Utilizing MIKC-type MADS-box protein SOC1 for yield potential enhancement in maize

Abstract

Overexpression of Zea mays SOC gene promotes flowering, reduces plant height, and leads to no reduction in grain production per plant, suggesting enhanced yield potential, at least, through increasing planting density. MIKC-type MADS-box gene SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) is an integrator conserved in the plant flowering pathway. In this study, the maize SOC1 (ZmSOC1) gene was cloned and overexpressed in transgenic maize Hi-II genotype. The T₀ plants were backcrossed with nontransgenic inbred B73 to produce first generation backcross (BC₁) seeds. Phenotyping of both transgenic and null segregant (NT) BC₁ plants was conducted in three independent experiments. The BC₁ transgenic plants showed new attributes such as increased vegetative growth, accelerated flowering time, reduced overall plant height, and increased grain weight. Second generation backcross (BC₂) plants were evaluated in the field using two planting densities. Compared to BC₂ NT plants, BC₂ transgenic plants, were 12-18% shorter, flowered 5 days earlier, and showed no reduction in grain production per plant and an increase in fat, starch, and simple sugars in the grain. Transcriptome comparison in young leaves of 56-day-old BC₁ plants revealed that the overexpressed ZmSOC1 resulted in 107 differentially expressed genes. The upregulated transcription factor DNA BINDING WITH ONE FINGER 5.4 (DOF5.4) was among the genes responsible for the reduced plant height. Modulating expression of SOC1 opens a new and effective approach to promote flowering and reduce plant height, which may have potential to enhance crop yield and improve grain quality.

Keywords: Dwarf plant; Flowering; SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1; Zea mays.

Fig. 1

Growth of transgenic ZmSOC1-OX (TR) and null segregant (NT) maize plants of BC₁ and BC₂ lines. A Data from nine BC₁ lines in experiment #3. B 60-day-old BC₂ plants planted in a low-density field plot. Transgenic and nontransgenic plants were randomly grown in each plot

Fig. 2

Flowering time of transgenic ZmSOC1-OX (TR) and null segregant (NT) maize plants of BC₁ and BC₂ lines. The BC₁ plants were from two independent BC₁ lines (c7 and c9). Rep1, rep2, and rep3 represent experiment #1, #2 and #3. A Tassel appearance time of BC₁ lines. B Silk formation time of BC₁ lines. Star(s) on each bar represent the comparison result between the TR and the NT plants of the same BC₁ line in the same experiment, n for c7_18rep1 NT = 3, n for c7_18rep1 TR = 3, n for c7_18rep2 NT = 4, n for c7_18rep2 TR = 4, n for c7_17rep3 NT = 8, n for c7_19rep3 TR = 6, n for c9_18rep1 NT = 2, n for c9_18rep1 TR = 5, n for c9_18rep2 NT = 5, n for c9_18rep2 TR = 3, n for c9_19rep3 NT = 3, n for c9_19rep3 TR = 8. C Tassel and silk appearance time of BC₂ lines. BC₂ seeds from three c7 lines and three c9 lines were grown in a field. For each line, 30 plants were randomly grown in each of the six plots, including three plots for a high planting density of 40,000 plants/acre (H) and another three for a low planting density of 32,000 plants/acre (L). Star(s) on each bar represent the comparison result between the TR and the NT plants of the same planting density. Bars indicate standard deviation. Significance codes: *** < 0.001, ** < 0.01, *< 0.05

Fig. 3

Plant height and leaf number of transgenic ZmSOC1-OX (TR) and, nontransgenic, null segregant (NT) maize plants of BC₁ and BC₂ lines. The BC₁ plants were from two independent BC₁ lines (c7 and c9). Rep1, rep2, and rep3 represent experiment #1, #2 and #3. BC₂ seeds from three c7 lines and three c9 lines were grown in field. For each BC₂ line, 30 plants were randomly grown in each of the six plots, including three plots for a high planting density of 40,000 plants/acre (H) and another three for a low planting density of 32,000 plants/acre (L). A Average plant height of mature BC₁ plants. B Average leaf number of mature BC₁ plants. C Average plant height of mature BC₂ plants. D Average leaf number of mature BC₂ plants. E Average grain dry weight per BC₁ plant; average grain dry weight per BC₂ plant. Star(s) on each bar represent the comparison result between the TR and the NT plants of the same BC₁ line in the same experiment, n for c7_18rep1 NT = 3, n for c7_18rep1 TR = 3, n for c7_18rep2 NT = 4, n for c7_18rep2 TR = 4, n for c7_17rep3 NT = 8, n for c7_19rep3 TR = 6, n for c9_18rep1 NT = 2, n for c9_18rep1 TR = 5, n for c9_18rep2 NT = 5, n for c9_18rep2 TR = 3, n for c9_19rep3 NT = 3, n for c9_19rep3 TR = 8. For the BC₂ plants, star(s) on each bar represent the comparison result between the TR and the NT plants of the same planting density. Bars indicate standard deviation. Significance codes: *** < 0.001, ** < 0.01, * < 0.05

Fig. 4

A Venn diagram illustrating overlap of the three transcriptomic comparisons of the annotated, differentially expressed transcripts (DETs) among BC₁ null segregant c7NT and two transgenic ZmSOC1_OX lines of c7TR and c9TR. B, C Comparison of the RT-qPCR analysis result and the RNA-seq data of the selected DETs (Table 2, Table S5). −∆∆Ct is an average of three biological and three technical replicates for each DET. ZmActin1 (SAC1_ARATH) was used to normalize the RT-qPCR results. Bars indicate standard deviation

Fig. 5

Major differentially expressed transcripts (DETs) and their potential functions (A) and gene networks of the shared DETs in leaf tissues of transgenic ZmSOC1_OX plants (B). The ontology file of GOSlim_Plants in BiNGO was used to identify overrepresented GO terms (p < 0.05). Bubble size and color indicate the frequency of the GO term and the p value, respectively