Genome duplication effects on pollen development and the interrelated physiological substances in tetraploid rice with polyploid meiosis stability

Variability of morphological features and nuclear DNA content in haploids and doubled haploids of androgenic callus lines of rice (<I>Oryza sativa</I> L.)

The work is relevant for understanding evolutionary processes in plant species. Twelve callus lines with multiple regeneration of haploids and doubled haploids were obtained in F1 hybrids of Oryza sativa L. through in vitro androgenesis. Intracallus variability of the morphological features of haploids was often accompanied by a decrease in the values of morphological features with an increase in the serial number (p < 0.05). The number of panicles on a plant and the number of flowers on a panicle on two callus lines in the second or third group were increased. No variability was detected in five callus lines, i.e., such a phenomenon was not a rule. The nuclear DNA content of doubled haploids in four groups of the same callus line was 1.03– 1.09 pg, and for haploids it was 0.53–0.58 pg. Intracallus variability of nuclear DNA content was detected between groups of haploids of the same line and among doubled haploids of the same line. Significant differences were found between the haploids of one callus line and the three other callus lines of the Sadko × Kuboyar hybrid towards an increase of nuclear DNA content (p < 0.0015). The theoretical possibility of the appearance of intraspecific variability among plants with a small number of chromosomes is considered. A scheme of genomic reorganization is proposed for such species: initial plant (2n) → aneuhaploid plants (n + 1) → megasporogenesis and microsporogenesis of the 0-n type, formation of fertile pollen (n + 1) → diploid plant (2n + 2). Aneuhaploid evolution explains the intraspecific variability of chromosome numbers among plant species with low ploidy. Aneuploid technologies can help in the artificial formation of new polyploid crops, and rice is given a primary role. 

A New Way of Rice Breeding: Polyploid Rice Breeding

Polyploid rice, first discovered by Japanese scientist Eiiti Nakamori in 1933, has a history of nearly 90 years. In the following years, polyploid rice studies have mainly focused on innovations in breeding theory, induction technology and the creation of new germplasm, the analysis of agronomic traits and nutritional components, the study of gametophyte development and reproduction characteristics, DNA methylation modification and gene expression regulation, distant hybridization and utilization among subspecies, species and genomes. In recent years, PMeS lines and neo-tetraploid rice lines with stable high seed setting rate characteristics have been successively selected, breaking through the bottleneck of low seed setting rate of polyploid rice. Following, a series of theoretical and applied studies on high seed setting rate tetraploid rice were carried out. This has pushed research on polyploid rice to a new stage, opening new prospects for polyploid rice breeding.

Integration of small RNA, degradome and proteome sequencing in Oryza sativa reveals a delayed senescence network in tetraploid rice seed

Seed of rice is an important strategic resource for ensuring the security of China's staple food. Seed deterioration as a result of senescence is a major problem during seed storage, which can cause major economic losses. Screening among accessions in rice germplasm resources for traits such as slow senescence and increased seed longevity during storage is, therefore, of great significance. However, studies on delayed senescence in rice have been based mostly on diploid rice seed to date. Despite better tolerance have been verified by the artificial aging treatment for polyploid rice seed, the delayed senescence properties and delayed senescence related regulatory mechanisms of polyploid rice seed are rarely reported, due to the lack of polyploid rice materials with high seed set. High-throughput sequencing was applied to systematically investigate variations in small RNAs, the degradome, and the proteome between tetraploid and diploid rice seeds. Degradome sequencing analysis of microRNAs showed that expression of miR-164d, which regulates genes encoding antioxidant enzymes, was changed significantly, resulting in decreased miRNA-mediated cleavage of target genes in tetraploid rice. Comparisons of the expression levels of small RNAs (sRNAs) in the tetraploid and diploid libraries revealed that 12 sRNAs changed significantly, consistent with the findings from degradome sequencing. Furthermore, proteomics also showed that antioxidant enzymes were up-regulated in tetraploid rice seeds, relative to diploids.

Fertile Tetraploids: New Resources for Future Rice Breeding?

Ploidy manipulation is an efficient technique for the development of novel phenotypes in plant breeding. However, in rice (Oryza sativa L.), severe seed sterility has been considered a barrier preventing cultivation of autotetraploids since the 1930s. Recently, a series of studies identified two fertile autotetraploids, identified herein as the PMeS (Polyploid Meiosis Stability) and Neo-Tetraploid lines. Here, we summarize their characteristics, focusing on the recovery of seed fertility, and discuss potential future directions of study in this area, providing a comprehensive understanding of current progress in the study of fertile tetraploid rice, a classical, but promising, concept for rice breeding.

