| Some cytological aspects of diploid wheat anther culture
B.H. TAN* and G.M. HALLORAN Plant sciences Section, School of Agriculture and Forestry, University of Melbourne, Parkville, Victoria 3052, Australia. Previous attempts (FUJII 1970, HESZKY and MESCH 1976) at inducing callus in anthers of diploid wheat (Triticum monococcum L., 2n=14) were unsuccessful. Recently we studied the responses of eight T. monococcum lines collected by one of us (G.M.H.) in Asia Minor. Callus induction was feasible only in three of these lines. In that preliminary study we noted the occurrence of pollen dimorphism (Fig. 14), which was manifest in the late uninucleate and later stages of pollen development. The phenomenon has also been reported in certain cultivar(s) of hexaploid wheat, barley, oats, tobacco and Paeonia (SUNDERLAND 1978). The cytoplasm of the larger and predominant delete form is rich in starch granules and stains readily with acetocarmine, both characteristics of which are absent in the smaller form. According to DALE (1975) only the latter type is potentially morphogenic in barley. The developmental sequences from meiosis to the tetrad in vivo (Figs. 1-2), and from early-uninucleate to the mature trinucleate pollen (Figs. 3-10) both in vivo and in vitro are similar to those already described for other cereal species. During anther culture a variable proportion of pollens at the mid- or late-uninucleate stage is capable of unrestrained mitotic division to form multi-cellular pollens (m.c.p's) within the anther sac - which in essence is the cytological basis toward callus formation in situ. The so-called 'A-Pathway' (SUNDERLAND and DUNWELL 1974), whereby the vegetative nucleus divides repeatedly in contrast to the quiescence of the generative nucleus(i), appeared to be the more common pathway in pro-callus morphogenesis (Figs. 16-19). Although the 'B-Pathway' was also represented it is insignificant in T. monococcum. The latter mode of morphogenesis is often arrested at the bicellular stage. Non-haploid nuclei are common in such pollens (Fig. 15), probably as a result of endo-mitosis or endoreplication. Free nucleus(i) are sometimes observed following the first mitotic division (Fig. 11); SUNDERLAND (1978) suggested that the parallel orientation of the spindles, in relation to the intine, could be responsible for the absence of cell wall formation. Of considerable interest was the observation that an inoculated anther was either inductive, in which case about 5-40% of (uninucleate) pollens developed into m.c.p.'s and pro-calli after about three weeks, or that it was otherwise non-supportive of morphogenesis (Fig. 12-13): m.c.p. induction is thus sine qua non. Rarely was more than one sister anther from the same floret found to be inductive, which suggests that the inductive stimulus is not ubiquitous. It is sheer speculation to suggest what the inductive stimulus (i) or substance (s) might be, but it probably holds the key to putting anther culture on a more predictable basis. It could be hormonal, or, it could involve the removal of an inhibitory substance (s) present in excess or only in some but not all the anthers. The presence of activated charcoal in the culture medium should be beneficial in the latter situation, but comparative studies examining the value of adsorbents are not always conclusive. Our current investigations, focusing on the influence of medium and effect of spike pre-treatment on the frequencies of anther induction and subsequent callus formation, will be reported at a later date. Ackuowledgements Financial support received from the Victorian Wheat Research Committee is hereby gratefully acknowledged. The work was conducted while the senior author held a Melbourne University Research Fellowship. Literature Cited DALE, P.J. 1975. Pollen dimorphism and anther culture in barley. Planta 127: 213-220. FUJII, T. 1970. Callus formation in wheat anthers. Wheat Inf. Serv. 31: 1-2. HESZKY, L. and J. MESCH 1976. Anther culture investigation in cereal bank collections. Z. Pflanzen- zucht. 77: 187-197. SUNDERLAND, N. and J.M. DUNWELL 1974. In 'Tissue Culture and Plant Science', ed. H.E. Street, Blackwell, Oxford. SUNDERLAND, N. 1978. Strategies in the improvement of yields in anther culture. Proc. Symp. Plant Tissue Culture, Peking p. 65-86. |
| * Present address: Department of Agronomy, Institute of Agriculture. University of Western Australia, Nedlands, W.A., 6009, Australia. |