Increasing seed size and quality by manipulating BIG SEEDS1 in legume species

Abstract

Plant organs, such as seeds, are primary sources of food for both humans and animals. Seed size is one of the major agronomic traits that have been selected in crop plants during their domestication. Legume seeds are a major source of dietary proteins and oils. Here, we report a conserved role for the BIG SEEDS1 (BS1) gene in the control of seed size and weight in the model legume Medicago truncatula and the grain legume soybean (Glycine max). BS1 encodes a plant-specific transcription regulator and plays a key role in the control of the size of plant organs, including seeds, seed pods, and leaves, through a regulatory module that targets primary cell proliferation. Importantly, down-regulation of BS1 orthologs in soybean by an artificial microRNA significantly increased soybean seed size, weight, and amino acid content. Our results provide a strategy for the increase in yield and seed quality in legumes.plant organ size | seed size | forage quality | Medicago | soybean A s a key crop worldwide, soybean not only provides up to 69% of proteins and 30% of oils to the human diet (1) but also requires a low input of fertilizers due to its symbiotic nitrogen fixation ability (2), making it a highly valuable crop to secure global food supplies while contributing to sustainable agriculture. It is clear that the rapid increase of world population requires significant increases in crop production (3).Seed size is a major agronomic trait that has been selected in crop plants during their domestication (4-6). Due to whole genome duplication events that occurred ∼59 and ∼14 Mya (million years ago) (2), soybean has a complex genome structure. Thus, the isolation of key genes or quantitative trait loci (QTL) that control seed size and weight in soybean using conventional approaches can be a challenge and has not been reported to date. To overcome this challenge, we took a molecular genetics approach, using the legume plant Medicago truncatula as a genetic model. We identified BIG SEEDS1 (BS1) as a key gene that controls size of lateral organs, including seeds, seed pods, and leaves, in M. truncatula. Based on these results, we further identified two BS1 orthologs in soybean (Glycine max). Our objectives were to understand the role and regulatory mechanism of the BS1 gene in lateral organ size control in legume plants. Down-regulation of soybean BS1 genes using an artificial microRNA resulted in increased size of seeds, seed pods, and leaves, thus revealing a key and conserved role of BS1 in the control of organ size in legumes. Results and DiscussionIsolation and Characterization of M. truncatula big seeds1 Mutants.In a screen of the fast neutron bombardment (FNB)-induced deletion mutant collection of M. truncatula [cultivar (cv.) Jemalong A17] (7), a unique mutant with large seeds was isolated and named M. truncatula big seeds1-1 (mtbs1-1) (Fig. 1A). The mtbs1-1 mutant also exhibited larger seed pods and leaves than WT plants, suggesting a key role of BS1 in determining lateral organ size (SI Appendix, Figs. S1 and S2). Time...

SlKIX8 and SlKIX9 are negative regulators of leaf and fruit growth in tomato

“…In line with our findings, several rosid eudicot species, in which the KIX or PPD genes were either mutated or downregulated, displayed increased seed (pod) size (Ge et al, 2016;Li et al, 2018a). Furthermore, orthologs of Arabidopsis SAP, which encodes an F-box protein that regulates the stability of the PPD-KIX complex, are positive regulators of fruit and flower size in cucumber and of flower size in Capsella rubella (Sicard et al, 2016;Yang et al, 2018).…”

“…In multiple eudicot species that belong to the rosid order of Fabales, orthologs of the KIX and PPD proteins have been reported to negatively regulate seed pod size (Ge et al, 2016;Kanazashi et al, 2018;Li et al, 2019;Naito et al, 2017). To examine whether the KIX proteins might also have a role in determining tomato fruit size in the asterid model species tomato, we investigated whether the development of reproductive organs was affected in tomato kix8 kix9 mutants.…”

“…In Arabidopsis, the asymmetric cell division of meristemoids, which are precursor cells of the stomatal lineage, positively affects final leaf size and is restricted by a transcriptional repressor complex in which the co-repressor TPL is recruited to PPD2 by KIX8/KIX9 (Gonzalez et al, 2015;White, 2006). Members of this repressor complex were shown to regulate leaf size and shape in a variety of species that belong to different orders of the rosids (Ge et al, 2016;Gonzalez et al, 2015;Kanazashi et al, 2018;Li et al, 2019;Naito et al, 2017), suggesting that the repressor complex is a conserved regulator of leaf growth among rosid eudicots. Here, we demonstrate that the tomato orthologs of KIX8 and KIX9 act as TPL adaptors for PPD proteins as well and, thereby, regulate leaf growth in tomato, a model species of the asterid clade that also includes tobacco (Nicotiana tabacum), carrot (Daucus carota) and sunflower (Helianthus annuus).…”

“…Several eudicot members, in which orthologs of the PPD or KIX genes were mutated or downregulated, including Medicago truncatula, soybean (Glycine max), blackgram (Vigna mungo), and pea (Pisum sativum), produced enlarged leaves (Ge et al, 2016;Kanazashi et al, 2018;Li et al, 2019;Naito et al, 2017). Moreover, overexpression of SAP orthologs in poplar (Populus trichocarpa) and cucumber (Cucumis sativus L.) also increased leaf size (Townsley et al, 2015;Yang et al, 2018;Yordanov et al, 2017).…”

Transcriptional repression of GIF1 by the KIX-PPD-MYC repressor complex controls seed size in Arabidopsis

“…18). Down-regulation of PPDs orthologs in legume Medicago truncatula and legume soybean leads to high expression of MtGIF1 and GmGIF1 in leaves, stipules, and seeds, respectively 40 . These results indicate that the expression of GIFs regulated by PPDs might be a common mechanism in dicotyledon plants.…”