Glendon D. Ascough

Micropropagation of iridaceae-a review

The Iridaceae contains many hundreds of attractive species that have been overlooked for horticultural development. The incredible variation in flower and leaf shape, size and colour suggest this little-tapped resource could offer great opportunities for developing new commercial ornamental products. Micropropagation has increasingly become a valuable tool assisting breeders to release new species and cultivars into the market more rapidly. Here we review the progress made in Iridaceae micropropagation genus by genus, and highlight the potential for future expansion in this field.

The regulation of plant growth and development in liquid culture

Plant liquid culture offers many benefits over solidified media. Growth and multiplication rate of shoots, roots, bulblets and somatic embryos is enhanced in liquid culture, as a consequence of better availability of water and nutrients resulting from a lower resistance to diffusion and closer contact between the explant and the medium. Morphogenic development, such as meriste- moid production and somatic embryogenesis, is influenced by the physical environment provided by the liquid culture, and benefits from reduced gradients within the medium and dilution of exuded toxins. Chemical growth regulating factors, including plant hormones, growth retardants and other bioregulators are also discussed in terms of their influence on growth and development. Automation and scaling up for bioreactor production of propagules requires optimisation before becoming commercially viable. Considerable potential exists for future research into the molecular aspects regulating developmental pathways such as somatic embryogenesis, storage organ formation and meristemoid induction.

A smoke-derived butenolide alleviates HgCl2 and ZnCl2 inhibition of water uptake during germination and subsequent growth of tomato – Possible involvement of aquaporins

Aquaporins, concentrated in zones of cell division and enlargement, play a major role in governing the movement of water between neighboring cells during seed germination. The enhanced germination and growth found with smoke-water and butenolide could be the result of better water uptake, suggesting the involvement of aquaporins. The effects of butenolide, known aquaporin inhibitors (HgCl(2) and ZnCl(2)), along with several chemical agents known to reverse the inhibitory effects of mercuric chloride on the activity of aquaporins were tested. Seedlings raised in the presence of butenolide had higher moisture content (93%) compared to those imbibed in water only (85%). This suggests enhanced activity of aquaporins. The presence of aquaporin inhibitors (HgCl(2) and ZnCl(2)) reduced seedling water content and altered root development. The presence of HgCl(2) (10, 20 or 30 microM) reduced the percentage imbibition of seeds by 11-12%. A corresponding gradual decline, from 17.5% (10 microM) to 22.6% (30 microM) (p0.05), in the root length was recorded. Addition of dithiothreitol (DTT, 500 microM), beta-mercaptoethanol (ME, 250 microM) and butenolide (0.1 microM) along with the HgCl(2) overcame the observed inhibitory effects. The presence of ZnCl(2) (12.5 or 25 microM) affected percentage imbibition as well as root length, which was reversed to some extent following addition of the butenolide. Though zinc chloride-mediated inhibition remained unaffected by the presence of DTT and ME, the butenolide reversed the effect. These results are interesting as they suggest additional avenues of research for uncovering the profound effect butenolide has on germination and growth.

Flower abscission: environmental control, internal regulation and physiological responses of plants

Many aspects of the abscission process have been reviewed in the past and it has been some time since a review on flower abscission has been published. Recent advances in this field thus permit a consolidation of the current research. The flowering syndrome in plants is an intricate and complex process that ensures survival of the species. As such, it is a strictly controlled and tightly regulated series of developmental events, the success of which is a major priority. Abscission of flowers and floral organs is an important part of the reproductive cycle, that is similarly under exact direction. Flowers may be shed in response to an internal or genetic timing mechanism as well as external factors such as environmental stimuli, pathogen attack and pollination. These cues are processed and converted into physiological responses that include alteration in hormone production, induction and repression of specific signal transduction pathways, activation of existing enzymes and transcription of new proteins. While much progress has been made in identifying components of abscission using mutants and molecular biological approaches, many gaps remain in our understanding, especially in terms of considering abscission and its related events in the light of energy (carbohydrate) supply and demand.

