Acknowledgements
T. L. Davenport
5.4 Flowering Mechanisms
Reproductive Physiology 105
1958b; L.B. Singh, 1960; Sturrock, 1966; Ravishankar et al., 1979; Scholefi eld, 1982; Scholefi eld et al., 1986). The complexes of primary to quaternary branch- ing lateral structures of the infl orescence each terminate with three cymose fl owers. The terminal fl ower opens fi rst, followed by two subtending lateral fl owers. These complexes form the lateral infl orescence structures emerging from the central axis of the panicle. The central axis extension also terminates in a similar fashion. Morphological stages of fl oral buds and panicle develop- ment were described by Shu (1981) and Oosthuyse (1991a). Reece et al. (1949) described the development of infl orescences initiated in lateral buds when the terminal bud is missing. There are more nodes in dormant apical buds and their bracts are more developed than in axillary buds; however, fl oral evocation is indistinguishable.
Generative shoot development in apical buds initially involves swelling of the lateral meristems and their bud scales. Each axillary meristem devel- ops as an infl orescence on a primary peduncle. The apical meristem then forms new lateral meristems and leaf primordia for the distal portion of pan- icle development if fl oral inductive conditions persist (Núñez-Elisea et al., 1996). Panicles may be open or compact, depending upon internode elonga- tion, which is cultivar dependent (L.B. Singh, 1960), but the architecture gen- erally conforms to that in Fig. 5.5. Mixed shoots develop under weak fl oral inductive conditions (i.e. in the low-latitude tropics). Both leaves and pri- mary pedunculate infl orescences develop from the same nodes (Fig. 5.5).
Leaf primordia and lateral meristems develop as leaf and fl oral structures, respectively.
T.L. Davenport 106
also stimulates shoot initiation. Reece et al. (1946, 1949), Mustard and Lynch (1946), Núñez-Elisea and Davenport (1992b), Núñez-Elisea et al. (1996) and Davenport et al. (2006a) observed that the vegetative or reproductive fate of mango buds remains undetermined until after shoot growth is initiated.
Reece et al. (1949) proposed that a putative signal that triggers initiation of shoot development is separate and different from the inductive signal, which determines the fate of the shoot. Removal of apical buds by pruning stimu- lates initiation of axillary shoots (Singh and Singh, 1956; Núñez-Elisea and Davenport, 1992b; Núñez-Elisea et al., 1996; Davenport et al., 2006a). Defolia- tion of the apical whorl of fi ve to ten leaves also stimulates shoot initiation in dormant apical buds (Núñez-Elisea et al., 1991; Núñez-Elisea and Daven- port, 1995). The fate of shoots that emerge in response to these initiation stim- uli, however, is determined by other factors that are prevalent at the time of initiation. Tip pruning, for example, during warm summer months results in initiation of vegetative shoots from axillary buds, whereas pruning during cool winter months usually results in initiation of axillary infl orescences.
Induction
Induction in mango is the temporary commitment of buds to evoke a par- ticular developmental pathway (i.e. vegetative shoot, generative shoot or mixed shoot) when growth is initiated. Initiation of herbaceous plant fl ower- ing refers to the onset of fl oral bud growth in actively growing vegetative shoots after the fl oral inductive event (Bernier et al., 1981, 1993; Halevy, 1985–
1986; Bernier, 1988; Huala and Sussex, 1993; Kinet, 1993). The inductive sig- nal is formed in leaves, but the responsive buds are in continuous vegetative growth at the time of fl oral induction in herbaceous plants and fl oral initia- tion follows; whereas mango buds are in rest. Although the mango bud must be initiated to grow, that growth is induced according to forces already present.
Whereas the fl oral inductive signal in mango may be present prior to bud initiation, it must be present at the time of initiation for fl owering to occur (Kulkarni, 1988a; Núñez-Elisea and Davenport, 1995; Núñez-Elisea et al., 1996; Davenport and Núñez-Elisea, 1997; Davenport et al., 2006a). The induc- tive signal can be shifted from fl oral (F) to vegetative (V) or vegetative to fl oral, forming F/V or V/F transition shoots, by altering temperatures dur- ing early shoot development (Batten and McConchie, 1995; Núñez-Elisea et al., 1996) (Fig. 5.5). This shift in morphogenic responses during shoot devel- opment demonstrates the plasticity and temporal nature of induction, indi- cating that cells of the apical meristem do not become irreversibly determined under inductive conditions. These results demonstrate that, rather than being irreversibly committed to a vegetative or reproductive fate at the onset of shoot initiation, the mango apical meristem provides progenitor cells, some of which differentiate into specifi c target cells at each node in the apex. The apical meristem, therefore, may not be directly involved in the fl owering process.
