Impaginato 399 Adv. Hort. Sci., 2021 35(4): 399­405 DOI: 10.36253/ahsc­11519 Strigolactone analogue GR24 reduces axillary bud out break and growth in tea tree, Melaleuca alternifolia (Maiden & Betche) Cheel G.E. Lowe (*), M. Shepherd, T. Rose (*), C. Raymond Southern Cross Plant Science, Southern Cross University, P.O. Box 157, Lismore, NSW 2480 Australia. Key words: auxin, axillary bud release, shoot architecture, tea tree. Abstract: Strigolactone acts with other plant hormones to influence shoot architecture by suppressing axillary bud outgrowth. The exogenous application of synthetic analogues of strigolactone, such as GR24, have been investigated as a way to manage plant architecture in a number of crops. In this study we test whether GR24 can be used to supress bud outgrowth in clonal propagules of tea tree (Melaleuca alternifolia) in order to retain a “single stem” form desir­ able for machine planting. GR24 was applied to decapitated rooted cuttings of tea tree at two rates (0.5 mg L­1 and 1.5 mg L­1), with and without auxin. By 21 days post ­treatment, GR24 at both rates had significantly (p<0.05), reduced the mean number of axillary buds (5.7±0.4 and 5.5±0.3 buds respectively) com­ pared to decapitated untreated control plants (8.9±0.6 buds). Suppression of buds was significantly higher again when auxin was applied in conjunction with GR24. Nonetheless, no exogenous hormone treatment was as effective at sup­ pressing bud outgrowth as the apical dominance that occurred in intact control plants (1.1±0.4 buds). 1. Introduction Tea tree, Melaleuca alternifolia (Maiden & Betche) Cheel, is a small tree, native to the subtropics of eastern Australia (ANPSA, 2012), which is cultivated in plantations for the production of a medicinally valuable essential oil (Carson et al., 2006). The Australian industry has developed around improved cultivars released as seed lines, but there is recognition that clonal deployment should offer advantages in plantation uniformity and productivity (Doran et al., 1997). Although tea tree has been regard­ ed as relatively easy to propagate by cloning (Shepherd et al., 2013), the production of high quality propagules with the desired form compatible with automated machine planters remains a challenge. Tea tree tip cuttings with single stem architecture suitable for mechan­ ical planting can be produced if the apical bud remains intact (Lowe et al., 2019). Nodal cuttings (apical tips removed) are preferred to tip cuttings by (*) Corresponding author: g.lowe.22@student.scu.edu.au terry.rose@scu.edu.au Citation: LOWE G.E., SHEPHERD M., ROSE T., RAYMOND C., 2021 ­ Strigolactone analogue GR24 reduces axil‐ lary bud out break and growth in tea tree, Melaleuca alternifolia (Maiden & Betche) Cheel. ­ Adv. Hort. Sci., 35(4): 399­405 Copyright: © 2021 Lowe G.E., Shepherd M., Rose T., Raymond C. This is an open access, peer reviewed article published by Firenze University Press (http://www.fupress.net/index.php/ahs/) and distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: All relevant data are within the paper and its Supporting Information files. Competing Interests: The authors declare no competing interests. Received for publication 21 December 2020 Accepted for publication 12 August 2021 AHS Advances in Horticultural Science Short note https://doi.org/10.36253/ahsc-11519 http://www.fupress.net/index.php/ahs/ http://creativecommons.org/licenses/by/4.0/ http://creativecommons.org/licenses/by/4.0/ http://creativecommons.org/licenses/by/4.0/ Adv. Hort. Sci., 2021 35(4): 399­405 400 some propagators for their robustness in the nursery but these tend to have a multi­stem habit which is unsuitable for machine planting as the “bushy” shoots get stuck in the delivery tubes of planters. Form­pruning of propagules prior to planting may be one way to overcome this compatibility issue, but this is labour intensive and costly on a large scale. Shoot architecture is a consequence of complex interplay of interacting plant hormonal signals involv­ ing auxin, cytokinin (CK) and strigolactone (SL) that regulate bud release and/or subsequent growth (Ferguson and Beveridge, 2009; Leyser, 2009). Auxin is produced in the shoot apex and is actively trans­ ported down the stem, restricting bud release. Auxin does not enter the axillary buds but indirectly impacts on bud release