Pascani, Republic of Moldova

Maize and Sorghum Research Institute

Differences in sensitivity to herbicide of maize lines obtained under disruptive selection for reversion frequency of the mutable allele o2-m(r) --Koterniak VV Earlier (MNL, 73: 76-79) we reported about the lines obtained under disruptive selection (started in F5 progeny of one ear) for low and high content of whole endosperm revertants (WER). These lines were designated as LFWER and HFWER respectively. The frequency of WER formation is conditioned by the frequency of reversion of the mutable allele o2-m(r) (as a result of excision of the receptor element rbg from the opaque2 (o2) locus in the presence of the regulatory element Bg) which takes place before the first meiotic division of the primary endosperm nucleus. We established that differences in WER content in lines obtained are determined by changes in state of the initial receptive allele o2-m(r):3449 and the regulatory element Bg-3449. Changed forms of the receptive alleles and the regulatory elements presented in LFWER and HFWER lines were respectively designated as o2-lf, Bg-lf and o2-hf, Bg-hf. Lines obtained differed not only in WER content but also in some quantitative traits (MNL, 73: 76-79).

In 1999 we observed significant differences between LFWER and HFWER lines in their sensitivity to herbicide "Buctril D", Rhone Poulenc Inc., (22.5% of bromoxinil + 22.5% of 2,4-DMA) applied for weed control in the concentration of 0.56 kg/ha of the active substance at the plant stage of 3-5 leaves. Lines obtained under selection for low WER content (LFWER lines) did not show visible symptoms of sensitivity to the herbicide. In contrast with this, HFWER lines were characterized by high sensitivity to the herbicide expressed in strong leaf twisting (Fig. 1).

Stronger sensitivity of the HFWER lines to the herbicide was quite unexpected since the plants of these lines in comparison with the plants of LFWER lines are more vigorous, have higher kernel weight, kernel volume and number of leaves on the main stalk (MNL, 73: 76-79).

Figure 1. Herbicide sensitivity (expressed in leaf twisting) of the lines obtained under selection for high frequency ("HF", left row) and low frequency ("LF", right row) of whole endosperm revertants.

Sensitivity to herbicide of HFWER lines is not connected directly with the frequency of reversion of the mutable allele o2-hf. Either the "standard" HFWER lines or the o2-hf; Bg-hf genotype and their o2-hf; +Bg derivatives lacking the regulatory element Bg with the typical o2 endosperm appearance and the absence of WER both were sensitive to herbicide. The same sensitivity to herbicide was observed in HFWER sublines with WER frequency close to that of LFWER lines obtained as a result of reverse selection for WER content. (The reverse selection, i. e. selection for low WER content at HFWER lines and high WER content at LFWER lines, was effective for the former and ineffective for the latter, suggesting deletion changes in the o2-lf allele and indicating that the change in state which led to formation of this allele was caused most likely by internal deletion of the rbg element (Maydica, 1999, in press)).

It seems that differences in sensitivity to herbicide between LFWER and HFWER lines are not linked with the possible differences in their developmental stages. Though LFWER lines are earlier in comparison with HFWER lines by 1-3 days (MNL, 73: 76-79). The big majority of the plants of these lines were in the same developmental stage as the plants of HFWER lines. Thus 79% (34 out of 43) of the plants of LFWER families had the same date of flowering of male inflorescences as the plants of HFWER families (indicated data were obtained on the families of LFWER and HFWER lines used in the reverse selection program). Notwithstanding this, the plants of LFWER lines did not show visible reaction to herbicide.

Since LFWER and HFWER lines were obtained under disruptive selection for WER content and are characterized by the different states of the Bg-rbg system components (which are responsible for WER content) we can suggest that differences between these lines in herbicide sensitivity (as well as differences in quantitative traits) are also connected either with disruptive selection for WER content or with the states of the Bg and rbg elements. In case of insertions of the rbg or Bg in the genes determining these traits (e. g. insertion in the gene(s) responsible for herbicide resistance) the activity of such genes can be changed and be dependent on the activity of the components of the Bg-rbg system of transposable elements. If the insertions of rbg or Bg elements in the mentioned genes were presented in the initial source of instability before the disruptive selection was started we can infer that later these elements could undergo spontaneous changes in state (as a result of internal deletions, intragenic transposition etc.) which could be picked up by the following disruptive selection. Such changes in state could have not only spontaneous character but be induced by the rbg or Bg elements responsible for low and high WER content in these lines (i. e. by the rbg elements presented in o2-lf and o2-hf alleles and by the Bg-lf and Bg-hf elements). This kind of interaction can be assumed taking into account the paper of Cuypers et al. (EMBO J., 1988, 7: 2953-2960) in which it was established that a defective En-I102 element reduced the excision frequency of both the autonomous En-1 element and the inhibitor element Spm-I5719A.

Bg and rbg elements could affect the activity of the genes responsible for the herbicide resistance and quantitative traits also through their insertions in such genes arising as a result of transpositions of the receptor elements from the o2-lf and o2-hf alleles and transpositions of the regulatory elements Bg-lf and Bg-hf. Because of differences of these Bgs and rbgs (due to their changes in state) we can expect the differences in activity of the genes in which they are inserted.

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