Reproductive Physiology 107
Target cells within leaf primordia and lateral meristems are competent to respond to inductive signals; for example when initiated to grow under veg- etatively inductive conditions, individual leaf primordia develop as leaves and subtending lateral meristems associated with each developing leaf develop as dormant axillary buds with protective bracts. These axillary buds may develop in subsequent fl ushes as vegetative shoots when initiated in vegetatively inductive conditions or as axillary infl orescences under fl oral inductive conditions. Under strongly fl oral-inductive conditions, leaf pri- mordia fail to develop beyond the bract stage, become dormant, and lateral meristems develop. Each lateral meristem forms nodes consisting of leaf pri- mordia and meristems that are infl uenced by the putative fl oral-inductive stimulus, which suppresses development of newly formed leaf primordia.
Subsequently formed meristems form pedunculate structures that terminate in cymose infl orescences borne on each tertiary peduncle (Fig. 5.4). Forma- tion of the primary, secondary, tertiary and quaternary peduncles, as well as pedicels of infl orescences are always accompanied by a subtending, aborted bract or vestigial leaf at each node (Fig. 5.4). Such development is attributed to a sequence of gene expression (Coen et al., 1990; Coen and Meyerowitz, 1991; Weigel et al., 1992; Coen and Carpenter, 1993; Lumsden, 1993; Yanofsky, 1995). Shoot initiation during weakly fl oral-inductive conditions activates growth of leaf primordia to develop leaves and the lateral meristems to pro- duce peduncles bearing lateral infl orescences in each node of mixed shoots.
The bases of each pedicel branch within each lateral infl orescence also bear a vestigial leaf.
Upon termination of cell divisions in the apical meristem at the end of a fl ushing period, no more nodes are formed. The apical bud of vegetative shoots becomes quiescent, and the resting leaf primordia, bracts and lateral meristems are poised to resume growth at a later date. When reproductive or mixed shoots become quiescent, the lateral meristems ultimately develop determinant cymose infl orescences. The most distally located meristem is possibly the determinant extension of the central axis forming the terminal cymose fl oral group.
Chimeric shoots (Fig. 5.5) can occur in mango trees when shoot initiation occurs during fl oral inductive conditions. They display infl orescences on one side of the longitudinally bisected shoot and leaves on the other. The shoot axis is red on the fl oral side of red fruiting cultivars (typical of panicles) and green on the vegetative side (typical of vegetative shoots). This difference in the two sides extends to the apical bud, which bears an undeveloped infl orescence on the fl oral side and leaf bracts on the vegetative side. The explanation for this spatial differentiation is that target nodes on each side of the apical bud respond to the different inductive signals at the same time.
The apical meristem is not implicated except to form more nodes for the lat- eral inductive responses on each side in the second portion of growth. Differ- ences in inductive signals on each side of an existing shoot probably cause the differential response. This phenomenon indicates that the fate of nodes on each side of the shoot cannot be attributed to a single mother cell in the apical meristem. The inductive response must involve cells formed in later
T.L. Davenport 108
cell divisions and would be determined by their location within nodes of the bud.
Florigenic promoter (FP) or stimulus
Early fl owering work provided evidence for the presence of a graft transmis- sible fl oral stimulus (i.e. fl origen) that was induced in leaves and was trans- located to buds to stimulate fl oral development (Chailakhyan, 1936; Zeevaart and Boyer, 1987). Florigen was functionally conserved across plant species (Lang, 1965, 1984; Zeevaart, 1976; Lang et al., 1977). Floral induction in most plants involves sensing of some environmental cue (i.e. daylength, water stress or vernalizing temperature) in some organ (e.g. leaves). A putative fl o- ral stimulus or alteration in the ratio of fl origenic to anti-fl origenic compo- nents may be translocated to target cells in meristems (Bernier et al., 1981).
Photoassimilate movement from leaves in phloem facilitates its transport to buds where it can interact to initiate fl owering (King and Zeevaart, 1973).
Until recently, a fl oral stimulus could not be identifi ed. Alternative hypoth- eses were proposed that nutrient diversion to the meristems could be involved (Sachs and Hackett, 1983) or that fl oral induction might be con- trolled by multiple factors, including the putative fl oral stimulus, photoas- similates and phytohormones (Bernier et al., 1993).
