Nitrogen (N), auxin and cytokinins (CTKs) play important roles in regulating the growth of rice tiller buds. However, little is known about the underlying. Cytokinin (CK), synthesized in the root cap, promot. Adventitious roots, apical dominance, Arabidopsis thaliana, auxin, cytokinin, ethylene, lateral .. The same basic rules on phloem and xylem relationships occur in both. In this work, a combined HPLC-ELISA technique was used to associate in vitrorooting capacity of tree peony micro-cuttings with contents of cytokinin and auxin;.
Roles of auxins and cytokinins in legume-rhizobia symbiotic interactions. The scheme represents an early rhizobial infection event in a legume Medicago truncatula root leading to the formation of an indeterminate nodule.
Rhizobia produce specific Nod factor signaling molecules as well as auxin and cytokinin hormones. A major contribution of Nod factor signals in establishing rhizobial infection events i.
Auxin–Cytokinin Interaction Regulates Meristem Development
The activation of these root epidermal cytokinin responses depends on the CRE1 Cytokinin Response 1 receptor LHK1, standing for Lotus Histidine Kinase1, being its functional homolog in Lotus japonicus whereas the activation of the auxin epidermal responses depends on ARF16a Auxin Response Factor 16a, a transcription factor.
A cytokinin response is additionally activated in dividing cortical cells and nodule primordia, also depending on the CRE1 LHK1 receptor, promoting nodule organogenesis even in the absence of symbiotic signals rhizobia or Nod factors. Ultimately, the activation of these auxin and cytokinin responses is linked to modulations of the cell cycle leading either to infection thread formation epidermis or cell divisions cortex.
In addition, divergences in these hormonal responses are observed between determinate and indeterminate nodule types e.
The involvement of cytokinin signaling pathways in the regulation of nodulation was first highlighted using a RNAi approach specifically targeting different putative cytokinin receptors in M. The fact that the lhk1 and cre1 mutants still form some nodules strongly suggest that a functional redundancy exists within the cytokinin receptor CHK family. Recently, Ng et al. Several RRA genes, used as markers of the activation of cytokinin responses, are associated with symbiotic nodulation in M.
As observed when affecting the cytokinin pool, the downregulation of the type A RR MtRRA9 reduces nodulation, suggesting a negative function of cytokinins in nodulation Op den Camp et al.
GUS fusions were detected in response to Rhizobium both in the root epidermis and in dividing cortical cells Held et al. In agreement, the auxin-response reporter DR5 fusion was detected in Rhizobium infected root hairs as well as the auxin-responsive GH3. Accordingly, an arf16a mutant shows a reduced rhizobial infection efficiency even though the number of nodule primordia and differentiated nodules remains unchanged, suggesting a positive role of auxins in the earliest stages of the rhizobial infection.
Using a dominant-negative version of an auxin signaling repressor gene expressed in actinorhizal nodules, IndoleAcetic Acid7 IAA7to specifically inhibit auxin signaling in Frankia-infected cells, an increased actinorhizal nodulation was observed Champion et al. This suggests a model where auxins, notably produced by Frankia symbiotic bacteria, induce the degradation of IAA7 and thus activate auxin-signaling, ultimately leading to an inhibitory negative feedback on nodulation.
Overall, these data indicate that in addition to their roles in nodule organogenesis, auxins, cytokinins and flavonoids may also regulate symbiotic bacterial infection Figures 1 and 2. Cytokinins and potentially auxins additionally likely regulate later symbiotic nodulation stages such as the nitrogen fixation metabolism, as suggested by transcriptomic analyses of laser-dissected differentiated nodule zones Roux et al.
Concerning auxins, it was recently reported that an auxin-overexpressing S. Changes in auxin metabolism and response occurring in a wild-type Rhizobium strain during its differentiation within nodules however remain to be demonstrated. Recent studies, respectively using a ckx3 mutant or cre1 and related chk cytokinin receptor mutants, indicate that cytokinins affect nitrogen fixation efficiency both in L.
This may imply that a potential direct connection between cytokinins and the nitrogen fixation metabolism exists. Finally, it was recently proposed that cytokinins could be involved in a systemic shoot-to-root negative regulation of nodulation Sasaki et al. More detailed analyses are therefore required to understand and integrate the various roles of auxins and cytokinins at these different symbiotic nodulation stages.
Auxin and Cytokinin Regulations in Plant—Pathogen Interactions In the past decades, most of the studies on plant pathogen interactions have focused on how the pathogens infect plant aerial organs e. Auxins and cytokinins have been recently highlighted to act in defense responses either depending on other defense-related hormones such as Salicylic Acid and Jasmonic Acid, or independently reviewed in Naseem and Dandekar, It should be noted that the function of auxins and cytokinins in defense and immunity responses largely depends on the plant and organ involved, probably because of their divergent developmental roles notably in shoots versus roots.
It is only recently that data have been gained using root pathosystems Chen Y. Some root pathogens are able to synthesize auxin- and cytokinin-like molecules Estruch et al. Amongst the best described examples, Agrobacterium tumefaciens and A. The integration of genes encoding these phytohormonal biosynthetic enzymes into the host plant genome leads to cell proliferation and a gall formation in the case of A.
Another well studied root pathogen is P. The clubroot disease is characterized by the formation of galls on infected roots Figure 1leading to plant premature senescence.
A microarray transcriptomic analysis performed on infected A. Cytokinins and auxins were additionally functionally associated with early steps of the P. Strikingly, an endophytic fungus, Heteroconium chaetospira, was described as a competitor for root cortical cell colonization, suppressing clubroot disease in rapeseed, and upregulating an auxin biosynthesis gene BnAAO1 for Ascorbic Acid Oxidase; Lahlali et al.
