Phytohormones control every aspect of plant growth and development, including seed germination, seedling growth, branching, plant height, flowering, seed development and senescence. A few major phytohormones and their roles in regulating plant growth and development are listed in Table 8.2.

Table 8.2 Selected phytohormones and the growth and developmental responses influenced by them

Growth/developmental responses

Maintenance of meristem identity in shoot and root apical meristems, organogenesis of leaves, flowers, floral organs and lateral roots Seed germination, leaf expansion, induction of flowering, flower development and seed development Seed germination, root and shoot development and senescence Cell expansion, vascular differentiation, reproductive development, leaf inclination

Seeds germination, hypocotyl growth and shoot branching

Also, phytohormones such as auxins, gibberellins, cytokinins and ethylene can modify fiber and wood formation during growth [22]. Auxin is required for cell division and axial plant growth and it helps to enforce apical dominance (where the shoot tip exerts inhibitory action on the axillary bud outgrowth). The primary site of biosynthesis of auxins is at the shoot tip. It is transported basipetally to other parts of the plant via an elaborate transport mechanism involving a number of members of the PIN family of proteins [23]. The involvement of auxins and cytokinins is proven in organ development and controlling organ size. Cytokinins help to break apical dominance and promote the outgrowth of lateral shoots. Hence, interactions of auxin and cytokinin control the shoot branching in plants [24, 25]. More recently, another phytohormone strigolactones has been shown to be necessary to inhibit shoot branching, and mutants in the biosynthesis pathway exhibit plants with more branches [26]. Increased gibberellin biosynthesis by ectopically expressing AtGA20ox promotes growth rate and biomass increase in hybrid aspen [27] and tobacco [28]. Furthermore, in a recent study silencing of AtGA2ox homolog in tobacco was demonstrated to enhance plant biomass [29].

The effect of phytohormones can be examined from the biosynthesis and their biological actions. Thus, plants exhibiting wide variations in structure have been observed when key genes involved in phytohormone biosynthesis and signaling have been mutated. The classic examples of gibberellin-deficient plants (e. g., Arabidopsis ga1-3 mutant; [30]) showing extreme dwarfism is a good illustration of the importance of this hormone in regulating plant architecture. This mutant arose from a deletion in the ent-kaurene synthase enzyme that catalyzes an early step in gibberellic acid biosynthesis. However, it retains the ability to respond to exogenously added gibberellins to grow to normal size. Mutants in other phyto­hormone biosynthetic pathways are also known to result in similarly striking changes in plant morphology.

The discovery of specific receptors for the different phytohormones and their elaborate signaling pathways [31-33] is another area of interest for this discussion. The signaling cascade for cytokinins involves sequential phosphorylation and activation of intermediate proteins [34]. There are generally multiple receptors and intermediate proteins for the phytohormones. Thus, for cytokinin signaling, more than three receptors, five phosphotransfer proteins (cytoplasm to nucleus shunting)

Downstream target genes regulating shoot and root development branching etc.

Fig. 8.1 Schematic representation of cytokinin signal transduction pathway (based on [34, 37]). This is an example of the signaling intermediates of one of the phytohormones. Similarly, the signaling pathways of other phytohormones have many intermediates, genes for which can be the targets of biotechnological improvements of biomass yield in selected plants. AHK2, 3, 4 Arabidopsis Histidine Kinase2, 3, 4 are cytokinin receptors on cell membranes. Dimers of the receptors bind cytokinins such as zeatin. AHP Arabidopsis Histidine Phosphotransfer proteins serve as phosphate shuttle from the cytoplasm to nucleus. ARR Arabidopsis Response Regulator proteins are the response regulators that affect the transcription of downstream target genes that are activated by cytokinins and over 20 response regulator proteins are known (Fig. 8.1). Similarly, auxin signaling cascade has multiple receptors and effector proteins [33]. Another major aspect of phytohormone signaling is the crosstalk between different phytohor­mones [35], which adds a new dimension of control of plant development by this group of rather simple chemical molecules. Mutants in various intermediates along the signaling pathway can lead to interesting agronomic traits such as altered organ size, altered branching and overall changes to plant architecture. Thus we observed that suppression of AtHOGl expression, which is a putative cytokinin signaling intermediate, leads to enhanced branching in Arabidopsis and petunia [36]. It is not our intention to review phytohormone signaling in detail here, but this brief description is used to illustrate the genetic complexity of phytohormone signaling. Therefore, the various intermediates of the phytohormone signaling pathways may be explored as targets for genetic modification to achieve desired plant architecture.