Molecular biology of fl owering in the facultative, long-day, model plant, Arabidopsis thaliana (reviewed in Zeevaart, 2006 and Aksenova et al., 2006), has provided insight into the nature of the fl oral stimulus (FP). A network of four interacting genetic signalling pathways may result in fl owering in response to photoperiodic, vernalization, gibberellin and autonomous envi- ronmental cues (Perilleux et al., 1994; Mouradov et al., 2002; Perilleux and Bernier, 2002; Boss et al., 2004; Komeda, 2004; Putterill et al., 2004; Corbesier and Coupland, 2005). The photoperiodic pathway involves activation of the CONSTANS (CO) gene that encodes a zinc-fi nger protein, which in turn induces expression of the FLOWERING LOCUS T (FT) gene in the phloem tissue of leaves. FT is the terminal, integrating gene of the four path- ways regulating fl owering in Arabidopsis. Its transcribed mRNA was initially thought to be the FP that is transported in phloem to buds (Huang et al., 2005); however, evidence indicates that the translated protein product of FT is translocated to Arabidopsis buds (Corbesier et al., 2007). Analogous proteins encoded by Hd3a, an ortholog of FT in rice (Tamaki et al., 2007), and the aspen ortholog, PtFT1, which along with CO regulates the timing of fl owering and growth cessation of Populus trichocarpa (Bohlenius et al., 2006), appear to be the FP. In the buds, the protein product of FT is thought to combine with the bZIP transcription factor (FD) protein to activate transcription of fl oral iden- tity genes (i.e. APETALA1) to begin fl oral expression (Abe et al., 2005; Wigge et al., 2005). Similar mechanisms are likely to exist in mango.
Zhang et al. (2005) and Davenport et al. (2006b) isolated a CONSTANS- like gene (MiCOL) from mango leaf DNA. CO is a circadian expression gene interacting with the photoperiodic pathway in Arabidopsis (Putterill et al.,
Reproductive Physiology 109
2004), and is central to activation of the FT gene in Arabidopsis during long days. Its role in mango fl owering is unclear. The mango ortholog has 79%, 76% and 62% homology with two apple CO genes, MdCOL2 and MdCOL1, and the Arabidopsis CO gene (AtCO), respectively. Isolation of the FT or homologous gene responsible for synthesis of the FP has been unsuccessful.
Studies with mango indicate that a FP is synthesized in leaves during exposure to cool, fl oral-inductive temperatures and moves to buds to induce fl owering (Reece et al., 1946, 1949; Singh and Singh, 1956; L.B. Singh, 1959, 1962, 1977; R.N. Singh, 1961; Sen et al., 1972; Núñez-Elisea and Davenport, 1989, 1992b; Davenport and Núñez-Elisea, 1990; Davenport et al., 1995, 2006a). Unlike receptor sites in buds of Thlaspi arvense (Metzger, 1988) and other plants requiring vernalization for fl oral induction (Zeevaart, 1976;
Bernier et al., 1981), mango leaves appear to be where the putative fl oral stim- ulus is produced. Complete defoliation of girdled branches during inductive conditions results in vegetative shoots instead of generative shoots (Reece et al., 1949; Sen et al., 1972; Núñez-Elisea and Davenport, 1989, 1992b; Núñez- Elisea et al., 1996; Davenport et al., 2006a). It appears to be transported over long distances from leafy branches to defoliated branches (Sen et al., 1972;
Núñez-Elisea et al., 1996).
The putative, temperature-regulated FP is short-lived in situ (Núñez- Elisea and Davenport, 1989, 1992b; Davenport et al., 1995; Núñez-Elisea et al., 1996). Leafl ess cuttings from trees during cool, fl oral inductive conditions produce infl orescences when stimulated to grow within 7 days of transfer to warm, non-inductive conditions; the infl uence of the removed leaves lasts for 13 days when cuttings are stored at cool temperatures (Davenport et al., 2001a). The same cuttings produce only vegetative shoots in both storage conditions after the initial loss of reproductive shoot production. There are more leaves on mango stems than are necessary for fl oral induction in cool temperatures. Stems bearing as little as one-quarter of a cross-sectioned leaf induce 95% generative shoots (Davenport et al., 2006a); the remaining shoots are vegetative. Half of a leaf or more resulted in 100% generative shoots.
Thus, the limiting amount of leaf necessary for fl oral induction is less than a quarter of a leaf per stem. Davenport et al. (2006a) demonstrated the quanti- tative movement of mango FP from half to fi ve leaves on a donor stem to fi ve leafl ess receiver stems located as far as 100 cm from the donor stem in isolated branches during exposure to cool, fl oral inductive temperatures. The FP moves with photoassimilates in phloem from donor leaves to buds in the receiver stems.
