II. 24. Chlorophyll-deficient chimeral frequencies in EMS, phytohormone and visible light alone and combination treatments.
A.S. Khalatkar and Y.R. Bhargava, Post Graduate Department of Botany, Nagpur University Campus, Nagpur 440 010, India.
The mutagen induced genetic alterations in the somatic tissue of plants are analysed as mosaicism or chimerism (Nilan and Vig, 1976), and these effects are considered to be deleterious, due to the high incidence of sterility and inhibited growth. The chlorophyll-deficient chimeras manifested in the form of white, yellow and brown streaks in Himalayan hull-less barley (Hordeum vulgare L.) plant population after 30 to 35 days of sowing, are reported to express wide differences in the frequency and the size of the altered sector. However, in the present study with hormonal pre-, during- and post-treatments in different light conditions, there were indications about the protection against the EMS induced Ml chlorophyll chimeras.
In the present study, phytohormones such as gibberellic acid (GA), indole acetic acid (IAA), naphthoxy acetic acid (NAA), 2,4-dichlorophenoxy acetic acid (2,4-D) and kinetin (Kn), were utilized for the pre-, during- and post-treatments of ethyl methan sulfonate (EMS). All these treatments were carried out in the presence of the different wave-lengths (450-670 nm) of visible light.
Except for 2,4-D (Khalatkar and Bhargava, 1982), the other hormone and light controls did nod induce the chlorophyll-deficient chimeras in Ml generation; whereas, with the involvement of EMS in the pre-, during- and post-treatments, the plants with chlorophyll-deficient sectors were induced. EMS pre-treatment control induced the maximum frequency of chimeral streaks and this was true with a few exceptions for most of the EMS controls.
Considerable reduction in the chimeral frequency was obtained with NAA pre-treatment in white light in contrast to the maximum frequency of chimeras with EMS alone in similar light condition. A steep reduction in the chimeral frequency with GA pre-treatment in red light was noted. The pre-treatment with all hormones indicated significant reduction in the chimeral frequency and of the different hormones NAA was outstanding (Fig. 1). However, the data on the during-treatment indicated a limited protection against the induction of chimeras with phytohormones. In this again, NAA was more effective in all the light conditions. Besides, GA in blue as well as in green and Kn in green also exhibited a steep fall in the chimeral frequency (Fig. 2). While NAA post-treatment in all light conditions was significantly effective against the induction of chlorophyll-deficient streaks, similarly, Kn in blue light reduced the chimeral frequency considerable (Fig. 3).
The data in the present investigation indicates considerable protection with GA, IAA, NAA, 2,4-D and Kn pre-treatments for the EMS induced genetic alterations, resulting in the production of chlorophyll-deficient chimeras. These hormones could either be providing direct protection against the EMS effect or the EMS induced mutagenic lesions are repaired. The results obtained support both the contentions, as in the pre-treatment the hormones are already present in the cell before EMS finds an entry and therefore is protective. While in the post-treatment the EMS had already acted upon the genetic material and the hormonal entry followed, which might have repaired the EMS induced damage. However, in the case of during-treatment, the seeds were exposed simultaneously to EMS and hormones which brought about a smaller reduction in the frequency of EMS induced M1 chlorophyll-deficient chimeras.
Though the mode of occurrence of chlorophyll chimeras is not clearly known, one of the causes for the induction of the chlorophyll deficient chimeras could be the alteration of DNA in the chloroplasts. According to Chun et al. (1963), the chloroplast DNA is rich in the guanine-cytosine content in spinach, beet, Chlamydomonas and Chlorella. And, as EMS is known to react preferentially with guanine (c.f. Freese, 1963), there is a possibility that Ml chlorophyll-deficient chimeral streaks may be due to the alkylation of the chloroplast DNA. The data further indicate the modifying role of the different wave-lengths of visible light, which is apparent from the differential frequencies with the same phytohormone in various wave-lengths.
References:
Chun, E.H.L., M.H. Vaughn and A. Rich. 1963. The isolation and characterisation of DNA associated with chloroplast preparations. Jour. Mol. Biol., 7:130-141.
Freese, E. 1963. Molecular mechanism of mutations. In: 'Molecular Genetics', Academic Press, New York, pp. 207-269.
Khalatkar, A.S. and Y.R. Bhargava. 1982. 2,4-Dichlorophenoxy acetic acid - a new environmental mutagen. Mutat. Res. 103:111-114.
Nilan, R.A. and B.K. Vig. 1976. Plant test systems for detection of chemical mutagens. In: 'Chemical Mutagens - Principles and Methods for their detection', Plenum Press, New York, pp. 143-170.
