Cell type identity in<Emphasis Type="Italic">Arabidopsis</Emphasis> roots is altered by both ascorbic acid-induced changes in the redox environment and the resultant endogenous auxin response

Redox plays a critical role in controlling many cellular processes of plant growth and development. To understand the effect of changes in redox on cell-type determination in the root meristem, we examined the influence of a strong reducing agent -ascorbic acid (AA) - on both the expression patterns of several cell type-specific promoters and the endogenous auxin sensitivity of auxin-responsive DR5::GUS transgenic plants. AA treatment altered the regular expression of columella-specific markers. Moreover, when the same treatment was applied to the DR5::GUS lines, normal expression of the GUS reporter was completely abolished in the auxin maximum, while exogenous auxin restored AA-driven depletion of that maximum. Interestingly, the level of DHA (dehydroascorbate, an oxidized form of AA) in the AA-treated roots was greatly increased. This indicates that changes in cell-type specificity and the sensitivity to endogenous auxin may result from an increase in the cellular DHA that is metabolized from exogenously supplied AA. Therefore, we propose that redox changes in the root meristem alter auxin homeostasis, perhaps causing a change in cell types within the root meristem.     

The influence of cytokinin-auxin cross-regulation on cell-fate determination in Arabidopsis thaliana root development

Root growth and development in Arabidopsis thaliana are sustained by a specialised zone termed the meristem, which contains a population of dividing and differentiating cells that are functionally analogous to a stem cell niche in animals. The hormones auxin and cytokinin control meristem size antagonistically. Local accumulation of auxin promotes cell division and the initiation of a lateral root primordium. By contrast, high cytokinin concentrations disrupt the regular pattern of divisions that characterises lateral root development, and promote differentiation. The way in which the hormones interact is controlled by a genetic regulatory network. In this paper, we propose a deterministic mathematical model to describe this network and present model simulations that reproduce the experimentally observed effects of cytokinin on the expression of auxin regulated genes. We show how auxin response genes and auxin efflux transporters may be affected by the presence of cytokinin. We also analyse and compare the responses of the hormones auxin and cytokinin to changes in their supply with the responses obtained by genetic mutations of SHY2, which encodes a protein that plays a key role in balancing cytokinin and auxin regulation of meristem size. We show that although shy2 mutations can qualitatively reproduce the effect of varying auxin and cytokinin supply on their response genes, some elements of the network respond differently to changes in hormonal supply and to genetic mutations, implying a different, general response of the network. We conclude that an analysis based on the ratio between these two hormones may be misleading and that a mathematical model can serve as a useful tool for stimulate further experimental work by predicting the response of the network to changes in hormone levels and to other genetic mutations.     

SIZ1 regulation of phosphate starvation-induced root architecture remodeling involves the control of auxin accumulation

Phosphate (Pi) limitation causes plants to modulate the architecture of their root systems to facilitate the acquisition of Pi. Previously, we reported that the Arabidopsis (Arabidopsis thaliana) SUMO E3 ligase SIZ1 regulates root architecture remodeling in response to Pi limitation; namely, the siz1 mutations cause the inhibition of primary root (PR) elongation and the promotion of lateral root (LR) formation. Here, we present evidence that SIZ1 is involved in the negative regulation of auxin patterning to modulate root system architecture in response to Pi starvation. The siz1 mutations caused greater PR growth inhibition and LR development of seedlings in response to Pi limitation. Similar root phenotypes occurred if Pi-deficient wild-type seedlings were supplemented with auxin. N-1-Naphthylphthalamic acid, an inhibitor of auxin efflux activity, reduced the Pi starvation-induced LR root formation of siz1 seedlings to a level equivalent to that seen in the wild type. Monitoring of the auxin-responsive reporter DR5::uidA indicated that auxin accumulates in PR tips at early stages of the Pi starvation response. Subsequently, DR5::uidA expression was observed in the LR primordia, which was associated with LR elongation. The time-sequential patterning of DR5::uidA expression occurred earlier in the roots of siz1 as compared with the wild type. In addition, microarray analysis revealed that several other auxin-responsive genes, including genes involved in cell wall loosening and biosynthesis, were up-regulated in siz1 relative to wild-type seedlings in response to Pi starvation. Together, these results suggest that SIZ1 negatively regulates Pi starvation-induced root architecture remodeling through the control of auxin patterning.     

DAR2 acts as an important node connecting cytokinin,auxin, SHY2 and PLT1/2 in root meristem size control

Cytokinin and auxin antagonistically affect cell proliferation and differentiation and thus regulate root meristem size by influencing the abundance of SHORT HYPOCOTYL2 (SHY2/IAA3). SHY2 affects auxin distribution in the root meristem by repressing the auxin-inducible expression of PIN-FORMED (PIN) auxin transport genes. The PLETHORA (PLT1/2) genes influence root meristem growth by promoting stem cells and transit-amplifying cells. However, the factors connecting cytokinin, auxin, SHY2 and PLT1/2 are largely unknown. In a recent study, we have shown that the DA1-related protein 2 (DAR2) acts downstream of cytokinin and SHY2 but upstream of PLT1/2 to affect root meristem size. Here, we discuss the possible molecular mechanisms by which Arabidopsis DAR2 controls root meristem size.     

Symplastic communication in the root cap directs auxin distribution to modulate root development

Root cap not only protects root meristem, but also detects and transduces the signals of environmental changes to affect root development. The symplastic communication is an important way for plants to transduce signals to coordinate the development and physiology in response to the changing enviroments. However, it is unclear how the symplastic communication between root cap cells affects root growth. Here we exploit an inducible system to specifically block the symplastic communication in the root cap. Transient blockage of plasmodesmata (PD) in differentiated collumella cells severely impairs the root development in Arabidopsis, in particular in the stem cell niche and the proximal meristem. The neighboring stem cell niche is the region that is most sensitive to the disrupted symplastic communication and responds rapidly via the alteration of auxin distribution. In the later stage, the cell division in proximal meristem is inhibited, presumably due to the reduced auxin level in the root cap. Our results reveal the essential role of the differentiated collumella cells in the root cap mediated signaling system that directs root development.