Triploidy causes sexual infertility in Cyrtanthus breviflorus (Amaryllidaceae)

The balance between sexual and asexual reproduction can vary markedly in clonal plants. At one extreme, plants are sexually infertile and reproduction is solely clonal. Infertility can be caused by environmental and/or genetic factors, but the role of each is often unknown. Here we determine variation in sexual reproduction and explore the underlying factors causing sexual infertility in Cyrtanthus breviflorus Harv. We examined open- and cross-pollinated fruit set, ploidy using flow cytometry, pollen viability, pollinator visits to flowers and pollen deposition onto stigmas. One population was sexually infertile; no plants produced fruit. Three populations were sexually fertile; >98% of plants produced fruit. Percent pollen viability differed between infertile (18%) and fertile (97%) populations. The most likely cause of infertility was unequal ploidy. Plants in the infertile population were triploid, whereas those in fertile populations were diploid. Pollination factors were not related to infertility. In infertile and fertile populations, pollen-collecting insects visited flowers frequently, depositing 4-fold more pollen grains onto stigmas than the number of ovules per flower. Our study is the first to demonstrate infertility and triploidy in C. breviflorus. How triploidy became established despite high levels of pollinator activity remains a challenging question.

Hormonal and cell division analyses in Watsonia lepida seedlings

The regeneration ability, cell division activity, auxin and cytokinin content of seedling regions and hypocotyl subsections of Watsonia lepida were studied. A total of 21 different cytokinins or conjugates were found in seedlings, with the highest cytokinin content in meristematic regions (root and shoot apical meristems). The greatest contribution to the cytokinin pool came from the biologically inactive cZRMP, suggesting that significant de novo synthesis was occurring. Five different auxins or conjugates were detected, being concentrated largely in the shoot apical meristem and leaves, IAA being the most abundant. Analysis of hypocotyl subsections (C1-C4) revealed that cell division was highest in subsection C2, although regeneration in vitro was significantly lower than in subsection C1. Anatomically, subsection C1 contains the apical meristem, and hence has meristematic cells that are developmentally plastic. In contrast, subsection C2 has cells that have recently exited the meristem and are differentiating. Despite high rates of cell division, cells in subsection C2 appear no longer able to respond to cues that promote proliferation in vitro. Auxin and cytokinin analyses of these subsections were conducted. Possibly, a lower overall cytokinin content, and in particular the free-base cytokinins, could account for this observed difference.

Stimulatory effects of smoke and smoke-derived butenolide on crop plants

Plant-derived smoke is a powerful germination cue that promotes the germination of many species from fire-prone and non-fire areas [1]. A butenolide-type compound, 3-methyl-2H-furo[2,3-c]pyran-2-one, isolated from plant-derived smoke, was found to be the highly active germination cue present in smoke [2]. It promotes germination at concentrations as low as 10-9 M, and also promotes seedling vigour of several crop plants such as tomato, bean, okra and maize [3]. Preliminary toxicity testing has shown that the compound is not mutagenic or genotoxic at the concentrations tested (1×10-4 M and lower) [4]. Treatment of tomato (Solanum lycopersicum cv. Heinz-1370) seedlings with smoke-water and 1×10-9 M butenolide was conducted under greenhouse conditions. Smoke-water treatment resulted in maximum height, number of leaves and stem thickness from 57 to 78 days after sowing. For both smoke-water and butenolide-treated plants, the percentage of plants with fruit was higher than the control. In addition, smoke-water treatment significantly (P<0.05) increased the total number of marketable fruit. The butenolide-treated plants also yielded more fruit (although not significant). Thus, smoke technology and the smoke-derived butenolide hold promise for use in agriculture and horticulture for improving seed germination, seedling vigour and potentially increasing overall yield. Acknowledgements: National Research Foundation (NRF, Pretoria), University of KwaZulu-Natal. References: 1. Van Staden, J. et al. (2000) Plant Species Biol. 15:167–178. 2. Van Staden, J. et al. (2004) S. Afr. J. Bot. 70:654–659. 3. Van Staden, J. et al. (2006) Field Crops Res. 98:98–105. 4. Verschaeve, L. et al. (2006) Mutat. Res. 611:89–95.