Abstract
Seed size is an important component of grain yield and a key determinant trait for crop domestication. The Krug Yellow Dent long-term selection experiment for large and small seed provides a valuable resource to dissect genetic and phenotypic changes affecting seed size within a common genetic background. In this study, inbred lines derived from Krug Large Seed (KLS) and Krug Small Seed (KSS) populations and reciprocal F1 crosses were used to investigate developmental and molecular mechanisms governing seed size. Seed morphological characteristics showed striking differences between KLS and KSS inbred lines, and the reciprocal cross experiment revealed a strong maternal influence on both seed weight and seed size. Quantification of endosperm area, starchy endosperm cell size, and kernel dry mass accumulation indicated a positive correlation between seed size, endosperm cell number, and grain filling rate, and patterns of grain filling in reciprocal crosses mirrored that of the maternal parent. Consistent with the maternal contribution to seed weight, transcriptome profiling of reciprocal F1 hybrids showed substantial similarities to the maternal parent. A set of differentially expressed genes between KLS and KSS inbreds were found, which fell into a broad number of functional categories including DNA methylation, nucleosome assembly, and heat stress response. In addition, gene co-expression network analysis of parental inbreds and reciprocal F1 hybrids identified co-expression modules enriched in ovule development and DNA methylation, implicating these two processes in seed size determination. These results expand our understanding of seed size regulation and help to uncover the developmental and molecular basis underlying maternal control of seed size in maize.
Highlights
Seed size is a key determinant for evolutionary fitness and is a crucial agronomic trait selected during crop domestication (Doebley et al, 2006)
By using a unique genetic resource derived from the Krug Yellow Dent long-term selection experiment for seed size in maize, we identified remarkable reciprocal differences due to large maternal
Our observation that reciprocal F1 crosses closely mirrored the phenotype of the self-pollinated maternal parent in terms of seed weight, seed size, and seed development provides strong support for a maternal influence on seed size in maize
Summary
Seed size is a key determinant for evolutionary fitness and is a crucial agronomic trait selected during crop domestication (Doebley et al, 2006). Seeds produced by cereal crops are a major source of staple food, livestock feed, and biofuel (Makkar, 2012). Seed size has been proposed to be a key contributor to grain yield in crop plants (Kesavan et al, 2013; Zhang et al, 2014). The embryo and endosperm develop within the maternal tissues of the ovule, and the integuments of the ovule give rise to the coat of the mature seed (Chaudhury et al, 2001). Seed size is co-ordinately determined by the growth of the triploid endosperm, the diploid embryo, and resources and developmental cues provided by the maternal plant (Sundaresan, 2005)
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