by inhibiting auxin export from axillary buds and by regulating stem CK and SL to either promote or suppress branching, respective­ ly (Gomez­Roldan et al., 2008; Muller and Leyser, 2011; Dun et al., 2012). The influence of SL upon plant architecture was unravelled with the aid of studies on mutants with increased branching, including the ramosus (rms) mutant in pea (Pisum sativum L.), and the more axil­ lary growth (max) mutant in Arabidopsis (Arabidopsis thaliana L.) (Brewer et al., 2009; Dun et al., 2013). Mutants have exaggerated branching habits relative to wild type plants because they are deficient in SL due to genetic changes in transcription or hormonal pathways (Umehara et al., 2008; Yaish et al., 2010). Potential horticultural applications of SL include, management of plant architecture and control of fruit ripening, although high cost of SL production and regulatory approvals remain a limitation to large scale commercial use (Vurro et al., 2016; Ferrero et al., 2018). In this study we investigated the use of exogenous hormones to manage shoot form of tea tree cuttings during propagation. The aim was to test whether application of an exogenous synthetic SL, GR24, or a combination of GR24 and auxin, can suppress axillary bud release in tea tree. Suppression by SL was stud­ ied in decapitated plants where axillary bud out­ growth was triggered by removal of the apical bud. 2. Materials and Methods Plant growth environment and reagents Experiments were conducted at Southern Cross University, Lismore Campus, NSW, Australia during 2020, inside controlled environment growth cabi­ nets, under 16/8 hour photoperiod, and with tem­ perature set at 26°C. Stock hormone solutions were prepared by following Brewer et al. (2009) for indole ­3 acetic acid (IAA), and Manandhar (2016) for GR24. Working solutions of 1.75 mg L­1 (10 µM) for IAA, and 0.5 mg L­1 (1.68 µM), and 1.5 mg L­1 (5 µM) for GR24, were prepared by dilution with distilled water and stored at 4°C. Method and reagent validation using pea plants Initial tests were carried out using the P. sativum type rms1 mutant (provided by Professor C. Beveridge, University of Queensland, Australia) to confirm competency of reagents and methods in our laboratory. Testing of mutant pea followed methods of Manandhar, (2016). Application of 0.3 mg L­1 and 1 mg L­1 GR24 to the rms1 mutant of pea significantly reduced side branching compared to the untreated rms1 control, thus validating the method in our labo­ ratory. Testing GR24 applications on tea tree The experiment used a Randomised Complete Block Design, with four blocks (replicates) of five plants in line plots, subject to one of six treatments (four hormone or two control treatments, giving a total of 120 plants). Plants subject to hormone treat­ ments either had a low or high treatment of GR24 (0.5 mg L­1 or 1.5 mg L­1) with or without supplemen­ tal IAA (1.75 mg L­1). Control treatments consisted of decapitated plants, and plants with an intact apex in distilled water. Scion from a clonal line was set in January 2020 using the mini cutting technique with intact apical buds (Lowe et al., 2019). The GR24 experiments were conducted 5 months post­setting when cuttings had rooted and possessed a single main stem with no visi­ ble axillary buds detectable with a 10x magnification hand lens (Fig. 1 left). Roots were washed free of media and trimmed to 5 cm in length. Except for con­ trol plants, the shoot apex was removed (approxi­ mately 2 cm) above node 35 (counted basipetaly from the lowest detectable node) to provoke release from apical dominance (Thimann and Skoog, 1934). All plants were then positioned upright in clear 50 mL Falcon tubes containing 5 mL of treatment solution, or distilled water, so that only the roots were in con­ tact with treatment solutions (Fig. 1 middle). The experiment was conducted over 21 days in June 2020. Treatment solutions were applied on day 1 and replaced every 7 days. For plants subjected to a Lowe et al. ‐ Manipulating shoot branching in tea tree 401 hormone combination treatment (GR24 and IAA), a 0.5mL droplet of IAA was placed on top of the decap­ itated stem to simulate the presence of the shoot apex and natural auxin production (Cline, 1996) (Fig. 1 right). All plants were given 1 mL of Yoshida nutri­ ent solution (Yoshida et al., 1976) 2 days