Morphological Characterization of a New and Easily Recognizable Nuclear Male Sterile Mutant of Sorghum (Sorghum bicolor)

RESEARCH ARTICLE Morphological Characterization of a New and Easily Recognizable Nuclear Male Sterile Mutant of Sorghum (Sorghum bicolor) Zhanguo Xin1, Jian Huang2, Ashley R. Smith2, Junping Chen1, John Burke1, Scott E. Sattler3, Dazhong Zhao2* a11111 1 Plant Stress and Germplasm Development Unit, USDA-ARS, Lubbock, Texas, United States of America, 2 Department of Biological Sciences, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin, United States of America, 3 USDA-ARS-PA-Grain, Forage, & Bioenergy Res. Unit, 251 Filley Hall/Food Ind. Complex, Lincoln, Nebraska, United States of America * Abstract OPEN ACCESS Citation: Xin Z, Huang J, Smith AR, Chen J, Burke J, Sattler SE, et al. (2017) Morphological Characterization of a New and Easily Recognizable Nuclear Male Sterile Mutant of Sorghum (Sorghum bicolor). PLoS ONE 12(1): e0165195. doi:10.1371/ journal.pone.0165195 Editor: Wei Wang, Henan Agricultural University, CHINA Received: August 3, 2016 Accepted: October 8, 2016 Published: January 4, 2017 Copyright: This is an open access article, free of all copyright, and may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose. The work is made available under the Creative Commons CC0 public domain dedication. Data Availability Statement: The ms8 seeds will be deposited to the National Genetic Resources of USDA-ARS (http://www.ars-grin.gov) where users can request the seeds. The ms8 seeds can be also requested from Zhanguo Xin (Zhanguo.Xin@ars. usda.gov) before the ms8 seeds become available in the National Genetic Resources. Funding: This work was partially supported by the National Science Foundation (IOS-0721192 and IOS-1322796), the Research Growth Initiative (RGI) at the University of Wisconsin-Milwaukee, and the UW-Madison/UW-Milwaukee Intercampus Sorghum (Sorghum bicolor L. Moench) is one of the most important grain crops in the world. The nuclear male sterility (NMS) trait, which is caused by mutations on the nuclear gene, is valuable for hybrid breeding and genetic studies. Several NMS mutants have been reported previously, but none of them were well characterized. Here, we present our detailed morphological characterization of a new and easily recognizable NMS sorghum mutant male sterile 8 (ms8) isolated from an elite inbred BTx623 mutagenized by ethyl methane sulfonate (EMS). Our results show that the ms8 mutant phenotype was caused by a mutation on a single recessive nuclear gene that is different from all available NMS loci reported in sorghum. In fertile sorghum plants, yellow anthers appeared first during anthesis, while in the ms8 mutant, white hairy stigma emerged first and only small white anthers were observed, making ms8 plants easily recognizable when flowering. The ovary development and seed production after manual pollination are normal in the ms8 mutant, indicating it is female fertile and male sterile only. We found that ms8 anthers did not produce pollen grains. Further analysis revealed that ms8 anthers were defective in tapetum development, which led to the arrest of pollen formation. As a stable male sterile mutant across different environments, greenhouses, and fields in different locations, the ms8 mutant could be a useful breeding tool. Moreover, ms8 might be an important for elucidating male gametophyte development in sorghum and other plants. Introduction Sorghum (Sorghum bicolor L. Moench) is the fifth most important grain crops in the world, providing food of subsistence to over 500 million people in Africa and South East Asian [1]. As a C4 crop with excellent tolerance to drought and high temperature stresses as well as adaptation to marginal soils, sorghum is becoming increasingly important as a promising bioenergy crop for sugar, biomass, and biofuel production [2, 3]. With a moderate sequenced diploid genome (~730 Mb), sorghum is also an emerging model for highly productive C4 crops [4, 5]. PLOS ONE | DOI:10.1371/journal.pone.0165195 January 4, 2017 1 / 14 A New Sorghum Male Sterile Mutant Research Incentive Grants Program to DZ. DZ also thankfully acknowledges supports of the Shaw Scientist Award from the Greater Milwaukee Foundation and the Bradley Catalyst Award from the UWM Research Foundation. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: I have read the journal’s policy and the authors of this manuscript have the following competing interests: Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer. This does not alter our adherence to PLOS ONE policies on sharing data and materials. Male sterile mutants are key tools for hybrid breeding. Male sterility in flowering plants is often attributed to a failure in pollen production or shedding due to defective anther development or dehiscence, while the development of female floral organs is normal [6]. There are two types of male sterility: cytoplasmic male sterility (CMS) and nuclear male sterility (NMS) or genic male sterility. CMS, which is maternally inherited, is caused by incompatibility of the cytoplasmic genome with the nuclear genome [6]. The CMS must be maintained by pollination with a companion line that has a nearly identical nuclear genome but a compatible cytoplasm. Genetic defect in the nuclear genome results in NMS, which is usually segregated as a recessive trait in the self-pollinated F2 offspring. Male sterility has played a major role in production of hybrid seeds in large scale to exploit the phenomenon of heterosis. At present, CMS is predominantly used for hybrid production in crops. In the CMS breeding system, three lines are required for hybrid seed production [7]. The female parent (A line) is completely male sterile. A maintainer line (B line) is needed to pollinate the A line for maintaining the absolute male sterility of the A line. During hybrid seed production, a male parent (R line) is required to provide pollen to the A line, restoring the fertility of the F1 plants by complementing the cytoplasmic defect of the A line with dominant restoring factors in the nuclear genome [8]. Simultaneous development of all three lines in breeding hybrids is very complicated and expensive. Moreover, strict requirements for A, B, and R lines severely limit the use of germplasm accessions for generating all possible hybrid vigor. In rice, two-line hybrid breeding systems using conditional nuclear male-sterility have been explored to simplify the hybrid seed production procedure and to expand the possibilities of making hybrids between accessions that are not possible with the three-line breeding system [9–11]. In the two-line breeding system, a conditional male sterile line is u (...truncated)