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, 20 (8), 2130-45

Brassinosteroids Regulate Grain Filling in Rice

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Brassinosteroids Regulate Grain Filling in Rice

Chuan-yin Wu et al. Plant Cell.

Abstract

Genes controlling hormone levels have been used to increase grain yields in wheat (Triticum aestivum) and rice (Oryza sativa). We created transgenic rice plants expressing maize (Zea mays), rice, or Arabidopsis thaliana genes encoding sterol C-22 hydroxylases that control brassinosteroid (BR) hormone levels using a promoter that is active in only the stems, leaves, and roots. The transgenic plants produced more tillers and more seed than wild-type plants. The seed were heavier as well, especially the seed at the bases of the spikes that fill the least. These phenotypic changes brought about 15 to 44% increases in grain yield per plant relative to wild-type plants in greenhouse and field trials. Expression of the Arabidopsis C-22 hydroxylase in the embryos or endosperms themselves had no apparent effect on seed weight. These results suggested that BRs stimulate the flow of assimilate from the source to the sink. Microarray and photosynthesis analysis of transgenic plants revealed evidence of enhanced CO(2) assimilation, enlarged glucose pools in the flag leaves, and increased assimilation of glucose to starch in the seed. These results further suggested that BRs stimulate the flow of assimilate. Plants have not been bred directly for seed filling traits, suggesting that genes that control seed filling could be used to further increase grain yield in crop plants.

Figures

Figure 1.
Figure 1.
Expression Pattern of an Arabidopsis pAS Promoter in Rice. (A) aHAP fluorescence in roots and leaves. (B) aHAP fluorescence in mesophyll cells. (C) and (D) aHAP in flower buds (no fluorescence). (E) aHAP fluorescence in filaments. (F) aHAP fluorescence in seed (15 d after pollination). (G) and (J) uHAP fluorescence in flowers and seed. (H) and (I) aHAP fluorescence in germinating seed. (K) to (M) mRNA levels. Primers were designed to the coding sequence and OCS terminator of each of the transgenes. Primers to the Os-CYP transgene were used for the wild type and did not reveal mRNA levels for the endogenous gene. The plasmid in (L) contained At-gCYP and in (M) contained At-CYP. The top At-gCYP bands in (L) are from unspliced pre-mRNA. Emb, embryo; End, endosperm; F1 and F2, flower buds; P, pith; R, root; SC, seed coat.
Figure 2.
Figure 2.
Greenhouse Phenotypes. (A) Leaf height of seedlings. (B) Leaf sheath. The boxes show the positions of the nodes. (C) Leaf angle. (D) Leaf angle in the wild type. (E) Leaf angle in Zm-CYP-1. (F) Tillering. (G) Panicle size (the panicles are among the largest three for each plant). (H) and (J) Seed size in the wild type (branch 9). (I) and (K) Seed size in Zm-CYP-1 (branch 9). Arrows in (D) and (E) point to leaf joints.
Figure 3.
Figure 3.
Expresssion of At-gCYP in Seed. (A) Fluorescence in seed. (B) iHAP × uAt-gCYP fluorescence in F2 seed. (C) gHAP × uAt-gCYP fluorescence in F2 seed. (D) Weights of F2 seed. −, +, and ++ represent segregants: wild type, hemizygous, and homozygous for GFP. Bars indicate se.
Figure 4.
Figure 4.
Effects of At-gCYP on Seed Filling. The data were calculated by weighing two sets of 100 double hemizygous aHAP1 × uAt-gCYP F2 seed and 100 aHAP1 × uAt-gCYP F2 seed, ordering each group of weights from heaviest to lightest, and subtracting from them the weights of 100 wild-type seed. The heavier seeds are to the left and the lighter to the right. Bars indicate se.
Figure 5.
Figure 5.
Field Phenotypes. (A) Seed size in the wild type. (B) Seed size in Zm-CYP-1. (C) Tiller number in the wild type. (D) Tiller number in Zm-CYP-1. (E) Mature plants in the field. (F) and (G) Representative RT-PCR of mRNA levels. The RT-PCR primers in (F) were the same as those for Zm-CYP in Figure 1L, but the primers in (G) were different from those for At-gCYP in Figure 1L. All primers were designed to coding and terminator sequences.
Figure 6.
Figure 6.
Microarray Gene Expression Analysis. (A) Induction of the 33 genes expressed only in the flag leaf, including 18 genes encoding protein kinases (red), and induction of the 28 genes expressed only in the seed, including 12 genes encoding hypothetical proteins (red). (B) Microarray data represented as colored lines for UDP-glucose pyrophosphorylase (1), trehalose-phosphate synthase (2), and trehalose phosphatase (3) in the shunt from glucose-1-phosphate to trehalose in the flag leaf. (C) Microarray data represented as colored lines for phosphoglucomutase (1), UGPase (2), sucrose-phosphate synthase (3), sucrose synthase (4), glucose-1-phosphate adenylyltransferase (5), starch synthase (6), and 1,4-α-glucan branching enzyme (7) in the pathway from glucose-6-phosphate to sucrose and starch. (D) Microarray data for farnesyl-diphosphate synthase (1), geranylgeranyl-pyrophosphate synthase (2), squalene synthase (3), squalene monooxygenase (4), cycloartenol synthase (5), and lanosterol synthase (6) in the pathway from geranyl pyrophosphate and geranylgeranyl-pyrophosphate to cycloartenol and lanosterol in the seed. Orange and red, >2-fold and >5-fold induction, respectively; blue, >2-fold repression; yellow, no change; gray, no data.
Figure 7.
Figure 7.
Effects of At-gCYP on Photosynthesis. (A) Maximum quantum efficiency (Fv/Fm). (B) CO2 uptake. Bars indicate se.

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