before treatment solutions, in order to promote growth. Data collection and analysis The number of visible axillary buds and axillary shoot length for the 10 uppermost nodes (nodes 1 to 10, with 1 being furthest from the roots) on each plant was recorded at the same time on day 2, 5, 7, 9, 12, 14, 16, 19, and 21. Shoot length was deter­ mined to the nearest mm with digital callipers. One­ way analysis of variance (ANOVA) was used to test for treatment effects on the number of axillary buds and shoot growth. Where treatment effects were sig­ nificant (p<0.05), a Duncan’s multiple range test was performed on treatment means. All statistical analy­ ses were performed using Genstat, Release 19.1 (VSN International, 2018). 3. Results Effects of GR24 on axillary bud growth in tea tree Rates of bud release over a 21 day time course exper‐ iment Treatment effect was significant at each of the 9 days counts were undertaken (p<0.05) (Fig. 2). No buds were detected on control intact plants until Day 9, and only 40% of control intact plants had one or more detectable buds by 21 days post­treatment (Fig. 2). The rate of bud release on control intact plants was due to normal hormonal regulation in rooted cuttings of tea tree, and it established a base­ line for the slowest and least degree of axillary bud development in this study (Fig. 2). In contrast, control decapitated plants had the highest rate of bud devel­ opment of any of the six treatments as bud release was not mitigated by normal endogenous regulation or due to the presence of exogenous hormonal treat­ ment (Fig. 2). The axillary buds on control decapitat­ ed plants began to initiate 2 days after decapitation and continued to develop for the duration of the experiment, with significantly more buds detected at each recording date than any other treatment (Fig. 2). Where exogenous hormones were applied (either GR24, or GR24 + IAA) the bud numbers were inter­ mediate to the control intact and control decapitated extremes (Fig. 2). All four hormone treatments tend­ ed to track together until Day 9, but thereafter, the additional action of auxin apparently moderated the detectable number of buds further (Fig. 2). There was no difference in the number of detectable buds between the low and high GR24 treatment, regard­ less of IAA presence (Fig. 2). At the end of the 21­day time course experiment, the control intact plants had significantly less buds (mean + SE 1.1 ± 0.4), and control decapitated plants had a significantly more buds (mean + SE 8.9 ± 0.6 ), than all other treatments (Fig. 2). While the rate of GR24 (high versus low) had no effect, the addition of Fig. 1 ­ Preparing rooted cuttings of tea tree M. alternifolia for the GR24 experiment (left); A rooted cutting five months after setting and prior to decapitation (middle); Rooted cutting (control intact) in 50 mL clear plastic tube at the beginning of the experiment, and (right) a droplet of IAA applied on top of decapitated stem stump in GR24 and IAA. Fig. 2 ­ Mean number of detectable axillary buds over a 21­day time course for tea tree M. alternifolia propagules subjected to GR24 or GR24 plus IAA compared to control intact and decapitated plants. Error bars represent SE of means. Adv. Hort. Sci., 2021 35(4): 399­405 402 auxin with GR24 resulted in significantly fewer buds relative to GR24 at either rate alone (Fig. 2). Size and patterns of bud development along the stem When considering all assessed axillary buds (pooled data from all 10 nodes), as measured on Day 21, hormone treatments of GR24, or GR24 + IAA, sig­ nificantly reduced the bud size compared to control decapitated plants (p<0.05) (Table 1). The average bud length for hormone treatments ranged between 2.5 ± 0.5 mm and 3.3 ± 0.9 mm and was significantly less than the average for control decapitated plants (4.2 ± 0.6 mm), and significantly higher than for con­ trol intact plants (0.03 ± 0.08 mm) (Table 1). Assessment of bud development node by node revealed further differences in developmental pat­ terns. On control intact plants, buds on the upper­ most nodes (nodes 1 to 3), if present, were not of a measurable size. In control decapitated plants, buds were longest in the upper most few nodes, and grad­ ually reduced in size to the lowest measured node (node 10) (Table 1). In hormone treated plants, the longest buds occurred at the uppermost nodes (node 2 was generally longer than node 1), and again there was a progressive decline in size moving down the stem (Table 1). Bud length was not typically shorter in hormone treated plants relative to control decapi­ tated plants at the uppermost nodes, but below node 5, hormone treated plants tended to have significant­ ly shorter buds (Table 1). A representative example of the budding response showing bud position and length is presented in figure 3. 