Role of glucose in spatial distribution of auxin regulated genes

Abstract

Plants have the ability to adjust its physiology and metabolism to the changes of nutrient availability in the environment. Since a number of common responses are regulated by sugar and auxin, the obvious question arises is whether sugar and auxin act interdependently to bring about changes in plant morphology. In the February issue of the PLoS ONE,¹ we presented detailed investigation of glucose and auxin signaling interaction in controlling root growth and development in Arabidopsis thaliana seedlings. Further analysis of tissue specific regulation of glucose auxin signaling interaction may provide some insight as to how these two signaling molecules interact to control the morphogenic changes during seedling development.

In literature there are several reports about sugar and phytohormones interaction. The possible link of sugar with auxin was suggested by some sugar related mutants that were also perturbed in auxin responses. The glucose sensor HXK mutant gin2 also display resistance towards exogenous auxin.² The turanose insensitive, tin mutant encodes for a WOX5 gene that is induced by both turanose and auxin. WOX gene plays an important role in maintaining auxin maxima at root tip and root formation and patterning by acting as a negative trigger of IAA homeostasis.³ A new allele of hls1 (hookless1) mutant is shown to be resistant to both sugar and auxin responses.⁴ Along with these reports, a number of common responses are reported to be controlled both by sugar and auxin defining the need to further study the mechanism of interaction between these molecules.

In our study, we have observed that the increasing concentration of glucose can also bring more randomization in root growth direction along with changes in other parameters such as root length, lateral root production and root hair formation. These responses are dependent on HXK mediated signaling to different extents as suggested by the response of gin2 (glucose receptor hexokinase) mutant towards exogenous sugars.

In order to find out the extent of interaction between auxin and glucose, whole genome transcript profiling was done. Microarray analysis suggested that total 604 genes were two or more fold up/downregulated by IAA. Glucose alone could regulate 377 (62%) of these 604 IAA regulated genes suggesting a large overlap between the two signaling molecules and could be a possible explanation for controlling a large number of common responses shared by glucose and auxin. Out of these 377 genes 68% were regulated agonistically and 32% were regulated antagonistically by glucose. Glucose could also modulate the extent of regulation of almost 63% genes induced or repressed by IAA.

Most of the IAA regulated genes, which were agonistically affected by glucose, were regulated transcriptionally by glucose alone, whereas most of the IAA regulated genes in which glucose antagonizes IAA mediated up or downregulation were not regulated transcriptionally by glucose alone. This suggests that antagonistic action of glucose on IAA regulated genes either requires auxin regulated factor/s or may control gene expression through some non transcriptional pathways in presence of auxin. The GUS expression of HS:AXR3NT::GUS line was found to be more stabilized in presence of glucose both in presence or absence of IAA suggesting degradation of AXR3 protein was reduced in presence of glucose in the medium.

Glucose can affect almost all steps of auxin metabolism since it can modulate auxin biosynthetic gene YUCCA2, auxin receptors TIR1 and ABP1, auxin transporter PIN1, auxin response factors ARF4, ARF8 and a number of genes belonging to auxin induced gene families such as AUX/IAA, GH3 and SAUR.¹ Auxin perception and signaling mutants tir1, slr1, axr2, axr3 showed differential response towards exogenous glucose in terms of root growth, lateral root induction, root hair elongation and gravitropism,¹ suggesting glucose may use auxin signal transduction elements to affect root architecture.

Glucose could increase PIN2::GFP accumulation on the plasma membrane and could also cause increased accumulation of PIN2::GFP in lateral walls. The basipetal auxin transport was also found to be more prominent in glucose treated seedlings.¹ Further to check if glucose application can affect accumulation/spatial distribution of auxin inducible genes within the seedling, 5 d old SAUR::GUS seedlings were treated with 0% and 3% glucose containing medium for 3 h and GUS activity was checked. While in the 0% glucose containing medium only a very few seedlings (11%) show GUS expression only in one cotyledon, in 3% glucose containing medium a large number of seedlings (67%) show GUS expression only in one cotyledon (Fig. 1). The differential distribution of SAUR::GUS may mean differential auxin transport/signaling activities leading to activation of auxin induced responses only in one of the two cotyledons. This finding may implicate a role of glucose in affecting spatial distribution of auxin regulated genes and thus altered morphogenesis. This is a matter of further investigation as to how this altered expression of auxin inducible genes in turn affect seedling architecture.

Figure 1.

5d old light-grown Saur::GuS seedlings treated for 3h with 0% glucose or 3% glucose containing 1/2 MS medium followed by gus staining.

Addendum to: Mishra BS, Singh M, Aggrawal P, Laxmi A. Glucose and auxin signaling interaction in controlling Arabidopsis thaliana seedlings root growth and development. PLoS ONE. 2009;4:4502. doi: 10.1371/journal.pone.0004502.