Maomao Li

Objective Nitrogen is an important nutrient element that affects the growth，development，yield and quality of rice.Improving the low nitrogen tolerance of rice varieties contributes to the green and efficient development of agricultural production.The identification and screening of rice germplasms with low nitrogen tolerance is a key method for cultivating varieties that can withstand low nitrogen conditions. Method A total of 3，000 rice germplasms from southern rice-growing areas were used as experimental materials.From 2017 to 2018，biological yield per plant and grain weight per plant at the maturity stage were used as indexes to identify low nitrogen tolerance.Combined with field performance，624 low nitrogen tolerant rice germplasms were preliminarily selected.In 2019，the panicle and yield correlation traits of the primary screening germplasm were determined for the accurate identification of low nitrogen tolerance.A total of 153 germplasms demonstrating excellent yield performance and low nitrogen tolerance were screened.On this basis，the plant height，number of effective tillers per plant，panicle length，spikelets per panicle，full spikelets per panicle，1 000-grain weight，grain density and theoretical yield were investigated in 2020.Tianyouhuazhan，the control variety of Jiangxi rice regional trial，was used as the control to identify and evaluate the 153 low nitrogen tolerant rice germplasms under low nitrogen stress. Result There are significant differences in plant height，number of effective tillers per plant，panicle length，spikelets per panicle and theoretical yield among different nitrogen application treatments.However，there are no significant differences in the full spikelets per panicle，1，000-grain weight and grain density.Under different nitrogen treatments，theoretical yield is significantly or extremely significantly positively correlated with the number of effective tillers per plant，panicle length and spikelets per panicle，but not significantly correlated with plant height and 1，000-grain weight.This indicates that low nitrogen stress has a significant impact on the number of effective tillers per plant，panicle length，spikelets per panicle，and theoretical yield.The coincidence rate between the two methods—clustering analysis and the membership function method—is 86% when applied to evaluate the low nitrogen tolerance characteristics of 153 excellent germplasms，using panicle number，panicle length，total grain number per panicle，and theoretical yield as evaluation indexes. Conclusion A total of 30 cultivars with strong low nitrogen tolerance have been identified using two methods，and the results of comprehensive evaluation may be more reliable.The selected germplasms with strong low nitrogen tolerance will have important application value not only in the study of molecular genetic mechanism of low nitrogen tolerance in rice，but also in the breeding of rice cultivars with low nitrogen tolerance and green efficiency.

Seed aging is the key factor leading to the loss of genetic integrity. In this study, the seeds of Dongxiang wild rice, Xianggu, 9194 and Nipponbare were kept in a plant incubator with constant temperature and humidity for artificial aging treatment. The genetic integrity of germplasm resources with different germination gradients were analyzed using 44 SSR markers. The results suggested that different accessions could be ranked in order of aging resistance from highest to lowest as common wild rice > Xianggu > 9194 > Nipponbare. In order to maintain the genetic diversity of rice, the population size for reproduction and regeneration should be between 60 and 140. After aging, the number of polymorphic alleles, the number of specific single plant, the ratio of polymorphic bands, the number of alleles, the number of effective alleles, gene diversity index and Shannon index of different accessions all decreased with the decrease of germination rate. The germination rate of 60% was the critical value to maintain genetic integrity. Besides, the genetic integrity of eighteen SSR markers was rapidly lost or significantly increased. The regions of these markers were closely related to seed viability or genetic integrity. This study provides a theoretical basis for determining the population size for reproduction and regeneration and the critical value of germination rate of rice resources.

