Htd1

High Tillering Dwarf 1(htd1), encoding carotenoid cleavage dioxygenase OsCCD7, inhibiting rice branching, is a negative regulator of rice tillers. (from reference)

Annotated Information

In earlier studies, a high-tillering dwarf indica rice, Aitaiyin 2, was found to carry two nonallelic semidwarf genes, sd-1 and sd-t(t) (from reference). Through the segregation analysis, we realized that the sd-t(t)gene was actually responsible for its high tillering and partially its severely dwarfing in Aitaiyin 2(data not shown). To clearly define this gene with the related phenotypes, we rename sd-t(t) as the high-tillering dwarf 1 htd1.

Function

The htd-1 mutant exhibits excessive tillers

Figure 1 Fig. 1 Tillering phenotypes of Nanjing 6 and the htd-1 mutant. Comparison of tillering abilities between Nanjing 6 (a, c, e) and the htd-1 mutant (b, d, f) at the onset of tillering stage (a, b), at the peak of tillering stage(c, d) and at the heading stage (e, f). The arrows point to the outgrowth of axillary buds. (from reference)

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Figure 2 Fig. 2 Comparison of tiller numbers at different leaf stages between htd-1 mutant and Nanjing 6. Tiller numbers were counted at different stages and shown as mean±SE (n=10). Numbers below the abscissa indicate different leaf stages defined by leaf numbers that the main culm of rice has outgrown. (from reference)

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The htd-1 mutant exhibits excessive tillers as a predominant phenotype in its whole life stage. Early in the third leaf stage, the htd-1 seedlings begin to grow out axillary buds from their subtending leaves, followed by the emergence of tillers, whereas the Nanjing 6 seedlings do not grow out their tillers at this stage. In addition, compared to Nanjing 6, the htd-1 mutant has no more axillary buds at its each leaf axil, i.e., it has only one axillary bud in each leaf axil just as that of Nanjing 6 (Fig. 1). At the maturity stage, the number of tillers of each htd-1 plant (99.5±12.2) is about nine times as that of each Nanjing 6 plant (11.1±0.74; Fig. 2). Usually, wild-type varieties increase their tiller numbers until the onsets of culm elongation and panicle initiation, and then the tiller numbers of each plant become immobile. Nevertheless, at this stage, the htd-1 mutant still keeps the high tillering capacity (Fig. 2). These observations suggest that the enhanced tillering capacity of the htd-1 mutant should result from the release of axillary buds from their dormant stage, but not from the increase of axillary bud numbers.

The dwarfing pattern of the htd-1 mutant is independent of GA

Besides excessive tillers, the htd-1 mutant displays the reduction of plant height. The htd-1 mutant is only 60% of Nanjing 6 in plant height (82.6±3.2 vs 140.5±3.3) after heading (Fig.1e, f). According to the classification scheme of rice dwarf mutants (Fig.3a; Takeda 1977), the panicle and the upper five internodes of the htd-1 mutant and Nanjing 6 were measured, respectively. In addition, the internodes of the other semidwarf (sd-1) variety, Nanjing 11, were also measured and compared with those of the htd-1 mutant. As shown in Fig.3b, c, the htd-1 mutant belongs to the dn-type of dwarfism, i.e., shows average shortening of each internode, whereas the sd-1 plant with its lower three internodes significantly shortened seems not fit into any dwarf types defined by Takeda (from reference). In rice, internode elongation is caused by cell division in the intercalary meristem, followed by cell elongation in the cell elongation zone (from reference). Usually, dwarfing culms could be the result of a defect in one or both of the two processes. When the culms cells in Nanjing 6 and the htd-1 plants were observed, we could not find obvious difference in the culm cell size between them (data not shown). Therefore, the dwarf of the htd-1 mutant could not be attributed to a defect in cell elongation. To explore the possible connection between the dwarfing in the htd-1 mutant and the GA pathway, we examined the effects of the shoot elongation and the endosperm a-amylase activity induction by the GA application to the htd-1 mutant, both of which are GApromoted physiological processes (from reference). The experiment with Nanjing 6 as a control showed that the htd-1 mutant had an almost same ratio of the lengths of the second leaf sheath with and without prior application of GA as Nanjing 6 (Table 1). To examine if any defect in the signal transduction of GA exists in the htd-1 mutant, agar plate assay for amylase activity was conducted using the embryoless half-seeds. The same patterns of a-amylase activity induction by GA were observed in Nanjing 6 and the htd-1 mutant (Fig. 4). Based on these observations, the htd-1 mutant is thought to be neither a GA-deficient dwarf nor a GAinsensitive dwarf. We concluded that the dwarf of the htd-1 mutant should be independent of GA.

Figure 3Fig. 3 a–c Characterizing the dwarf pattern of the htd-1 mutant. a Schematic representation of internode elongation patterns of wild-type (WT) and various rice dwarf mutants (d6-, dn-, dm-, nl- and sh- types; redrawn from Takeda 1977). b Phenotypic exhibition of the consisting components for the rice plant height in Nanjing 6 (WT, left), htd-1 (middle) and Nanjing11 (sd1, right). P, panicle; I – V, the respective internodes from top to bottom. c Schematic representation of the contribution of each internode and panicle to the plant height in Nanjing 6 (WT, left), htd-1 (middle) and Nanjing11 (sd1, right) (from reference)

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Table 1 Effect of GA3 on elongation of the second leaf sheath. Mean lengths ± SD, n=5 (from reference)

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Figure 4 Fig. 4 a-Amylase production from embryoless half-seeds of Nanjing 6 (top) and the htd-1 mutant (bottom). The half-seeds were placed on starch plates containing 1 lM GA3(+GA3) or no GA3 (-GA3)s. (from reference)

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The high-tillering phenotype of the htd-1 mutant is not related to the levels of IAA and ABA

Considering that both IAA and ABA have been previously proposed to play roles in inhibiting lateral bud growth over a long distance(from reference), we measured the levels of IAA and ABA in the htd-1 mutant and Nanjing 6 seedlings. The levels of IAA and ABA in the two types of seedlings were nearly the same (IAA: Nanjing 6, 11.71±0.91 ng/g FW; htd-1, 11.79±0.86 ng/g FW. ABA: Nanjing 6, 65.1±1.50 ng/g FW; htd-1, 65.9±1.45 ng/g FW). In addition, when exogenous IAA or ABA was applied to the htd-1 mutant seedlings, its abnormal tillering phenotype could not be rescued. Therefore, we presume that the high tillering capacity of the htd-1 mutant may not result from a defect in synthesis of IAA and ABA, respectively.

Labs working on this gene

National Plant Gene Research Centre (Beijing), Institute of Genetics and Developmental Biology, Chinese Academy of Science, Beijing, 100101, China.( Junhuang Zou, Zengxiang Chen,Shuying Zhang)

References

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