| Allopolyploidization is an essential process of speciation, also is considered one important driving force of plant evolution. Allopolyploids often show superiority relative to their progenies, such as larger organs and enhanced stress tolerance. The successful cross and subsequent genome duplication between C. hystrix and cucumber was made and a newly synthesized allotetraploid in Cucumis, C. ×hytivus (Cucumis ×hytivus Chen and Kirkbride,2n = 38),was obtained. C.×hytivus represents a breakthrough in the wide hybridization of Cucumis and is a valuable resource for the improvement of cucumber.However, newly synthesized allopolyploid suffer from the "genomic shock", which leads to the extensive genetic and epigenetic changes. These changes can cause significant effect on the phenotypes of allopolyploids. On top of that, the phenotypic stable C. xhytivus obtained through many generations' self-pollination is a very useful model system to study the effect of allopolyploidization on speciation and phenotypic variation for their clear genetic relationship. C. ×hytivus displays yellow-green leaf and can be consistently observed in the later self-pollinated generations, suggests that it is stably inherited in C. ×hytivus.Chlorophyll is essential for photosynthesis. The physiological consequence of the chlorophyll shortage in C. ×hytivus remains unclear. microRNAs are a group of small RNAs that were found and extensively studied recently. microRNAs have been stated to play crucial roles in the phenotypic variation in allopolyploids. Therefore,with the focus on the yellow-green leaf, we try to the reveal the photosynthetic physiological and epigentic consequense of allopolyploidization on phenotypic variation in C. xhytivus, the main results are as follows:1. Allopolyploidy in Cucumis leads to the divergence of phenotypes in response to low light and extended photoperiodsIn order to get knowledge of the effect of allopolyploid on phenotypes of C. ×hytivus,we characterized the morphology and photosynthesis of C. ×hytivus and its parents. The results show that the C. ×hytivus only partly is an intennediate hybrid and it has not only chlorophyll deficiency, which recovers during leaf development, but also lower carotenoid content. However, the growth and photosynthesis of C. ×hytivus seems to be unaffected by the low pigments content, indicated by the growth and chlorophyll fluorescence parameters,e.g., Fv/Fm. The consistent order of Psat and DWs being lowest in C. hystrix, medium in C.×hy,tivus and highest in C. sativus suggests the three species to have genetically different photosynthetic efficiency, which relates well with the natural habitats of the parent species and the hybrid as intermediate. We also investigated their photosynthetic response to low light and long photoperiod. The shade tolerance of C. hystrix was confirmed by the high total chlorophyll content, low ratio of chl alb, low Psat and Rd, and the response under low light treatment. C. ×hytivus appears to be inhibited by the low light levels to the same extent as the cultivated 'BeijingJietou',which indicates neither improvement of shade tolerance nor hypothetical heterosis effect in C. xhytivus. However, unexpectedly, the PSII of C. hystrix was affected by the long photoperiod in the long term, suggested by the decrease of Fv/Fm. This sensitivity towards day length has not been passed on to C.xhytivus.2. Evaluation of genotypic variation during leaf development in four Cucumis genotypes and their response to high light conditionsPhysiological responses to differences in light intensity were studied in four genotypes of Cucumis with differences in leaf greenness. The four genotypes were C. ×hytivus(synthesized allotetraploid, yellow-green); its parents, C, hystrix (wild Cucumis species,dark green) and C. sativus L. 'BeijingJietou' (cultivated cucumber, green); and M1 (a chlorophyll deficient mutant of cultivated cucumber, yellow-green). The plants were subjected to a photosynthetic photon flux density (PPFD) of either 800 ?mol m-2 s-1 or 200?mol m-2 s-1 in climate chambers for 20 days. Plant growth, chlorophyll (Chl) content, gas exchange, Chl fluorescence parameters and carbohydrate partitioning in the four genotypes were studied. The four genotypes showed different amounts of Chl accumulation and the genotypic differences led to divergent photosynthetic capabilities, carbohydrate partitioning and photosynthetic responses to high light. However, none of the four genotypes showed significant light inhibition. The dependency of chorophyll to photosynehsis differed among genotypes, which can be related to the disturbances caused by allopolyploidy and mutantion. Despite the similar leaf chlorosis and delay chlorophyll accumulation in C.×hytivus and M1, the differed photosynthetic response to high light suggests that the leaf chlorosis in C. ×hytivus is essentially different from common leaf chlorosis mutants. The reduction of chorophyll content in C. ×hytivus under high light indicates that there may be an unknown endogenous regulation mechanism that reduced the chlorophyll content of C.×hytivus to avoid excess light absorption. Moreover,the yellow-green young leaves of C.×hytivus could be due to this endogenous regulation in order to protect the young leaves from photoinhibition.3. The role of microRNAs in allopolyploidization in CucumisIn order to find out how the distinct small RNAs profiles were maintained in the C.×hytivus that contain two distinct genomes and how these small RNAs affect gene expression and phenotypes, we used high-throughput sequencing to compare miRNA expression profiles between C. xhytivus and its parents and between different leaf developmental stages of C. ×hytivus. Following high-throughput sequencing, 546 conserved and 287 novel miRNAs were identified. The expression levels of four miRNAs and five target genes obtained by quantitative real-time PCR (qRT-PCR) were consistent with the sequencing results. We detected that 15 miRNAs were divergently expressed between the parent species, and 24 miRNAs were differentially expressed in C. ×hytivus compared to either of its parents or both, which suggest effect of allopolyploidization on miRNAs accumulation. Remarkably, 26 conserved and 13 novel miRNAs differed in expression between the young and mature leaves of C. ×hytivus, indicates an essential role of miRNA-mediate regulation of leaf development following allopolyploidization. Previous study have demonstrated that allopolyploidization affected the chlorophyll biosynthesis in C. ×hytivus, leading to yellow-green leaves. Meanwhile, five of the 26 conserved miRNAs were from the miR171 family, which has been reported to play a role in the inhibition of chlorophyll biosynthesis. This study enriches the context of allopolyploidy effect on miRNAs and lays a foundation for the elucidation of the miRNA-mediated regulatory mechanism in chlorophyll biosynthesis. |
PDF Full Text Request