Characterization of novel multi-seeded (msd) mutants of sorghum for increasing grain number

Authors: Burow, Gloria; Xin, Zhanguo; Burke, John; Hayes, Chad

Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/26/2014
Publication Date: 7/21/2014
Citation: Burow, G.B., Xin, Z., Burke, J.J., Hayes, C.M. 2014. Characterization of novel multi-seeded (msd) mutants of sorghum for increasing grain number. Crop Science. 54:1-8.

Interpretive Summary: Seed number per panicle is a major determinant of grain yield in sorghum [Sorghum bicolor (L.) Moench] and other cereal crops. This yield trait (number of seeds per panicle) is proposed to be directly related to developmental mechanisms that controls inflorescence architecture and panicle size. Typically in a sorghum panicle, the basic repeating structure is a spikelet. The prevalent condition (wild type phenotype) is that spikelets are composed of a single sessile (directly attached to branch), fertile floret that develop into viable seed and one or two adjacent sterile pedicellate (attached to branch by a short peduncle) florets (Monoseeded [MSD] trait). Based on total number of florets per panicle, most sorghum germplasm and cultivars exhibit between30-40% seed set. We report the development and detailed characterization of a stable and recessive multi- seeded mutant (msd1) in sorghum (BTx623 background, WT) which produces fertile sessile and pedicellate florets that develop into seeds resulting in increased seed set from 40 to 90% and had greater number of primary and secondary panicle branches. Additionally, msd1 mutant displayed increased overall panicle length, number of primary and secondary inflorescence branches per panicle. Based on the results of total seed weight per panicle, msd1 mutant have a distinct potential to increase grain production thru an increase in seed number but also as parent to use for crosses with large seeded germplasm. Hybridization of msd1 with the large-seeded sorghum lines could potentially improve the seed size and further increase yield production of msd1 mutant. More importantly, msd1 could also be utilized in F1 hybrid seed production, where both parents (female and male inbreds) will have to harbor the mutation and yield could potentially benefit from heterosis. Finally, genes defined by the msd mutation will be necessary to understand the mechanism of suppressed seed development in pedecillate florets and the overall process of seed set in sorghum panicles.

Technical Abstract: The tribe Andropogoneae of the Poaceae family exhibits highly branched inflorescence known as panicle or tassel. Characteristically, each spikelet in a panicle or tassel comprise of a combination of sessile/fertile and sterile florets. In sorghum, (Sorghum bicolor L. Moench), the existing cultivars and wild type genetic stocks display basic spikelet feature of a single sessile, fertile floret that develop into viable seed and one or two adjacent sterile pedicellate florets resulting in overall seed set of 30-40% (mono seed trait, MSD). Here, we report on the isolation of 6 independent recessive mutants lines with multi seeded (msd) condition in sorghum (BTx623 background), which produce fertile sessile and pedicellate florets that develop into seeds resulting in increased overall seed set per panicle to 60 to 90% and had greater number of primary and secondary branches in a panicle, thus increasing branching complexity. Pedicellate florets of msd mutants exhibited complete flowers with full development of functional gynoecium (ovary, style and stigma) and androecium (complete set of 3 anthers with copious amount of pollen) at a rate of 60-95%, compared to none (0%) in wild type (WT). This fundamental alteration of flower development in msd mutants resulted in doubling of grain number per panicle compared to WT. Additionally, msd1 mutant displayed increased overall panicle length; number of primary and secondary inflorescence branches per panicle. The seeds are smaller as reflected by lower seed weight as possible trade-off from the increased seed number. However, on a per panicle basis, msd1 plants produced greater total seed weight per panicle, displayed similar germination rate as WT, undergo normal developmental process and complete reproductive stage without any perturbation, in the field and under controlled conditions. To explore the possible molecular genetic basis of msd mutation we analyzed and sequence putative candidate genes based on information from analogous phenotype in barley, six-rowed barley, causal gene Hvvrs1 and a closely related gene from maize, Zmgt1. Sequencing of predicted sorghum orthologs, Sb02g037560 (Sbvrs1 like) and Sb01g0439102 (Sbgt1 like) did not detect mutation or polymorphism between WT and 6 independent msd mutants in these 2 genes. We hypothesize that the multi-seeded phenotype describe herein is likely result from alteration of the function of major genetic switch that controls or regulate both gynoecium and androecium development and panicle branching in Andropogoneae .