In this study, LCM was used to spefically isolate SCC of two clementine genotypes differing for the SI response. 'Comune' is a widespread self-incompatible variety, while 'Monreal' is a self-compatible variety originated from a spontaneous bud mutation [17
]. The microdissection of the SCC allowed to perform a highly specific study of the transcriptome of the cells implicated in the interaction between pistil and pollen tubes, with the main aim of identifying candidate genes involved in self-pollen rejection. The results of microarray analysis suggested that the differential regulation of few specific transcripts might have lead to the breakdown of SI in 'Monreal'. Based on functional information retrieved from the databases, these genes do not show any clear functional enrichment, and many of them have no annotation, making it difficult to compare our results with SI systems characterized on model plants. On the other hand, our experiment provided a new set of transcripts that are very likely to play a key role during the progamic phase in citrus. Searches in the databases revealed that most of these genes are not flower specific, suggesting that probably the mutation leading to the breakdown of SI did not affected the S-locus determinants. However, it is known that non S-locus genes are implicated in SI [30
]. Most of the preferentially expressed genes were low-represented in EST databases and showed a weak expression in whole styles with stigmas before the SI reaction occurred. This indicates the usefulness of the LCM in the identification of highly specific and/or low expressed genes.
Flowers for LCM were sampled one day after pollination, when pollen was just germinating. Self-pollinations were performed to assess whether the presence of pollen in the stigma might have elicited a differential response of the SCC in self-compatible vs. self-incompatible conditions. So, while pollen tubes where still in the stigma, we targeted the rectangular SCC located in the upper/middle style, which is the site where incompatibility occurs in 'Comune', without contamination of pollen tubes. Although we did not analyze mRNAs of microdissected SCC of non-pollinated flowers, qRT-PCR demonstrated that slight differences in gene expression were also present in whole styles with stigmas before pollination, indicating that, at least in the cases of cit.7568, cit.11563, cit.5456 and cit.5776, the different mRNA levels in the SCC between 'Comune' and 'Monreal' were not induced by the pollen germination in the stigma.
Among the identified genes, our analysis focused on four unigenes over-represented in 'Comune' (cit.7568, cit.11563, cit.5456 and cit.5776). A time course analysis coupled to histological observation of pollen tube elongation was performed to correlate pollen tube behaviour with the difference in gene expression between 'Comune' and 'Monreal'. Until the 4th DAP we could not observe any clear differences in the mRNA level of the 4 analyzed genes, and no clear differences were evident in pollen tube behaviour between the two varieties. Form the 5th DAP we observed an impressive up-regulation in the self-incompatible combination. Selected candidate genes showed a clear differential expression (even > 100 fold change) during pollen - style interaction (Figure ), in agreement with histological observation (pollen tube growth in 'Monreal' and no growth in 'Comune'). The peak of up-regulation was evidenced in concomitance with the pollen tube arrest, suggesting that all the analyzed genes have a key role in the stop of pollen tube elongation. The up-regulation was clearly induced by pollen tubes in the styles, as confirmed with the comparison of the expression levels of non-pollinated and self-pollinated flowers at T6 (Figure ). The candidate genes were weakly expressed in pollen tubes and no differences in the mRNA levels were observed between 'Comune' and 'Monreal' (Figure ). As a result, it is unlikely that the transcriptome of pollen tubes influenced the mRNA levels detected in the time course analysis. In fact, our expression data demonstrate that the drastic changes in mRNA levels occurred in the stylar tissues as a response to self-incompatible pollen tubes. Also, the drastic up-regulation of the four genes should not represent a downstream response to SI, since these genes are already differentially regulated in SCC before SI occurs.
Bioinformatic analysis provided useful information to hypothesize the role of the four unigenes. cit.7568 probe set matched to a putative F-box protein gene sharing homology to an Arabidopsis F-box (At5g04010) annotated as non-specific [28
]. This protein belongs to the C2 group of the F-box superfamily, of which SLY1 belongs. However, the similarity with SLY1 regards the F-box domain, while the C-terminus, responsible for targeting the substrate for ubiquitination, does not share any homology with SLY1.
F-box protein superfamily is one of the largest in plants [31
]. These proteins are key regulators of proteolysis, conferring specificity to the SCF (Skp1, Cullin, F-box) E3 ubiquitin ligase complex, which is responsible for recognizing the substrate for ubiquitin-mediated protein degradation. Despite the fact that it is not possible to correlate the role of the novel clementine F-box gene with any characterized orthologs, database searches and transcriptional data revealed that the clementine F-box gene was not previously identified in clementine cDNA libraries and evidenced highly specific expression patterns, since it was almost 250 fold up-regulated in SCC (Figure ) and about 50 to 300 fold up-regulated in whole styles with stigmas (Figure ) in concomitance with the arrest of pollen tube elongation. This drastic up-regulation, and the absence of identical ESTs in databases support the high specificity of this gene and point out to its possible involvement in highly specific proteolytic events occurring in the style during the self-incompatible response. Studies on the families where SI system have been already characterized (namely Brassicaceae, Solanaceae
) showed that proteolysis has a key role during self-pollen rejection. Usually proteolytic events follow the initial SI signal perception leading to the eventual death of the male gametophyte [33
]. Functional analysis will be necessary to shed light on the role of the clementine F-box during pollen-pistil interaction.