Amongst well-described plant pathogens infecting the root system, Aphanomyces euteiches is an oomycete causing strong damage to legume crops Gaulin et al. Infected roots become brown and necrotic, leading to a reduction in water and nutrient uptake, and later, to leaf chlorosis and plant death Gaulin et al.
Fusarium oxysporum is an ascomycete fungus belonging to a broad group containing non-pathogenic as well as pathogenic species. Root pathogenic strains are able to infect a wide range of plants including cotton, tomato, banana and Arabidopsis Chen Y.
A microarray transcriptomic analysis performed in cotton infected roots Dowd et al. Both in vitro and in vivo exogenous applications of auxins improve tomato root growth but also prevent F. Finally, one of the most famous and destructive soil-borne bacteria is Ralstonia solanacearum, causing a rapid vascular wilt disease to more than species, including legumes, tomato, potato, tobacco, banana, and Arabidopsis Genin and Denny, ; Peeters et al.
WAT1 is involved in auxin homeostasis in relation to vacuolar auxin transport, and the inactivation of WAT1 confers a broad spectrum resistance to several vascular pathogenic bacteria including R. Transcriptomic and metabolomic analyses demonstrated a repression of several genes linked to auxin metabolism in wat1 mutant roots, correlated with a decrease of a major form of auxin indole glucosinolate and to a reduction in the amount of the auxin precursor tryptophan.
Interestingly, crossing of the wat1 mutant with a trp5 mutant carrying a mutation of an anthranilate synthase ASA1 provokes an over-accumulation of tryptophan, and restores wat1 susceptibility to R.
Altogether, these results suggest a positive role of auxins in secondary wall formation, as well as in the susceptibility to pathogenic R. Accordingly, the cre1 mutant shows an increased resistance to R. Concluding Remarks Rhizospheric beneficial and detrimental microbes penetrate into root systems and tissues and trigger major modifications at organ, cellular and molecular levels, notably through modifications of developmental phytohormonal balances.
The Table 1 summarizes roles of auxins and cytokinins in different root—microbe interactions. A main feature is that as auxins and cytokinins are critical to regulate cell division and differentiation, these hormones are therefore tightly associated with the formation of new organs such as lateral roots, nodules on legume roots in response to rhizobia, as well as galls for example in response to A.
However, links likely existing with hormones controlling lateral root development, and notably auxins, remain to be identified.
Auxins, cytokinins, and their associated signaling pathways are also required for inducing root cortical cell divisions, either in legume plants to generate nodule primordia in response to rhizobia, or in Brassicaceae plants to form galls in response to the P. Interestingly, in these distantly related host plants, cortical cell divisions are similarly associated with the accumulation of naringenin and kaempferol flavonoids.
Cytokinin homeostasis is spatially and temporally regulated by a fine balance between synthesis and catabolism. The first enzyme identified in the Arabidopsis cytokinin biosynthetic pathway is adenosine phosphate-isopentenyltransferases IPTs. Another landmark is the identification of two cytochrome P monooxygenases, CYPA1 and CYPA2, which catalyze the hydroxylation at the prenyl side chain of the iP-nucleotides to synthesize tZ-nucleotides Takei et al.
In contrast to auxin, cytokinin is perceived in plants through a multi-step phosphorelay pathway similar to the bacterial two-component signaling system Kakimoto, ; To and Kieber, Recent analyses on these three receptors have revealed a largely overlap expression pattern and partially redundant functions in cytokinin perception Higuchi et al. Furthermore, the arr1, arr10, arr12 triple mutants show a strong reduction in cytokinin induction of multiple type-A ARR transcripts Mason et al.
Mutation analysis has indicated that at least eight of the 10 type-A ARRs are negative regulators of cytokinin signaling again with overlapping functions To et al. For the past few years, genetic and molecular evidence has revealed the interaction between auxin and cytokinin during plant development Dettmer et al. Many recent biochemical and genetic investigations have further confirmed that the intricate cross-talk and integration of hormone signaling are required for differentiation and maintenance of plant meristems.
Hereafter, we shall focus our discussion on the interaction between auxin and cytokinin in the regulation of meristem development.
The first step of embryonic patterning is the establishment of the apical—basal axis, in which asymmetric distribution of auxin mediated by PIN proteins plays a major role. A zygote undergoes an asymmetric division to produce a smaller apical cell and a larger basal cell. At this two-cell embryo stage, PIN7 is expressed in the basal cell to transport auxin to the apical cell Friml et al. After two more rounds of cell division, PIN7 localizes to the apical membrane of suspensor cells, resulting in the accumulation of auxin in the whole pro-embryos Friml et al.
This WUS induction might be related to auxin accumulation. However, PIN7 polarity is reversed at the cell stage, resulting in the transport of auxin towards the suspensor cells Friml et al.Plant Control
The transported auxin accumulates in the uppermost cell of the suspensor to form the hypophysis, the founder of the stem-cell niche of the embryonic root Friml et al. At a later transition stage of the embryo, auxin is directed towards the center of the cotyledon primordia in the apical domain to establish the cotyledons. Therefore, auxin transport is critical for the maintenance of the polar axis and the formation of two types of meristems in the embryo. Loss-of-function of MP or gain-of-function of BDL causes the aberrant specification of the apical cell, and prevented the formation of the embryonic root Weijers et al.
Embryos of the mp mutants are abnormal at their early globular stages. In addition, heart-stage mp embryos lack the central pro-vascular cylinder. The gain-of-function bdl mutants show milder defects than mp mutants and have a reduced vascular system and a hypocotyl of variable length without primary root meristem Hamann et al. Therefore, the primary auxin response mediated by MP and BDL is essential for root meristem initiation.