Molecular Control and Application of Male Fertility for Two-Line Hybrid Rice Breeding

Abstract

The significance of the climate change may involve enhancement of plant growth as well as utilization of the environmental alterations in male fertility (MF) regulation via male sterility (MS) systems. We described that MS systems provide a fundamental platform for improvement in agriculture production and have been explicated for creating bulk germplasm of the two-line hybrids (EGMS) in rice as compared to the three-line, to gain production sustainability and exploit its immense potential. Environmental alterations such as photoperiod and/or temperature and humidity regulate MS in EGMS lines via genetic and epigenetic changes, regulation of the noncoding RNAs, and RNA-metabolism including the transcriptional factors (TFs) implication. Herein, this article enlightens a deep understanding of the molecular control of MF in EGMS lines and exploring the regulatory driving forces that function efficiently during plant adaption under a changing environment. We highlighted a possible solution in obtaining more stable hybrids through apomixis (single-line system) for seed production.

Keywords: CRISPR/Cas9; EGMS; HGMS; PGMS; PTGMS; TGMS; apomixes; hybrid rice; male fertility or sterility; single-line; three-line; two-line.

Figure 1

Three-line hybrid-rice technology. (a) Three-line HR technology works through three different rice lines, as rice-line A (cytoplasmic-male-sterile line), rice-line B (maintainer line), and rice-line R (restorer line). (b) The regulatory factors that can restore rice fertility. Several sequences of the mitochondrial (mt) genome undergo multirecombination (MR) through evolution in rice to generate structural mutations. The flow of sub-stoichiometric due to the variations in copy number of a gene and leading to the emergence of a functional cytoplasmic-male-sterile (CMS) gene. Expanding clusters of the pentatrico-peptide repeat resulted in functional-Rf alleles used for cytoplasmic-male-sterility restoration. The nucleus genes as Rf converse function of CMS gene(s) at transcriptional (Tc) and/or protein (P) levels, but the recessive allele like rf17 is retrogradely (R) upregulated through CMS gene(s). In the figure, MR, HmR, PPR, f284, f288, f352, fH7, f79, -WA, -CW, -HL, -BT, -LD, and unk represent multi-recombination, homologous-recombination, pentatrico-peptide repeat, orf284, orf288, orf352, orfH7, orf79, CMS-WA, CMS-CW, CMS-HL, CMS-BT, CMS-LD, and unknown, respectively.

Figure 2

Two-line hybrid-rice technology.

Figure 3

Examination of the male reproductive part in rice plant. (a) Rice plant at the booting stage. (b) Rice flower. (c) Dissection of the single-spikelet reproductive parts. (d) Proposed ultra-structure examination of the anther development. Upper-circular-part displayed normal anther development in rice, middle-circular-part showed abnormal anther development due to disruption of the microspores, and lower-circular-part showed abnormal development of tapetum and microspores during anther development. These abnormalities in anther development lead to the male sterility phenotype in rice.

Figure 4

Regulation of the male fertility of EGMS-lines in two-line HR through photoperiod and temperature alterations. (a) The NK58s (Japonica genetic background) comprise the single nucleotide polymorphism (SNP) within two long-noncoding RNAs (lncRNAs), pms1 and pms3. The SNP in Pms1 induces miR2118 binding and enhances the small RNA (21nt) processing under long-day (LD) conditions that downregulate the target genes responsible for tapetum programmed cell death (PCD). Additionally, the SNP in the promoter region of the pms3 that elevates the DNA-methylation reduced pms3 transcription under LD conditions proceeding the premature-PCD in the anther (PGMS). The SNP within pms3 in tms12-1 regulates TGMS in PA64s (the NK58s generated line with Indica genetic background). (b) The mutation in the TMS5 gene generated a tms5 line (TGMS) which encrypts RNase Z (RNase Z^S1) short form. RNase Z^S1 produces mRNAs splicing that encodes Ub_L40 protein. The mRNAs of the Ub_L40 under high temperature (HT) could not be spliced and elevated accumulation levels of mRNAs, resulting in male sterility in the tms5.

Figure 5

Transitional regulation of male fertility and sterility in EGMS-Lines. (a) Regulation of sugar partitioning in anther via the carbon-starved. The transcription factor CSA is a key player for sugar regulation from leaf to anther through direct regulation of transporter OsMST8 for normal anther reproduction. Under a short day, CSA mutation can lead to downregulation of OsMST8 transcription and be unable to transport sugar from flag leaves to anthers, resulting in male sterility. While, long-day (LD) conditions along with other regulators might switch regulation of this process and lead to normal anther development and male fertility. (b) The hms1 mutants regulate male reproduction under varying relative humidity (RH) percentage. It showed male sterility at low RH (<60%) and restored male fertility at high RH (>80%) via normal development of the anther. (c) Functional regulation of UPD-glucose pyrophosphorylase 1 (Ugp-1) under fluctuating temperature. Under high-temperature (HT) Ugp-1-overexpression rice plants revealed a high accumulation of nonspliced transcript of Ugp1 leading to male sterile plants, whereas under low-temperature (LT) successful splicing of Ugp1 transcript restored male fertility and normal seed setting.

Figure 6

Apomixis technology for single-line HR.