The mango fl oral stimulus is graft transmissible (L.B. Singh, 1959, 1962;
Kulkarni, 1986, 1988b, 1991). Flowering of seedling stems is stimulated by grafting onto mature trees or by grafting mature stems onto juvenile plants (L.B. Singh, 1959, 1962). Some mango cultivars selected in the tropics can fl ower at higher temperatures than others and are not restricted to winter fl owering (Kulkarni, 1991). Transfer of the FP from tropical to subtropical selec- tions was accomplished using reciprocal grafts between the two cultivar types (Kulkarni, 1986, 1988b, 1991). Subtropical cultivars that seldom fl ower in warm temperatures fl ower in the ‘off’ season using these techniques. Three conditions
T.L. Davenport 110
were essential for summer fl owering to occur in the low-temperature-requiring cultivars (receptors) when grafted to the summer fl owering type (donors): (i) the summer-fl owering donor cultivar stocks or scions were in a fl owering cycle; (ii) buds on the receptor scions or stocks of grafted plants had initi- ated shoot growth during this cycle; and (iii) receptor stocks or scions had been completely defoliated for transfer and/or expression of the fl oral stimu- lus. The presence of any leaves on the receptor plants resulted in vegetative shoots.
Girdling experiments to isolate treated mango branches from the rest of the tree suggest that the FP is translocated via phloem to apical buds (King and Zeevaart, 1973; Bernier et al., 1981; Núñez-Elisea and Davenport, 1989, 1992b; Núñez-Elisea et al., 1996; Davenport et al., 2006a). Shading experiments to reduce photosynthate loading into the phloem also support this (Kulkarni, 1991). Reduced fl owering responses were observed in isolated leafy branches that were provided with 90% and complete shading, which stopped photosyn- thate production entirely, mimicked defoliation during cool, fl oral inductive conditions, resulting in a vegetative growth response (R. Núñez-Elisea, T.L.
Davenport and B. Schaffer, Florida, 1991, unpublished results).
Vegetative promoter (VP)
An independently regulated VP probably contributes to induction of vegeta- tive shoots as opposed to a fl oral inhibitor or expression of a default vegeta- tive status in the absence of suffi cient FP at the time of shoot initiation.
Grafting studies (L.B. Singh, 1959, 1962; Kulkarni, 1986, 1988b, 1991, 2004) demonstrated that complete removal of leaves from receptor stems is required to express fl owering of those receptors when they are grafted to fl owering donor stems. Kulkarni (1986, 1988b, 1991, 2004) considered that a putative fl oral inhibitor in leaves of the non-induced receptor stems might antagonize the infl uence of the fl oral stimulus from donor leaves. Others have noted a relationship between leaf age and the ability of shoots to be reproductive (Singh et al., 1962a; Scholefi eld et al., 1986). KNO3-stimulated early fl owering in the tropics is successful only on stems that are at least 4 (Davenport, 2003) to 7 months old (Astudillo and Bondad, 1978; Bondad and Apostol, 1979;
Núñez-Elisea, 1985). Young stems often produce vegetative shoots when ini- tiated under conditions that are fl oral inductive for more mature stems (Núñez-Elisea and Davenport, 1995; Davenport, 2003). The putative VP appears to be most active in leaves of young stems and slowly dissipates over time to allow expression of the FP when shoots are initiated to grow in warm conditions.
The VP may be a gibberellin or closely associated with the gibberellin synthesis pathway as indicated by enhanced fl owering responses of trees to plant growth retardants. Mangoes growing in wet and humid, low-latitude tropics tend to produce frequent vegetative fl ushes and fl ower sporadically, perhaps due to higher levels of the VP in the young stems combined with low levels of the putative FP when shoot initiation occurs. Paclobutrazol
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(PBZ) reduces the time in rest necessary to allow fl oral induction during warm temperature conditions by c.1 month (Davenport, 2003), thus increas- ing the potential to produce reproductive shoots in younger stems when ini- tiated to grow. PBZ and uniconazole, triazole compounds that inhibit kaurene oxidase in the gibberellin-synthesis pathway (Dalziel and Lawrence, 1984;
Rademacher, 1991), stimulate production of fl owering shoots during weakly inductive conditions (Burondkar and Gunjate, 1991, 1993; Tongumpai et al., 1991a; Voon et al., 1991; Nartvaranant et al., 2000; Yeshitela et al., 2004a).
Application of PBZ to mango trees bearing 1-month-old stems produced infl orescences when bud break was initiated 3 months later by foliar applica- tion of KNO3 (Davenport, 2003).
Vegetative or reproductive induction at the time of shoot initiation is governed by the ratio of the putative fl oral promotive to inhibitory compo- nents (Lang et al., 1977; Lang, 1984; Kulkarni, 1988a; see Bernier et al., 1981 for additional references). The mango fl oral inhibitor should be viewed as an age-dependent VP. The presence of an age-regulated VP in mango leaves, which moves with the temperature-regulated FP and photoassimilates in phloem, may explain the induction of specifi c receptors by this promoter in targeted leaf primordia to cause development of leaves in vegetative or mixed shoots. A gradual decrease in the level or infl uence of the VP may cause vegetative shoots to develop when initiation occurs on 2-month-old stems, and generative or mixed shoots when initiation occurs in stems from 4- to 7-month-old stems, given the constantly warm daily temperatures maintaining a low level of FP in both situations.