Ⅲ-4. Radiation Chemistry & Biology

RIKEN Accel. Prog. Rep. 47 (2014)

The wheat plastochron mutant, fushi-darake, produced by heavy-ion beammutagenesis†
C. Tahira,＊1 N. Shitsukawa,＊1 Y. Kazama,＊2 T. Abe,＊3 and K. Murai＊1
Grasses such as wheat (Triticum aestivum) are the major
source of carbohydrates for humans, and the yield of grainfrom these crops is largely dependent on inflorescencearchitecture. A detailed understanding of development inwheat plants is of value not only to wheat breeding but alsofor basic scientific research. From the large scale mutant
panel of diploid einkorn wheat (Triticum monococcum)developed by heavy-ion beam irradiation1), we identified amutation that had an abnormally large number of nodes; wetermed this mutation fushi-darake (fdk), which means too
many nodes in Japanese.
The fdk showed drastic changes to their structuralorganization compared to wild type (WT) plants in the field(Fig. 1). Contrary to WT, fdk plants had 1/2 alternate
phyllotaxy with rapid leaf emergence. Consequently, the fdk
plants had a larger number of nodes and leaves compared tothe WT plants. In the fdk plants, vegetative shoot branches
emerged from the nodes in the upper part of the culm of
most tillers (Fig. 1). In these ectopic shoots, normal leaves
were produced with 1/2 alternate phyllotaxy. The culms of
fdk plants were unable to support the heavy upper
vegetative shoots, with the result that the plants collapsed
onto the ground (Fig. 1).
We examined the timing of leaf unfolding in WT and fdk
seedlings grown in a growth chamber. The rate of leaf
emergence was more rapid in fdk compared to WT after the3-leaf stage. This indicates that rapid leaf emergence in fdk
resulted from a rapid rate of leaf initiation: the plastochronof fdk plants was estimated to be half of that in WT.

fdk

WTFig. 1. fushi-darake (fdk) mutant plant grown in the fieldand green house. WT: wild-type wheat strain KU104-1

To investigate further the morphological differences
between WT and fdk plants, we carried out an SEM(scanning electron microscope) analysis of shoot apicalmeristem (SAM) development. SAM development in fdkplants was very different to that of WT plants. The SAM
elongated but its branch meristems (BMs) resembled leaf
primordia rather than spikelet meristems (SMs). Ectopic flat
dome-like BMs that were similar to leaf primordia were
initiated with a1/2 alternate pattern. These observations,
together with those on the morphology of the fdk, suggestthat the flat dome-like BMs develop into vegetative shoots.
The inflorescence of grass species is composed of aunique unit called the spikelet. When the wheat planttransits from the vegetative to reproductive growth phase(flowering), the SAMs are elongated and spikelet meristems(SMs) initiate as lateral branches. Our SEM analysis of thefdk mutant indicated that differentiation of SMs wasdelayed and the leaf primordia were initiated from branchmeristems (BMs) with 1/2 alternate phyllotaxy. Theseobservations suggest that 1/2 alternate phyllotaxy with rapid
leaf emergence produced the shortened plastochron in the
fdk mutant. The SAMs further elongated and produced flat
dome-like BMs at the position of the original SMs. We also
found that fdk plants had vegetative shoot branchesemerging from the nodes of upper part of culm of almost alltillers. Thus, our results suggest that these vegetative shoots
are likely to be developed from the BMs of elongated
SAMs. In conclusion, our findings indicate that theabnormal phenotype of the fdk mutant resulted fromtransformation of SMs into vegetative shoots.
Three plastochron mutants, plastochron 1 (pla1) 2), pla23)
and pla34), have been identified in rice (Oryza sativa).
Among them, we found that wheat fdk and rice pla1
mutants show similar phenotypes. These facts indicate thatsome common genetic cascades are involved in thephenotype of wheat fdk and rice pla1. The WT gene,
PLASTOCHRON 1 (PLA1), encodes a member of aplant-specific subfamily of cytochrome P450, CYP78A11,
which potentially catalyzes substances controlling plantdevelopment5). The similar phenotypes of the fdk with pla1,
suggest that PLA1 or related genes may be candidates forthe fdk mutation in wheat.
References

1) K. Murai et al.: Nuclear Instruments and Methods in Physics
Research B 314. 2013, 59.
2) J.-I. Itoh et al.: The Plant Cell 10. 1998, 1511.
3) T. Kawakatsu et al.: The Plant Cell 18. 2006, 612.
4) T. Kawakatsu et al.: The Plant Journal 58. 2009, 1028.
5) K. Miyoshi et al.: PNAS 101, 2004, 875.

†Condensed from the article in American J. Plant. Sci. 4, 28 (2013)
Department of Bioscience, Fukui Prefectural University＊2
RIKEN Innovation Center＊3
RIKEN Nishina Center＊1- 292 -