Cold stress is a major threat to the sustainability of rice yield. Brassinosteroids (BR) application can enhance cold tolerance in rice. However, the regulatory mechanism related to cold tolerance and the BR signaling pathway in rice has not been clarified. In the current study, the seedling shoot length (SSL), seedling root length (SRL), seedling dry weight (SDW), and seedling wet weight (SWW) were used as the indices for identifying cold tolerance under cold stress and BR-combined cold treatment in a backcross recombinant inbred lines (BRIL) population. According to the phenotypic characterization for cold tolerance and a high-resolution SNP genetic map obtained from the GBS technique, a total of 114 QTLs were identified, of which 27 QTLs were detected under cold stress and 87 QTLs under BR-combined cold treatment. Among them, the intervals of many QTLs were coincident under different treatments, as well as different traits. A total of 13 candidate genes associated with cold tolerance or BR pathway, such as BRASSINAZOLE RESISTANT1 (OsBZR1), OsWRKY77, AP2 domain-containing protein, zinc finger proteins, basic helix-loop-helix (bHLH) protein, and auxin-induced protein, were predicted. Among these, the expression levels of 10 candidate genes were identified under different treatments in the parents and representative BRIL individuals. These results were helpful in understanding the regulation relationship between cold tolerance and BR pathway in rice.

Low temperature is one of the major abiotic stresses limiting seed germination and early seedling growth in rice. Brassinosteroid (BR) application can improve cold tolerance in rice. However, the regulatory relationship between cold tolerance and BR in rice remains undefined. Here, we constructed a population of 140 backcross recombinant inbred lines (BRILs) derived from a cross between a wild rice (Dongxiang wild rice, DXWR) and a super rice (SN265). The low-temperature germination rate (LTG), survival rate (SR), plant height (PH), and first leaf length (FLL) were used as indices for assessing cold tolerance under cold stress and BR-combined cold treatment at seed germination and bud burst stages. A high-resolution SNP genetic map, covering 1,145 bin markers with a distance of 3188.33 cM onto 12 chromosomes, was constructed using the GBS technique. A total of 73 QTLs were detected, of which 49 QTLs were identified under cold stress and 24 QTLs under BR-combined cold treatment. Among these, intervals of 30 QTLs were pairwise coincident under cold stress and BR-combined cold treatment, as well as different traits including SR and FLL, and PH and FLL, respectively. A total of 14 candidate genes related to cold tolerance or the BR signaling pathway, such as CBF/DREB (LOC_Os08g43200), bHLH (LOC_Os07g08440 and LOC_Os07g08440), WRKY (LOC_Os06g30860), MYB (LOC_Os01g62410 and LOC_Os05g51160), and BRI1-associated receptor kinase 1 precursor (LOC_Os06g16300), were located. Among these, the transcript levels of 10 candidate genes were identified under cold stress and BR-combined cold treatment by qRT-PCR. These findings provided an important basis for further mining the genes related to cold tolerance or the BR signaling pathway and understanding the molecular mechanisms of cold tolerance in rice.

Foxtail millet (Setaria italica), which originated in China, has a strong tolerance to low nutrition stresses. However, the mechanism of foxtail millet tolerance to low-nitrogen stress is still unknown. In this study, the transcriptome of foxtail millet under low-nitrogen stress was systematically analyzed. Expression of 1891 genes was altered, including 1318 up-regulated genes and 573 down-regulated genes. KEGG (Kyoto Encyclopedia of Genes and Genomes) analysis revealed that 3% of these genes were involved in membrane transport and 5% were involved in redox processes. There were 74 total transcription factor (TF) genes in the DEGs (differentially expressed genes), and MYB-like transcription factors accounted for one-third (25) of the TF genes. We systematically analyzed the characteristics, expression patterns, chromosome locations, and protein structures of 25 MYB-like genes. The analysis of gene function showed that Arabidopsis and rice overexpressing SiMYB3 had better root development than WT under low-nitrogen stress. Moreover, EMSA results showed that SiMYB3 protein could specifically bind MYB elements in the promoter region of TAR2, an auxin synthesis related gene and MYB3-TAR2 regulate pair conserved in rice and foxtail millet. These results suggested that SiMYB3 can regulate root development by regulating plant root auxin synthesis under low-nitrogen conditions.