Comparative study on cytogenetics and transcriptome between diploid and autotetraploid rice hybrids harboring double neutral genes

Interspecific Hybridization Enhanced Tolerance to Salinity and Cadmium Stress Through Modifying Biochemical, Physiological, and Resistance Gene Levels, Especially in Polyploid Rice: A Sustainable Way for Stress-Resilient Rice

Polyploid plants exhibit strong resistance to salt and cadmium (Cd) stress, which can adversely affect their growth, reducing crop quality and yield. Transcriptome analysis, antioxidant enzymatic activities, physiological measurements of reactive oxygen species, and heterosis analysis were performed on hybrids with neo-tetraploid rice and its progenitors. The results showed that diploid hybrids had fluctuating yields in early and late seasons, while tetraploid hybrids had consistent grain yield throughout. Transcriptome analysis revealed that gene expression related to sugar metabolism processes increased in tetraploid hybrids. Transcriptome analysis revealed several genes associated with heterosis and stress, including OsEAF6, which is associated with heterosis, and OsCIPK14, which is involved in defense signalling pathways. Furthermore, compared to the parents, hybrids have a much higher number of genes associated with abiotic stress. Consequently, diploid and tetraploid hybrids were treated with Cd (0 and 100 µM) and NaCl (200 mM) in the present study. Under Cd toxicity, the levels of carotenoids were reduced by 33.31% and 45.59%, while the levels of chlorophyll a declined by 16.00% and 27.81% in tetraploid and diploid hybrids, respectively, compared to the control. Tetraploid hybrids had the highest germination rate under salt stress and the lowest Cd uptake compared to diploid hybrids and their parents. In general, the activities of antioxidant enzymes exhibited a considerable drop, whereas the levels of H2O2 and MDA showed a remarkable increase in parents compared to hybrids. Under cadmium toxicity, the expression of OsERF1 in tetraploid rice was increased, and OsABCC1 and OsHMA3 were highly expressed in neo-tetraploid rice. Interspecific hybrid (indica and japonica) displayed enhanced tolerance to cadmium and salinity stress, potentially serving as a natural resource to improve rice resilience. These findings provide a basis for understanding polyploid rice's gene expression pattern, environmental tolerance, and heterosis.

Physiologic and molecular responses of indica-japonica subspecies tetraploid rice seed germination to ion beams

Ionizing radiation can not only reduce the yield of rice but also cause rice toxicity, and consumption of this kind of rice threatens human health. Moreover, the production and application of freon has further caused a hole in the earth's ozone layer, increasing the amount of ionizing radiation from the sun affecting rice. To select and breed new radiation-resistant rice varieties, dry seeds of the indica-japonica subspecies of tetraploid rice subjected to different doses of ionizing radiation were investigated for their responses during germination. The results showed that the relative water absorption, seed vigour and GA₃ content sharply decreased in response to three different doses of ionizing radiation, and the regulation of the expression of genes related to α-amylase synthesis and gibberellin metabolism was disrupted. Moreover, the degree of inhibition increased with increasing dose. Notably, under 3.0 × 10¹⁷ ions/cm² radiation, an upregulation of OsGA3ox2 expression resulted in a sharp increase in GA₃ content in the indica-japonica tetraploid rice, and upregulated expression of OsAmy3A and OsAmy3D resulted in sharp increase in α-amylase activity, water absorption, and sucrose and fructose contents, which resulted in the seed vigour being greater than that of its parents. The results indicate that additional research on the physiological and molecular features of indica-japonica tetraploid rice seed germination in response to ionizing radiation is needed.

Developmental Differences between Anthers of Diploid and Autotetraploid Rice at Meiosis

Newly synthetic autotetraploid rice shows lower pollen fertility and seed setting rate relative to diploid rice, which hinders its domestication and breeding. In this study, cytological analysis showed that at meiosis I stage, an unbalanced segregation of homologous chromosomes, occurred as well as an early degeneration of tapetal cells in autotetraploid rice. We identified 941 differentially expressed proteins (DEPs) in anthers (meiosis I), including 489 upregulated and 452 downregulated proteins. The DEPs identified were related to post-translational modifications such as protein ubiquitination. These modifications are related to chromatin remodeling and homologous recombination abnormalities during meiosis. In addition, proteins related to the pentose phosphate pathway (BGIOSGA016558, BGIOSGA022166, and BGIOSGA028743) were downregulated. This may be related to the failure of autotetraploid rice to provide the energy needed for cell development after polyploidization, which then ultimately leads to the early degradation of the tapetum. Moreover, we also found that proteins (BGIOSGA017346 and BGIOSGA027368) related to glutenin degradation were upregulated, indicating that a large loss of glutenin cannot provide nutrition for the development of tapetum, resulting in early degradation of tapetum. Taken together, these evidences may help to understand the differences in anther development between diploid and autotetraploid rice during meiosis.

Thermo-Sensitive Genic Male Sterile Lines of Neo-Tetraploid Rice Developed through Gene Editing Technology Revealed High Levels of Hybrid Vigor

Neo-tetraploid rice, which developed from the progenies of autotetraploid hybrid by our research group, is a useful germplasm with high fertility and strong heterosis when they crossed with other autotetraploid rice lines. The CRISPR/Cas9-mediated TMS5 gene editing system has been widely used in diploid rice, but there are few reports in tetraploid rice. Here, we used CRISPR/Cas9 technology to edit the TMS5 gene, which is a temperature sensitive gene controlling the fertility in diploid rice, in neo-tetraploid rice to develop male sterile lines. Two mutant lines, H2s and H3s, were developed from the gene editing and displayed characteristics of thermo-sensitive genic male sterility. The daily mean temperatures of 23 °C to 26 °C were found to be critical for sterility (restrictive temperature) in H2s and H3s under both controlled (growth chambers) and natural growing conditions (field). Cytological observation showed the anther dysplasia appeared later in H2s and H3s than that of the TMS5 mutant of diploid rice (E285s) under the same conditions. Then these mutant lines, H2s and H3s, were crossed with tetraploid rice to generate F₁ hybrids, which exhibited obvious advantages for effective number of panicles, total grains and seed setting. The high levels of hybrids heterosis were maintained for several generations that can save seed cost. Our research provides an effective way of developing thermo-sensitive genic male sterility (TGMS) lines of tetraploid rice using gene editing, which will accelerate the utilization of polyploid heterosis.