Abstract
Auxin flows in a polar manner to target tissues and exert its morphogenic effect. Preventing auxin movement, with polar auxin transport (PAT) inhibitors, or increasing auxin levels in tissues through exogenous application can provide a means for assessing the importance of appropriate tissue distribution and concentration of this hormone during development. The formulation of culture media for micropropagation has been the primary focus of most orchid tissue culture research, a goal that unveils seedling hormone responses at a single point in development. This study was unique because it evaluated the auxin response of orchids during three stages of seedling development. Seedlings were grown on standard culture media for 10, 35 and 85 days. Each group was sub-cultured onto auxinand/or PAT inhibitor-containing media for an additional 10, 30 and 60 days, respectively. Data were collected on first leaf initiation, trichome formation and the appearance of propagative structures. In the 20-day seedlings, auxins and PAT inhibitors promoted precocious formation and random placement of protocorm hairs rather than in tufts, as seen in older, control seedlings. The 65-day seedlings formed protocorm-like bodies, rhizome-like growths from the stem, and fleshy leaves with trichomes. Seedlings cultured for 145 days developed microshoots or callus growth in the axils of older leaves and exhibited necrosis of original seedling roots and leaves. In general, exogenously applied auxin promoted the reversion of differentiated Spathoglottis plicata seedling tissue to a morphology that had propagative properties. Additionally, auxins commonly induced hair formation, which suggests that protocorm hairs may be root hair-like in nature. This work characterized three auxin growth responses in S. plicata seedlings that have not been reported in orchids: (i) the inhibition of first leaf initiation and abnormal first leaf morphology; (ii) the promotion of trichome formation; and (iii) the induction of rhizome-like structures and microshoots at a specific stage in seedling development.
Highlights
Phytohormones are key ingredients in the design of orchid culture media due to their effect on tissue and organ morphogenesis
Polar auxin transport inhibitors and exogenously applied auxins inhibit first leaf formation, promote hair growth and increase protocorm diameters in young seedlings (20 days after culture (DAC)) Protocorms grown from seed sown on standard media for 10 days were sub-cultured onto media that had been supplemented with polar auxin transport (PAT) inhibitors or auxin, and seedlings were grown for an additional 10 days
In contrast to the control seedlings, auxin- and PAT inhibitortreated seedlings had protocorms that either lacked a first leaf or generated short, fleshy leaves (Fig. 1). Those exposed to 2 mM 2,4-D were most affected, having only 5 % first leaf emergence compared with 80 % in the control (Table 1). 3-Indoleacetic acidtreated protocorms had first leaf percentages comparable to those of seedlings grown on 100 mM monensin or triiodobenzoic acid (TIBA), 40 % (Fig. 1B, D; Table 1). 2,3,5-Triiodobenzoic acid and monensin exhibited a dosage-dependent response, such that higher levels more effectively prevented first leaf formation or stunted first leaf growth (Table 1; Fig. 1D)
Summary
Phytohormones are key ingredients in the design of orchid culture media due to their effect on tissue and organ morphogenesis. Many of these hormone-containing recipes induce the formation of propagative structures, such as protocorm-like bodies (PLBs), rhizomes, callus tissue and microshoots (Arditti 2008). Understanding hormonal impacts on seedling morphogenesis will provide insights about development and enable researchers and growers to be more prescriptive in media design for future studies and cultivation. This work addressed the impact of auxin distribution on the initiation of first leaves and the effect of exogenous auxin application on the formation of trichomes and propagative structures in S. plicata seedlings at three stages of development
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