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. 2011 Jun;52(6):1083-94.
doi: 10.1093/pcp/pcr059. Epub 2011 May 12.

Flowering time genes Heading date 1 and Early heading date 1 together control panicle development in rice

Naokuni Endo-Higashi  1 , Takeshi Izawa
Affiliations

Flowering time genes Heading date 1 and Early heading date 1 together control panicle development in rice

Naokuni Endo-Higashi et al. Plant Cell Physiol. 2011 Jun.

Abstract

Although flowering time is often associated with plant size, little is known about how flowering time genes affect plant architecture. We grew four rice lines having different flowering time genotypes (hd1 ehd1, hd1 Ehd1, Hd1 ehd1 and Hd1 Ehd1) under distinct photoperiod conditions. By using genotype-treatment combinations that resulted in similar flowering times, we were able to compare the effects of flowering time genes on traits related to plant architecture. The results revealed that the combination of Heading-date 1 (Hd1) and Early heading date 1 (Ehd1) can reduce the number of primary branches in a panicle, resulting in smaller spikelet numbers per panicle; this occurs independently of the control of flowering time. In addition, expression of the Hd3a and Rice Flowering-locus T 1 (RFT1) florigen genes was up-regulated in leaves of the Hd1 Ehd1 line at the time of the floral transition. We further revealed that Hd1 and/or Ehd1 caused up-regulation of Terminal Flower 1-like genes and precocious expression of panicle formation-related genes at shoot apical meristems during panicle development. Therefore, two key flowering time genes, Hd1 and Ehd1, can control panicle development in rice; this may affect crop yields in the field through florigen expression in leaf.

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Figures

Fig. 1
Fig. 1
(A) Schematic summary of rice flowering time genes tested in this work. Hd1 promotes Hd3a (and RFT1) expression and flowering only under SD conditions, whereas Ehd1 promotes Hd3a (and RFT1) expression and flowering under both SD and LD conditions. (B) Experimental design. SD, short-day conditions (10 h light, 14 h dark); LD, long-day conditions (14.5 h light, 9.5 h dark); 30LD, 30 d under LD conditions; 60LD, 60 d under LD conditions. Each of the three plant materials was subjected to each of the four growth conditions. The experiments were repeated independently four times. (C) Days to heading of T65, T65 + Hd1, T65 + Ehd1 and T65 + Hd1 + Ehd1 under different daylength conditions (mean ± SE, n = 3). Data represent a single experiment. Similar results were obtained in three additional independent experiments.
Fig. 2
Fig. 2
Number of spikelets per panicle. The upper panel in each part shows all data from four independent experiments (n = 9–12). The lower panel shows average values and standard errors (n = 9–12, except for T65 + Hd1 under 150LD→SD conditions, shown as ‘x’, for which n = 3). In T65 + Hd1 under LD, some plants did not flower. The statistical significance of these data is summarized in Supplementary Table S1.
Fig. 3
Fig. 3
Comparisons of spikelet number per panicle of plants that flowered with similar timing. Comparisons between (A) T65 and T65 + Hd1 (SD and LD); (B) T65 and T65 + Ehd1; (C) T65 + Hd1 and T65 + Hd1 + Ehd1; (D) T65 + Ehd1 and T65 + Hd1 + Ehd1; and (E) All four lines. The left-hand panels show all data from four independent experiments; the right-hand panels show mean values ± SE (n = 9–12). Circles are data from SD conditions. Triangles are data from LD conditions. Diamonds are data from 60LD→SD. Statisical significance (P < 0.01 t-test): T65 (SD) vs. T65 + Hd1 (LD), T65 + Hd1 (SD) vs. T65 + Hd1 + Ehd1 (SD), T65 + Ehd1 (SD) vs. T65 + Hd1 + Ehd1 (SD) For (E), refer to Supplementary Table S1.
Fig. 4
Fig. 4
Changes of panicle development. (A) A scheme of the rice panicle. (B) Correlation between the number of spikelets per panicle and the number of primary rachis branches per panicle (left) and number of spikelets per primary rachis branch (right) under SD conditions. Data were collected from the main stem. **Correlation is significant at the 1% level.
Fig. 5
Fig. 5
Temporal expression patterns of Hd3a and RFT1 in mature leaves. Quantitative RT–PCR analyses of Hd3a (left panel) and RFT1 (right panel) in T65, T65 + Hd1, T65 + Ehd1 and T65 + Hd1 + Ehd1 under SD conditions. Timing of transition was determined by the microscopic observation of SAM parts of test samples for all sampling dates. Day 2 indicates the timing of the start of primary rachis differentiation. mRNAs were prepared from the corresponding leaf samples grown under SD conditions. The results are the mean ± SE (n = 3 individual plants). Three RT–PCRs were done for each cDNA sample from one plant. Leaf samples were collected 3 h after dawn from leaf blades. The graph shows the result of a single experiment (means ± SE, n = 3). Another experiment also gave similar results.
Fig. 6
Fig. 6
Temporal gene expression patterns of rice panicle development-related genes in SAM regions during early stages of panicle development. Levels of mRNA accumulation were examined by quantitative RT–PCR. Timing of transition was determined by the microscopic observation of SAM parts of test samples for all sampling dates. Day 2 indicates the timing of the start of primary rachis differentiation. The results are the mean ± SE (n = 3 individual plants). Three RT–PCRs were done for each cDNA sample from one plant. mRNAs were prepared from the corresponding SAM samples grown under SD conditions. Similar data were obtained when normalized by number of SAMs for mRNA preparation.
Fig. 7
Fig. 7
Effects of ectopic Hd3a expression on days to flowering and panicle size traits under SD conditions. Segregating T1 plants from three independent lines were grown. Open symbols indicate the spikelet numbers on the main stem for transgenic plants [n = 2–4 main panicles (plants) per line]; filled circles indicate spikelet numbers on the main stem for the non-promoter control (in which only Hd3a cDNA without promoter was transformed) plants (n = 4 samples). (A) Correlation between ‘days to flowering’ and OsMADS14 expression in leaves. The ectopic Hd3a expression was able to induce endogenous OsMADS14 in rice leaves. (B) Correlation between ‘days to flowering’ and number of spikelets per panicle. (C) Correlation between number of spikelets per panicle and number of primary rachis branches. (D) Correlation between number of spikelets per panicle and number of spikelets per primary rachis branch. Data were collected only from main stems. Heat-shock treatment was not given. **Correlation is significant at the 1% level.
Fig. 8
Fig. 8
Relationship among biomass, grain number and flowering time. (A) Correlation between ‘days to flowering’ and dry weight of shoot. SD conditions (open symbols), 30LD→SD, 60LD→SD, 150LD→SD and LD conditions (filled symbols). (B) Correlation between days to flowering and number of spikelets per plant. Shoot dry weight excludes the weight of panicles. The panels in each part show all data from four independent experiments. **Correlation is significant at the 1% level.
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