytohormone cytokinin as determinants of S-RNAse-based self-incompatibility-induced PCD in Petunia hybrida L

S-RNAse-based self-incompatibility (SI) in petunia (Petunia hybrida L.) is a self-/non-self-recognition system underlying the pistil rejection of self-pollen. Using different methods, including a TUNEL assay, we have recently shown that programmed cell death (PCD) is a factor of the SI in petunia. Here, we show that the growth of self-incompatible pollen tubes in the style tissues during 4 h after pollination is accompanied by five-sixfold increase in a caspase-like protease (CLP) activity. Exogenous cytokinin (CK) inhibits the pollen tube growth and stimulates the CLP activity in compatible pollen tubes. The actin depolymerization with latrunculin B induces a sharp drop in the CLP activity in self-incompatible pollen tubes and its increase in compatible pollen tubes. Altogether, our results suggest that a CLP is involved in the SI-induced PCD and that CK is a putative activator of the CLP. We assume that CK provokes acidification of the cytosol and thus promotes the activation of a CLP. Thus, our results suggest that CK and CLP are involved in the S-RNAse-based SI-induced PCD in petunia. Potential relations between these components in PCD signaling are discussed.