Genome duplication improves rice root resistance to salt stress

BACKGROUND

Salinity is a stressful environmental factor that limits the productivity of crop plants, and roots form the major interface between plants and various abiotic stresses. Rice is a salt-sensitive crop and its polyploid shows advantages in terms of stress resistance. The objective of this study was to investigate the effects of genome duplication on rice root resistance to salt stress.

RESULTS

Both diploid rice (HN2026-2x and Nipponbare-2x) and their corresponding tetraploid rice (HN2026-4x and Nipponbare-4x) were cultured in half-strength Murashige and Skoog medium with 150 mM NaCl for 3 and 5 days. Accumulations of proline, soluble sugar, malondialdehyde (MDA), Na(+) content, H(+) (proton) flux at root tips, and the microstructure and ultrastructure in rice roots were examined. We found that tetraploid rice showed less root growth inhibition, accumulated higher proline content and lower MDA content, and exhibited a higher frequency of normal epidermal cells than diploid rice. In addition, a protective gap appeared between the cortex and pericycle cells in tetraploid rice. Next, ultrastructural analysis showed that genome duplication improved membrane, organelle, and nuclei stability. Furthermore, Na(+) in tetraploid rice roots significantly decreased while root tip H(+) efflux in tetraploid rice significantly increased.

CONCLUSIONS

Our results suggest that genome duplication improves root resistance to salt stress, and that enhanced proton transport to the root surface may play a role in reducing Na(+) entrance into the roots.

Different responses to N(+) beam implantation between diploid and autotetraploid rice

The objective of the study is to investigate the biological effects of N(+) beam implantation in different ploidy rice. N(+) beam implantation had increase effect in tillers number, spikelet fertility, grain yield per plant, si-phellem cell size, photosynthetic rate, transpiration rate, flag leaf dry weight, flag leaf culm dry weight, stomatal length, vascular bundle area, and protein and starch content and decrease effect in 1,000-grain weight, stomatal width and chlorophyll, calcium, sodium, and zinc content for all rice lines. N(+) beam implantation had opposite effect on diploid and autotetraploid rice lines in vascular bundle area, stomatal complexes areas, epidermal cell size, era length, area of air spaces, midrib length, papilla number, flag leaf length, flag leaf width, flag leaf area, and cadmium, copper, ferrum, magnesium and phosphorus content. Twenty traits of diploid line and ten traits of autotetraploid line are significantly increased by N(+) beam in this experiment, ranging from 6.8 to 362.7 % in diploid line and 7.9 to 131.7 % in autotetraploid line. Six traits of diploid lines and 15 traits of autotetraploid line are significantly decreased by N(+) beam implantation in this experiment, ranging from 8.9 to 87.4 % in diploid line and 5.6 to 88.5 % in autotetraploid line.

Transcriptomic analysis of differentially expressed genes during anther development in genetic male sterile and wild type cotton by digital gene-expression profiling

BACKGROUND

Cotton (Gossypium hirsutum) anther development involves a diverse range of gene interactions between sporophytic and gametophytic tissues. However, only a small number of genes are known to be specifically involved in this developmental process and the molecular mechanism of the genetic male sterility (GMS) is still poorly understand. To fully explore the global gene expression during cotton anther development and identify genes related to male sterility, a digital gene expression (DGE) analysis was adopted.

RESULTS

Six DGE libraries were constructed from the cotton anthers of the wild type (WT) and GMS mutant (in the WT background) in three stages of anther development, resulting in 21,503 to 37,352 genes detected in WT and GMS mutant anthers. Compared with the fertile isogenic WT, 9,595 (30% of the expressed genes), 10,407 (25%), and 3,139 (10%) genes were differentially expressed at the meiosis, tetrad, and uninucleate microspore stages of GMS mutant anthers, respectively. Using both DGE experiments and real-time quantitative RT-PCR, the expression of many key genes required for anther development were suppressed in the meiosis stage and the uninucleate microspore stage in anthers of the mutant, but these genes were activated in the tetrad stage of anthers in the mutant. These genes were associated predominantly with hormone synthesis, sucrose and starch metabolism, the pentose phosphate pathway, glycolysis, flavonoid metabolism, and histone protein synthesis. In addition, several genes that participate in DNA methylation, cell wall loosening, programmed cell death, and reactive oxygen species generation/scavenging were activated during the three anther developmental stages in the mutant.

CONCLUSIONS

Compared to the same anther developmental stage of the WT, many key genes involved in various aspects of anther development show a reverse gene expression pattern in the GMS mutant, which indicates that diverse gene regulation pathways are involved in the GMS mutant anther development. These findings provide the first insights into the mechanism that leads to genetic male sterility in cotton and contributes to a better understanding of the regulatory network involved in anther development in cotton.