In this article we will discuss about the rapid gene regulation by auxin.
Naturally occurring hormone indole acetic acid (IAA) is the fastest known hormonal action among plant in terms of rapid cell elongation within a short span of 10-20 minutes. Auxin exhibits two types of response. One is short-term effect with 20 min leads to cell elongation. Second is long-term response such as cell division, differentiation and morphogenesis.
Auxin can rapidly alter specific mRNA sequences. It is well documented that Auxin elevates rRNA synthesis due to increased RNA polymerase I molecules, for example, synthetic auxin 2, 4-D enhances the level of a translatable mRNA, which are associated with ribosomal subunits.
There is considerable increase in mRNA level after 15 minutes when plant tissues are treated with auxins. The auxin induced mRNA change has been attributed to an increase in the amount of translatable mRNA brought about by activation of transcription, post transcriptional processing and even mRNA stability.
Regulation of auxin responsible genes seems to be dependent on positive correlation between the level of expression and auxin concentration (Fig. 3.2). For example, auxin regulated mRNA concentration is higher in the basal part of the soyabean seedling when auxin concentration is low.
Similarly, low profile expression of auxin regulated genes in the root region of etiolated pea seedling is due to higher level of auxin. Plant tissues treated with 2-4, D induced mRNA in elongating soyabean hypocotyls. The 2, 4-D mediated induction of pJcw1 and pcw2 mRNA after 30 and 15 minutes, respectively, have been documented. Induction of specific mRNA (within 15 min) is earlier than the initiation of cell elongation which has a latent period of 21-23 minutes.
The recent work on auxin controlled gene activation concedes that the receptor and signal cascade for transcriptional control are upstream of the auxin/IAA-ARE complex. ARF is auxin responsive factor that binds to auxin responsive element Aux RE.
Regulations of auxin responsive genes have been proposed based on negative and positive model of regulations. According to the negative model, represents protein interacts with promoter region of auxin responsive genes are precluding its transcription. Presence of auxin on the other hand, binds to the repressor and relieves its effect.
According to positive control model, an activator protein molecule is inactive in absence of auxin facilitates binding of activator to promoter region and initiate transcription. On the other hand, activator molecules in absence of auxin interact with repressor molecule and transcription is blocked due to non-availability of activator.
However, when auxin is present, it binds to repressor molecules alter its structure to get dissociated from activator; consequently, free activator binds to promoter region and initiate transcription (Fig. 3.3).
Earlier experiments discover that auxin induced gene expression and mRNA synthesis is mediated by auxin mediated H+ secretion. However, subsequent evidence overwhelmingly rejects any role of auxin induced H+ secretion in the rapid mRNA induction.
Subsequent work shows that the whole concept was re-examined and proposed that in the whole relationship between auxin induced mRNA accelerators, H+ secretion and cell elongation, the mRNA accumulation takes place much earlier than H+ secretion and cell elongation. Therefore, it is represented as.
According to current model on the control of gene expression by auxin, the auxin responsive factors (ARE) and Aux/IAA control transcription is through ubiquitin mediated proteolysis. More precisely, it can be interpreted that at low auxin levels ARE activates, combines with Aux/IAA repressors.
When auxin level is increased which gave signalling stimulates from receptors, induces the Aux/IAA complex to dissociate leading to depression. Another impact of dissociation is the phosphorylation of ARE and Aux/IAA, making them subtrate for proteolysis.