Figure 1.
Photographs of the sorghum lines 100 M and 58 M for flowering time phenotype.
(A) Photograph of 100 M (left) and 58 M (right) grown for 109 days in LD (14 h light/10 h dark). 100 M and 58 M flowered after 126 days and 62 days respectively. (B) Photograph of 100 M (left) and 58 M (right) grown in a greenhouse in SD for 53 days (10 h light/14 h dark). 100 M flowered after 59 days and 58 M flowered after 48 days. LD: long days. SD: short days. DTF = number of days to flowering time. Scale bar is 8.6 cm.
Table 1.
Sequence analysis of PHYB coding alleles in different sorghum lines.
Figure 2.
Flowering time QTL and analysis of epistasis in populations derived from 58MxR.07007.
(A) Flowering time QTL labeled Ma3, Ma5 and Ma6, were identified through analysis of flowering time variation in LD in the F2 population derived from 58MxR.07007. LOD values are shown on the Y-axis and sorghum chromosome numbers on the X-axis. The percent of the variance explained by each QTL is noted. The additive plot is shown in the lower portion of 2A where a positive value corresponds to alleles from R.07007 that delay flowering time. (B) Boxplot of flowering time distribution in the subset of the population with Ma1Ma5- genotypes but varying for alleles of Ma3/ma3R and Ma6/ma6. (C) Boxplot of flowering time distribution in the subset of the population having Ma1Ma3- genotypes but varying for Ma5/ma5 and Ma6/ma6. Median values for flowering time are represented by horizontal lines within boxes.
Table 2.
Information on flowering time QTL identified in the 58MxR.07007 F2 population.
Table 3.
Flowering time of F2/F3 progeny from 58MxR.07007 in LD.
Table 4.
Sequence analysis of PHYC coding alleles in different sorghum lines.
Figure 3.
Relative expression of SbPRR37 and SbGHD7 in 100 M (Ma3/PHYB) and 58 M (ma3R/phyB-1) in LD and SD.
100 M (solid black line) and 58 M (dashed red line) plants were entrained LD (14 h light/10 h dark) or SD (10 h light/14 h dark) and sampled for one 24 h cycle, followed by 48 h in LL (continuous light and temperature). The grey background corresponds to time when plants are in darkness. Relative gene expression was determined every 3 hours by qRT-PCR. Arrows represent morning peaks of expression and arrowheads represent evening peaks of expression. (A) In LD, the second peak (arrowhead) of SbPRR37 expression in the evening (∼15 h) is missing in the phyB deficient line, 58 M. (B) In SD, the second peak (arrowhead) of SbPRR37 is absent in both 100 M and 58 M. (C) In LD, the second peak (arrowhead) of SbGHD7 expression in the evening (∼15 h) is attenuated in 58 M. (D) In SD, the second peak of SbGHD7 is attenuated in both 100 M and 58 M. Each data point of relative expression was based on data from three technical replicates and three biological replicates. Error bars indicate SEM.
Figure 4.
Expression of SbCO, SbEhd1, SbCN8/12/15 in 100 M (Ma3/PHYB) and 58 M (ma3R/phyB-1) in LD and SD.
Relative RNA levels in leaves of 100 M (solid black lines) and 58 M (dashed red lines) entrained and sampled in LD (14 h light/10 h dark) or SD (10 h light/14 h dark) for 24 h followed by 24 h in LL (continuous light and temperature). Relative expression levels were determined every 3 hours by qRT-PCR analysis. The gray shaded areas represent the dark periods. (A) SbCO, (B) SbEHD1, (C) SbCN8, (D) SbCN12, (E) SbCN15. Each data point of relative expression is based on three technical replicates and three biological replicates. Error bars indicate SEM.
Figure 5.
Model of the photoperiod flowering time pathway in sorghum.
Phytochrome B (PhyB) is mediates light signaling that modulates flowering time in response to photoperiod in sorghum. In LD, PhyB up-regulates the expression of PRR37 and GHD7, two central floral repressors, during the evening phase of LD but with minimal influence in SD. Induction at this time of day is also dependent on output from the circadian clock. PhyB may stabilize and interact with PhyC, a candidate gene for Ma5 a locus that also contributes to photoperiod regulation of flowering time. SbPRR37 activates SbCO expression peaking at dawn. SbPRR37 and SbGhd7 repress expression of the floral inductors SbEHD1, SbCN8, SbCN12 and SbCN15, leading to delayed flowering in long days. In SD or 58 M (phyB-1), expression of the floral repressors SbPRR37 and SbGHD7 is reduced which results in floral initiation once plants have satisfied other requirements for flowering. PhyB was found to mediate repression of SbCN15 regardless of day length.