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Identification of exe Mutants

Seeds of the diploid einkorn wheat (Triticum monococcum L., 2n = 2x = 14, genome constitution AmAm) strain KU104-1 were given 50 Gy of 50 keV μm−1 LET (linear energy transfer) carbon ion beams and then sown in the field. The spikes of M1 plants were bagged and the harvested selfed seeds of each spike were used to produce the M2 lines. From approximately 1,200 M2 lines, we identified plants showing an abnormal extra early-heading phenotype; we termed these mutants extra earlyflowering (exe). The original wild type (WT) strain KU104-1 is a spring habit einkorn wheat having a dominant VRN1 allele and a null VRN2 allele. Therefore, the

exe mutants, exe1, exe2, exe3, exe4, identified in this study have no active gene at the VRN2 locus.

Table 19.1 shows the heading time of the WT and exe mutants in the field. Based on the heading time, the exe mutants were classified into two groups: Type I showed moderately extra early-heading type (exe1 and exe3); and Type II showed extremely extra early-heading type (exe2 and exe4). In the field, Type I and Type II headed about 30 and 45 days earlier than the WT, respectively.

Table 19.1 Heading time of the exe mutants and wild type (WT) plants grown in the field

Lines

Heading time

Difference from WT

Type

WT

6 June

exe1

7 May

−30

Type I

exe2

22 April

−45

Type II

exe3

7 May

−30

Type I

exe4

25 April

−37

Type II

Data from season 2011/2012

Morphological Characteristics of the exe Mutants

WT plants and exe mutants were grown in the experimental field at Fukui Prefectural University and their morphological phenotypes were characterized during the maturation stage. Three agronomic characters were assessed: internode length, spike length, and spikelet number per spike.

The internode lengths of exe mutants were shorter than those of WT plants (Fig. 19.1). In particular, Type II exe mutants showed a significantly shorter first internode length than WT plants. Compared to WT plants, exe mutant plants produced smaller spikes with fewer spikelets (Fig. 19.2). As a consequence of the smaller numbers of spikelets, spike lengths in exe mutants were shorter than in WT plants. Furthermore, the spikes of Type II exe mutants were smaller than those of Type I mutants. These observations indicate that shortened culm lengths and fewer spikelets per spike are associated with the extra early-flowering phenotype in the exe mutants.

Hypothetical Model for Extra Early-Flowering Phenotype

The expression analysis of VRN1 indicated that VRN1 is highly expressed at earlier stages in Type II mutants than in Type I mutants under both short day (SD) and long day (LD) conditions (data not shown). This clearly indicates that the difference in earliness between Type I and Type II mutants is associated with the level of VRN1 expression. Thus, VRN1 is essential for flowering in wheat, and the level of expression of VRN1 determines flowering time (Fig. 19.3).

Fig. 19.1 Comparison of internode lengths of exe mutants and wild type (WT) plants grown in the field.

First to fourth internode lengths are shown. The mean from the main culms of five plants of each line are shown. Asterisks indicate significant P values (Student's t-test):

*p < 0.05 and **p < 0.001 vs WT

Fig. 19.2 Spikes of exe mutants and wild type (WT) plants grown in the field. The exe mutants showed a significantly decreased spike length compared to the WT

Fig. 19.3 Schematic outline of the proposed mechanism for the extra early-flowering phenotype in exe mutants. This is based on the model of Shimada et al. (2009) in which VRN1 acts as an integrator of the vernalization and photoperiodic pathways that are coordinated with the autonomous pathway. VRN1 acts by up-regulating the florigen gene VRN3. In exe mutants, the mechanism for suppressing expression of VRN1 under SD conditions must be disrupted. Levels of accumulation of VRN1 transcripts induce VRN3 expression, resulting in the extra early-flowering phenotype. Note that the exe mutant has no VRN2 gene, because the original strain KU104-1 lacks VRN2 locus. Arrows and T-bars indicate promotion and suppression effect, respectively. Arrows indicated by bold lines show stronger effects

 
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