Search

Iron (Fe) is an essential plant micronutrient, being a major limiting growth factor in calcareous soils. To increase Fe uptake, plants induce lateral roots growth, the expression of a Fe(III)-chelate reductase (FCR), a Fe(II)-transporter and a H+-ATPase and the secretion of flavins. Furthermore, auxin hormone family is involved in the Fe-deficiency responses but the action mechanism remains elusive. In this work, we evaluated the effect of the auxin-precursor indole-3-acetaldoxime (IAOx) on hydroponically grown Medicago truncatula plants under different Fe conditions. Upon 4-days of Fe starvation, the pH of the nutrient solution decreased, while both the FCR activity and the presence of flavins increased. Exogenous IAOx increased lateral roots growth contributing to superroot phenotype, decreased chlorosis, and delayed up to 3-days the pH-decrease, the FCR-activity increase, and the presence of flavins, compared to Fe-deficient plants. Gene expression levels were in concordance with the physiological responses. Results: showed that IAOx was immediately transformed to IAN in roots and shoots to maintain auxin homeostasis. IAOx plays an active role in iron homeostasis delaying symptoms and responses in Fe-deficient plants. We may speculate that IAOx or its derivatives remobilize Fe from root cells to alleviate Fe-deficiency. Overall, these results point out that the IAOx-derived phenotype may have advantages to overcome nutritional stresses. This work was supported by the Spanish Research Agency [grant AGL2017-83358-R funded by MCIN/AEI/10.13039/501100011033 and by “ERDF A way of making Europe”] and the Government of Aragón [grant A09_23R], co-financed with FEDER funds; and the CSIC [grant 2020AEP119] to Y.G. This research was partially supported by [grants AGL2017-52396-P funded from MINECO (co-financed with FEDER funds) and PID-2020-1177 03GB-I00 from the Public University of Navarre] to J.F.M. J.M., M.U. and J.B. were the recipients of pre-doctoral contracts awarded by the Government of Aragon, Government of Navarre, and Public University of Navarre, Spain, respectively. A.R. was hired [grants AGL2017-83358-R and 2020AEP119]. We acknowledge support of the publication fee by the CSIC Gold Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI).

Plants differ widely in their growth and tolerance responses to ammonium and urea nutrition, while derived phenotypes seem markedly different from plants grown under nitrate supply. Plant responses to N sources are complex, and the traits involved remain unknown. This work reports a comprehensive and quantitative root proteomic study on the NH4+-tolerant legume Medicago truncatula grown under axenic conditions with either nitrate, NH4+ or urea supply as sole N source by using the iTRAQ method. Sixty-one different proteins among the three N sources were identified. Interestingly, among the proteomic responses, urea nutrition displayed greater similarity to nitrate than to ammonium nutrition. We found remarkable differences in membrane proteins that play roles in sensing the N form, and regulate the intracellular pH and the uptake of N. Also, several groups of proteins were differentially expressed in the C metabolism pathway involved in reorganizing N assimilation. In addition, enzymes related to phenylpropanoid metabolism, including the peroxidases POD2, POD6, POD7 and POD11, which were up-regulated under ammonium nutrition, contributed to the reinforcement of cell walls, as confirmed by specific staining of lignin. Thus, we identified cell wall lignification as an important tolerance mechanism of root cells associated with the stunted phenotype typical of plants grown under ammonium nutrition. This work was supported by the grants AGL2014-52396-P and AGL2017-86293-P from the Spanish Ministry of Economy and Competitiveness-MINECO. RE received a Juan de la Cierva-incorporación grant IJCI-2014-21452. JB is a holder of a PhD fellowship from the Public University of Navarre. The Proteomics Unit of Navarrabiomed is a member of ProteoRed, PRB3-ISCIII-Spain, and is supported by grant PT17/0019/009, of the PE I + D+I 2018-2020 funded by ISCIII and FEDER.

Indole-3-acetaldoxime (IAOx) is a particularly relevant molecule as an intermediate in the pathway for tryptophan-dependent auxin biosynthesis. The role of IAOx in growth-signalling and root phenotype is poorly studied in cruciferous plants and mostly unknown in non-cruciferous plants. We synthesized IAOx and applied it to M. truncatula plants grown axenically with NO3 -, NH4 + or urea as the sole nitrogen source. During 14 days of growth, we demonstrated that IAOx induced an increase in the number of lateral roots, especially under NH4 + nutrition, while elongation of the main root was inhibited. This phenotype is similar to the phenotype known as superroot previously described in SUR1- and SUR2-defective Arabidopsis mutants. The effect of IAOx, IAA or the combination of both on the root phenotype was different and dependent on the type of N-nutrition. Our results also showed the endogenous importance of IAOx in a legume plant in relation to IAA metabolism, and suggested IAOx long-distance transport depending on the nitrogen source provided. Finally, our results point out to CYP71A as the major responsible enzymes for IAA synthesis from IAOx, while they exclude indole-3-acetaldehyde oxidases. © 2019 Elsevier B.V. This work was supported by the grants AGL2017-86293-P and CGL2017-84723-P from the Spanish Ministry of Economy and Competitiveness (MINECO) , AGL2014-52396-P from the Ministry of Science Innovation and Universities (MICINN) , and IT932-16 from the Basque Government, Spain . JB and PL-G are holders of PhD fellowships from the Public University of Navarre. ACh received a Juan de la Cierva initiation grant FJCI-2016-27905 and RE received a Juan de la Cierva incorporation grant IJCI-2014-21452. This research was also supported by the Basque Government through the BERC 2018-2021 program, and by the Spanish Ministry of Science, Innovation and Universities through the BC3 María de Maeztu excellence accreditation (MDM-2017-0714).