Abstract

The A3 type of CMS in sorghum is one of the most difficult to restore fertility because of the low frequency of fertilityrestoring genes among sorghum accessions, the complex mechanism of fertility restoration that occurs with the complementary interaction of two gametophytic genes Rf3 and Rf4, and the sensitivity of their expression to air and soil drought. In order to test the hypothesis of the sporophytic type of fertility restoration in CMS lines with A3 type cytoplasm developed in our laboratory, we analyzed segregation in the self-pollinated progeny of fertile F1hybrids grown under different water availability conditions (in a dryland plot, in plots with additional irrigation, in a growth chamber, and in an experimental field with a natural precipitation regime) and in their backcrosses to the maternal CMS-line. The presence of sterile plants in the F2 and BC1 families with the maternal CMS line grown in all tested water availability conditions argues for the sporophytic mechanism of fertility restoration. Cytological analysis of fertile F1 hybrids revealed a significant amount of degenerating pollen grains (PGs) with impaired starch accumulation and detachment of the PG contents from the cell wall. It is assumed that the expression of the fertility-restoring genes Rf3 and Rf4 in the hybrids with studied CMS lines starts already in the sporophyte tissues, normalizing the development of a certain part of the PGs carrying the recessive alleles of these genes (rf3 and rf4), which are involved in fertilization and give rise to sterile genotypes found in F2 and BC1 families. For the first time, the transgenerational effect of water availability conditions of growing a fertility-restoring line on male fertility of the F2 generation was detected: a pollinator grown in a plot with additional irrigation produced more fertile and less sterile individuals compared to the same pollinator grown under a rainfall shelter (p < 0.01), and the segregation pattern changed from digenic to monogenic, indicating heritable inhibition of the expression of one of the fertility-restoring genes (kind of “grandfather effect”). The possibility of selection for the stability of the fertility restoration system of the A3 cytoplasm to functioning under conditions of high vapor pressure deficit during the flowering period was shown. These data may contribute to the creation of effective fertility restoring lines for this type of CMS in sorghum.

Highlights

  • The development of the reproductive structures is the stage of plant ontogeny most sensitive to environmental stresses

  • We showed in our previous study that severe vapor pressure deficit during flowering inversely correlated to the level of fertility of F1 hybrids in the A3 cytoplasm; the cultivation of hybrid populations with artificial irrigation favored the selection of lines restoring fertility for this type of cytoplasmic male sterility (CMS) (Kozhemyakin et al, 2017)

  • Analysis of seed sets in F1 ♀ А3 КP-70 × ♂ КVV-96 hybrids grown under different water availability conditions shows that the KVV-96 line can restore the fertility of A3 type CMS (Table 2)

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Summary

Introduction

The development of the reproductive structures is the stage of plant ontogeny most sensitive to environmental stresses. Mitochondria are the primary targets of environmental stresses, which disrupt their functioning and information exchange between the mitochondrial and nuclear genomes (Atkin, Macherel, 2009; Jacoby et al, 2012; Ng et al, 2014; Liberatore et al, 2016) This fact is of particular importance for hybrids with cytoplasmic male sterility (CMS), resulting from remote hybridization, in which the information exchange between the nuclear and mitochondrial genome, established during coevolution, is disturbed (Touzet, Meyer, 2014) and the resistance to environmental stresses is debilitated (Li et al, 2012). Among the different types of sterile cytoplasm found in sorghum (Reddy et al, 2005), A3 cytoplasm, the source of which is IS1112C accession, is one of the most difficult to restore fertility This difficulty arises from the low frequency of fertility-restoring genes among sorghum accessions (Worstell et al, 1984; Torres-Cardona et al, 1990; Dahlberg, MaderaTorres, 1997) and the sensitivity of their expression to air and soil drought (Kozhemyakin et al, 2017). F1 hybrids with restored male fertility, heterozygous for the Rf3 and Rf4 genes, yield 25 % of the fertile pollen that reduces the seed set and limits the practical use of A3 cytoplasm in sorghum breeding, compared to other types of sterile cytoplasms (A1 and A2), providing the 100 % seed set in F1 hybrids

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