In this article we will discuss about the three main plant hormone signalling involved in differential development and other physiological process in plants:- 1. Cytokinin Signalling 2. Auxin Signalling 3. Gibberellin Signalling.
1. Cytokinin Signalling:
Cytokininis have been registered as one of the key plant hormones involved in differential development and other physiological process in plants, such as cell division, root and shoot growth, chlorophyll development, leaf senescence and including biotic and abiotic stress response.
Comprehensive progress has been made in analysing cytokinin perception and signalling. Several painful efforts resulted in the identification of three cytokinin receptor proteins such as GK11, a receptor histidine kinase, CRE1/AHK4 histidine kinase and AHK2/AHK3 histidine kinase receptor in Arabidopsis due to the availability of its genome sequence.
In addition cytokinis down-stream transmitters like AHPs (Arabidopsis Histidine Phosphotransfer protein) and ARR (Arabidopsis Responsive Regulator) have been characterized. Expression of cytokinin receptors takes place mainly in the roots whereas remaining two is expressed in all other major organs.
Three cytokinin receptors contain two to three trans membrane domains at the N-tranasminal part, transmitter (histidin Kinase) and two receiver domains. The extracellular ligand-binding regions of 210 amino acid long, have drawn special attention as they are the main recognition sites for cytokinins.
These domains are present in variety of functionally diverse membrane receptor proteins that recognise cytokinin like adenine derivations or peptide ligands. This domain has been coined as the CHASE domain (cyclases/histidine-kinase-associated sensory extracellular domain). This domain is specific for AHK2, AHK3 and CRE1/AHK4 receptors in Arabidopsis.
Cytokinin signal-transduction pathway consists of Arabidopsis five histidine phospho-transmitters (AHPs) and 22 responsive regulators (ARRs). Five AHPs genes encodes proteins of 12 kDa, transmit the signal from the receptor, which is localised in the plasma membrane to ARRs, which are probably present in the nucleus. The ARRs are divided into two major classes, such as A and B-type.
There are 22 predicted responsive regulator (ARR) genes. Presence of receiver domain is the characteristic feature of type A-ARR genes. By contrast, the type B-ARR contains a C-terminal output domain in addition to the receiver domain. Transcription of A- type ARR increases rapidly within 10 min in response to cytokinin.
When cytokinin binds to the receptor, it undergoes dimerization and autophosphorylation. Following activation of receptor complex, phosphoryl group is transferred to AHPs which transport the signal from the cytoplasm to type B ARRs in the nucleus.
Following transmission of signal inside the nucleus type-B response regulators transcribe target genes, such as type-A ARR genes. In this signal process, type-A response regulations may down regulate the primary cytokinin signal response via a negative feedback process (Fig. 4.2).
2. Auxin Signalling:
Indole 3 acetic acid (IAA) is a naturally occurring auxin known to involve in cell division elongation or expansion, besides participation in other morphogenesis. During past several decades intensive research have been done in the identification of receptors for the auxin signal. The possible auxin receptor known as auxin-binding protein (ABP1), has recently been characterized.
In addition, several other auxin binding proteins such as glutathione S-transferase, β-glucanase, and a cytokinin glucosidase are known to express in stressed plants. ABP1 is a small family of 23 kDa protein that bind to auxins like Indole-3-acetic acid (IAA) and napthalene-1-acetic acid (NAA). ABP1 contains a C-terminal HDEL endoplasmic reticulum retention domain.
Over expression of ABP1 in transgenic tobacco evidenced its role of controlling cell expansion. Recent studies have established that AtP1N1, a 67 kDa protein is a trans membrane component involved in polar auxin transport. Similarly another member of the auxin efflux carrier family AtP1N2 is probably involved in controlling movement of auxin in elongation zone.
3. Gibberellin Signalling:
Gibberellins (GA) are associated with plant growth, seed germination, stem elongation, flowering and fruit development. Besides, it is also involved in the regulation of gene expression in the cereal aleurone layer.
Recent findings on the GA signalling pathways shows GAMYB is a GA-regulated MYB transcription factor, which is involved in the activation of a-amylase expression in barley aleurone cells and anther development. In addition, GAMYB (GA regulated MYB transcription factor) interacts with KGM (KINASE-associated with GAMYB). KGM is a member of protein kinase sub group, represses GAMYB functions in barley aleurone.
Recent studies on GA signalling shows that PHORI (photoperiod-Responsive 1) acts as positive regulator in GA signalling particularly in the expression of ent-Kaurene oxidase gene. When GA binds to an unidentified GA receptor (s), activates G proteins (D1) that enhance the GA signal.
Enhanced GA signal then mediates PHORI transport into the nucleus, where it acts as a positive regulation by GA signalling. Meanwhile nucleus located protein kinases and GID1 (GA insensitive dwarf 1) are also activated by GA signal, which in turn trigger GID2/SLY1-mediated degradation of DELLA proteins (SLR/RGA).
These DELLA proteins directly or indirectly inhibit the expression of GA-induced genes in the absence of GA in cells. Therefore in presence of GA, PHORI regulates expression of GA-responsive genes. Similarly 14.3.3 proteins regulates the sub-cellular localisation of RSG (Repression of Shoot Growth), which in turn controls the expression of the GA-response genes and GA mediated action take
lecules are known to be involved in signalling pathway. These specialised photoreceptors are involved in bundantly synthesised in the dark as inactive form and are degraded upon exposure to light.
Phytochrome undergoes inter convertable form from inactive red (R) light absorbing Pfr form into active far red (FR) absorbing Pfr form after absorption of a light photon. Phytochrome in this active status in turn can be transformed back into the Pr form after absorption of FR.
There are five distinct phytochromes in Arabidopsis designated phy A to phy E and play a key role in different photomorphog PIF3 (Phytochrome Interacting Factor). In this process, phy B can bind specifically to PIF3 that is already bound to light responsive G-box DNA sequence site (CACGTG) and facilitates transcriptional activation of specific gene (Fig. 4.4).