4. Discussion and Conclusions Strigolactone suppressed axillary bud growth in tea tree Relative to control decapitated plants without GR24 application, application of a synthetic strigolac­ tone analogue, GR24, at rates of 0.5 mg L­1 or 1.5 mg L­1, suppressed axillary bud number and expansion in decapitated rooted cuttings of tea tree. This response was consistent with its effects on other species such as calla lily (Zantedeschia L. sp.) (Manandhar, 2016) where synthetic SL GR24 was typically applied at con­ centrations between 0.3 mg L­1 (1 µM) and 3 mg L­1 (10 µM) (Umehara et al., 2008; Manandhar, 2016). Where larger plants at later stages of ontogenetic development (i.e. reproductive maturity) were stud­ ied, such as the study of Chrysanthemum morifolium Ramat, rates as high as 15 mg L­1 (50 µM) were required to inhibit bud growth (Dierck et al., 2016). A moderate response was evident in our tea tree plants at dosages towards the lower end of those used pre­ viously with herbaceous species. Both tested rates gave a similar response and, a synergistic effect between SL and auxin was evident, so that bud out­ growth was significantly reduced relative to the application of SL alone. A further factor that may have contributed to the efficacy of the treatments in our study, may have been the retention of the root system. SL is synthesised both in the roots and the shoots and is transported acropetally to suppress bud outgrowth (Domagalska and Leyser, 2011). In most other studies, stem sections with roots Table 1 ­ Mean length of the axillary buds from node 1 to node 10 measured at day 21. Treatment effect was tested on an individual node basis Treatment means followed by the same letter within the same row are not significantly different at the 95% confidence level (­ denotes no bud present). Control intact Low GR24 (0.5 mg L­1) High GR24 (1.5 mg L­1) Low GR24+IAA (0.5 mg L­1) High GR24+IAA (1.5 mg L­1) Control decapitated Average growth (mm) 0.03 ± 0.08 a 2.5 ± 0.5 b 3.3 ± 0.9 b 2.7 ± 0.6 b 2.7 ± 0.6 b 4.2 ± 0.6 c Node 1 (uppermost) 0 a 7.1 ± 1.1 b 9.8 ± 1.2 bc 9.2 ± 1.5 bc 9.4 ± 1.1 bc 12 ± 1.3 c Node 2 0 a 8.7 ± 1.1 b 10.5 ± 1.1 bc 9.4 ± 1.3 bc 10.1 ± 0.9 bc 12 ± 0.8 c Node 3 0 a 5.4 ± 0.9 bc 7.4 ± 1.0 c 4.2 ± 0.9 b 4.5 ± 1.1 b 7.2 ± 0.8 c Node 4 0.03 ± 0.03 a 2.1 ± 0.9 b 3.2 ± 0.7 bc 2.7 ± 0.9 bc 1.6 ± 0.5 ab 4.6 ± 0.9 c Node 5 0.1 ± 0.1 a 0.8 ± 0.3 a 1.3 ± 0.4 a 0.8 ± 0.3 a 1.2 ± 0.4 a 2.8 ± 0.7 b Node 6 0.1 ± 0.1 a 0.2 ±0.05 a 0.7 ± 0.3 a 0.1 ± 0.1 a 0.4 ± 0. 2 a 1.4 ± 0.5 b Node 7 0.1 ± 0.1 ab 0.4 ± 0.3 bc 0.1 ± 01 ab 0.1 ± 0.04 ab 0 a 0.7 ± 0.2 c Node 8 0.03 ± 0.03 a 0.6 ± 0.4 b 0.1 ± 0.3 a 0.03 ± 0.03 a 0 a 0.5 ± 0.1 b Node 9 0 a 0.1 ± 0.1 a 0 a 0.03 ± 0.03 a 0 a 0.2 ± 0.1 b Node 10 (lower most) 0.1 ± 0.1 a 0.03 ± 0.03 a 0 a 0 a 0 a 0.2 ± 0.1 b Lowe et al. ‐ Manipulating shoot branching in tea tree 403 removed were used for experimentation, and SL was supplied to the basal stem stump. Therefore endoge­ nous SL may have reinforced any effect due to exoge­ nous GR24 in our case, potentially inducing a stronger (bud outgrowth inhibition) response in the lower range of concentrations of GR24 compared to studies on other plants. Axillary bud outgrowth further inhibited with the addition of an auxin supply When GR24 was applied in conjunction with auxin, suppression of axillary bud outgrowth was enhanced relative to exogenous GR24 alone, as reported in other species (Crawford et al., 2010; Liang et al., 2010; Ward et al., 2013). Liang et al. (2010) reported total inhibition of bud outgrowth when 1.5 mg L­1 (5 µM) of GR24 and NAA were applied to chrysanthemum (Dendranthema grandi‐ florum) stems. Although both GR24 and GR24 plus IAA (at the low and high rate) treatments inhibited bud outbreak in tea tree plants for 5 days, when the buds began to grow and elongate, the growth rate of the buds was comparable