Another striking finding was the identification of the three putative Asp-rich protein encoding genes, cit.11563, cit.5456 and cit.5776, up regulated in 'Comune'. In addition to the high similarity in their primary structure, they co-localize in the clementine genome (Figure ). The clustered genes displayed similar expression patters (Figure ) which might indicate that they are commonly regulated as already reported for other clusters [34
]. Although their function has not been investigated, it is known that the expression of the Arabidopsis
homologous At1g47400 is sharply up regulated in plants exposed to 3-(30,40-dihydroxyphenyl)-L-alanine (L-DOPA), a phytotoxic allelochemical [36
]. In other studies, this gene turns out to be strongly induced by iron deficiency [37
] and by treatment with the polycyclic aromatic hydrocarbon phenanthrene [38
]. These results support the hypothesis that the genes encoding the Asp-rich proteins might be triggered in response to different types of stresses. Since other information is still lacking, we attempt to assign a putative function to the Citrus
Asp-rich proteins and also propose a model which overall might explain the regulation of SI in the 'Comune' variety. Because of their richness of aspartic acid residues, the Asp-rich proteins are supposed to act as novel Ca2+
"entrapping" proteins. Although they do not show sequence similarity with other well known Ca2+
interacting proteins, such as calsequestrin, calreticulin and calmodulin, the Asp-rich proteins share with those proteins the aspartate residue abundance which has been related with the protein ability of Ca2+
]. Therefore, assuming that Ca2+
levels play a decisive role during pollen-pistil interaction, as already reported for several plant species [40
], the up-regulation of the Asp-rich encoding genes observed in 'Comune' might enhance the amount of proteins functioning as Ca2+
-trap elements, and lead to an exceptional decrease in Ca2+
availability, thus contributing to switch off the signal cascade usually induced by the increase in cytosolic Ca2+
concentration or to the alteration of Ca2+
gradient needed for pollen tube elongation. Pollen tube growth could represent, among others, the downstream physiological process dramatically affected by this missing triggering event. To support this hypothesis, it is worthwhile to mention that the aforesaid At1g47400 gene is up regulated in a T-DNA insertional mutant of a P-type ATPase cation pump, the MALE GAMETOGENESIS IMPAIRED ANTHERS
); the mutant shows reduced male fertility and imbalanced cation homeostasis [41
]. Moreover, a Ca2+
pump (auto-inhibited Ca2+
-ATPase - ACA), which presumably pumps Ca2+
out the cytosol [42
], is differentially regulated in the two genotypes during pollen-pistil interaction showing an up regulation in the 'Comune' genotype [15
]. Further mandatory work will be undertaken to validate both the supposed role and the functioning model of pollen-pistil interaction in Citrus
genotypes and, if they are proved, this might represent a case in which specific regulatory mechanism involving different loci rather than the S
-locus could be co-responsible of the SI determination.
The integration of the transcriptomic data with the synteny analysis denoted a specific genome region containing a cluster of genes activated during self-pollen rejection (Figure ). Specifically, the Asp-rich protein genes are linked to a DELLA gene which was previously isolated in the self-pollinated 'Comune' styles with stigmas [15
] and showed a preferential expression in the self-incompatible genotype. This raise intriguing questions about the possibility that the non-homologous genes located in this genome region might contribute to a common function related to self-pollen rejection. Examples of co-expressed and functionally related gene clusters in eukaryotes have emerged over the last decade [34
] and are likely to increase with the huge amount of data coming from the genome projects. The most investigated operon-like organizations in plants are secondary metabolic pathways [44
], mostly implicated in plant defence response. Clustering appears to have occurred de novo
through some form of convergent evolutionary process [43
]. In our case, it is unlikely that the genes are clustered by chance, since this cluster is conserved in at least two other plant species, cacao and castor bean (Figure ). The collinearity of the citrus genome segment comprising the Asp-rich protein genes and DELLA with other segments of unrelated genomes support the hypothesis that selection might have favoured the linkage of these genes. The advantage of clustering is related with the fact that tightly linked genes might be co-regulated at the levels of nuclear organization and/or chromatin [43
]. The co-localization of the three up-regulated Asp-rich protein genes as well as DELLA in the scaffold 9 of the clementine genome v0.9 suggests that the mutation leading to self compatibility probably affected the functionality of tightly linked genes. Further transcriptional analyses will be carried out on the other genes surrounding the Asp-rich protein genes to study their possible role during different stages of the pollen-pistil interaction.