to the control decapitated Fig. 3 ­ A representative M. alternifolia propagule from each treatment showing the pattern and size of axillary buds 21 days after treat­ ment. Figure 3a, is a control intact propagule, buds were not visible without magnification at the upper nodes. Buds were visible from node 4 or lower but did not develop (arrow). Figure 3b, is a control decapitated propagule, with axillary buds at upper nodes developed into side shoots (arrow). Figures 3c to 3f show a propagule from each of the GR24 and GR24 plus IAA treat­ ments, buds and developing side shoots visible only on the upper most nodes with generally less shoot growth compared to the control decapitated propagule in figure 3b. plants, at least for the upper four nodes for most plants (Table 2). These observations provide further support for the influence of auxin in the vascular stream of the main stem on bud release and the influence of auxin transport on SL inhibition of axil­ lary buds. Ultimately, our findings are consistent with both the canalisation (Brewer et al., 2009; Ljung et al., 2001) and second messenger (Domagalska and Leyser, 2011) models developed to explain the physi­ ological regulation of shoot architecture involving the interaction plant hormones including auxin, CK and SL (Gomez­Roldan et al., 2008; Domagalska and Leyser, 2011). Competition between buds The role of the GR24 in reducing bud release and growth needs to be considered along with the inhibitory effect active buds have on buds above or below it and those opposite to it (Thimann and Skoog, 1934). Lateral buds do not produce auxin while they remain dormant but produce considerable quantities when actively growing (Balla et al., 2011). Auxin synthesised in an active bud is transported into 404 Adv. Hort. Sci., 2021 35(4): 399­405 the main stem, and this auxin saturation prevents auxin movement out of axillary buds further down the stem, inhibiting bud growth (Leyser, 2005; Balla et al., 2011). Tea tree plants treated with GR24 or GR24 + IAA, buds tended to form on the upper nodes (node 1 to node 10), whereas buds formed down to node 14 on control decapitated plants (Table 1; data >node 10 not shown). The rapidly growing buds on the upper nodes of control decapitated plants did not appear to inhibit bud release on the nodes below them, but they did affect subsequent bud elongation, consistent with a requirement for SL for bud inhibi­ tion (Beveridge, 2000). The mean length of the buds on the control decapitated plants decreased signifi­ cantly for the majority of nodes, basipetally (p < 0.05) (Table 1). For plants given the higher rates of GR24 (with and without IAA), active buds may have con­ tributed to the suppression of bud outbreak on the lower nodes, as no bud outbreak occurred past node 6 and 8, for GR24 and GR24 +IAA respectively (Table 1). Implications for use of SL analogues as agents for manipulating tea tree nursery stock This study demonstrated the potential of synthet­ ic SL GR24 at relatively low concentrations of 0.5 mg L­1 and 1.5 mg L­1 to suppress bud outgrowth in tea tree. This was encouraging for a potential application of SL in an early intervention response to mitigate undesirable side branching of plants where shoot tips are inadvertently lost due to pest, heat or desiccation damage, or where tips are removed during propaga­ tion (i.e. use of nodal cuttings). None of the artificial treatments tested were as effective as the control intact treatment where apical dominance suppresses bud outgrowth, but a moderate response at relative­ ly low dosages, suggests there is room to explore whether higher dosages can elicit stronger suppres­ sion of side branching. For example, Liang et al., (2010) found higher concentrations of exogenous SL or SL with auxin suppressed bud outgrowth to the same degree as endogenous regulation induced by an intact apical buds in chrysanthemum. The response at lower dosages in this first investi­ gation was also encouraging because woody perenni­ al species do not appear to be recalcitrant to the influence of hormones, at least for our relatively small (less than 2 grams fwt of biomass) test plants, with root systems, and with low tissue specialisation (i.e. stems had low degrees of lignification and had not formed papery bark). 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