Accumulation of the Auxin Precursor Indole-3-Acetamide Curtails Growth through the Repression of Ribosome-Biogenesis and Development-Related Transcriptional Networks

Abstract

The major auxin, indole-3-acetic acid (IAA), is associated with a plethora of growth and developmental processes including embryo development, expansion growth, cambial activity, and the induction of lateral root growth. Accumulation of the auxin precursor indole-3-acetamide (IAM) induces stress related processes by stimulating abscisic acid (ABA) biosynthesis. How IAM signaling is controlled is, at present, unclear. Here, we characterize the ami1 rooty double mutant, that we initially generated to study the metabolic and phenotypic consequences of a simultaneous genetic blockade of the indole glucosinolate and IAM pathways in Arabidopsis thaliana. Our mass spectrometric analyses of the mutant revealed that the combination of the two mutations is not sufficient to fully prevent the conversion of IAM to IAA. The detected strong accumulation of IAM was, however, recognized to substantially impair seed development. We further show by genome-wide expression studies that the double mutant is broadly affected in its translational capacity, and that a small number of plant growth regulating transcriptional circuits are repressed by the high IAM content in the seed. In accordance with the previously described growth reduction in response to elevated IAM levels, our data support the hypothesis that IAM is a growth repressing counterpart to IAA.

Beyond the Usual Suspects: Physiological Roles of the Arabidopsis Amidase Signature (AS) Superfamily Members in Plant Growth Processes and Stress Responses

“…However, despite the severely reduced IAM contents of cyp79b2 cyp79b3 plants [62], both IAOx and AMI1 impaired mutants, exhibited minor IAA altered levels under standard conditions [71]. Herein, excluding the upregulation of YUC8 and ILL5/IAR3, no other significant differentially expressed auxin homeostasis-related genes were identified in ami1-2 [61].…”

“…Strikingly, it was found that, normally germinating homozygous ami1 rty parentals, set a reduced number of siliques producing non-viable seeds. The offspring aborted germination just after radicle extrusion [71]. Consistently, it had to be concluded that impaired nutrient acquisition during seed filling was most probably inherited from heterozygous rty ancestors, thus explaining the parentals sterility.…”

“…So, the initially hypothesized IAOx or IAM metabolic redirection into this pathway had to be finally discarded. On the contrary, the transcriptomics analysis provided evidence of significant repression of plant growth regulating processes in response to IAM treatment, for instance, hindering the expression of the growth-regulating factors GRF3 and GRF5 on IAM treated wt Arabidopsis seedlings [71]. Further, the identified downregulation of the TCP family members, TCP10 and TCP23 [71], underpins the observed AMI1-related growth and time flowering alterations, considering the key roles of these transcription factors in the control of shoot morphogenesis and developmental transitions [82,83].Of special note was also the downregulation of translation-related genes, involved primarily in ribosome biogenesis and assembly, as well as rRNA processing.…”

“…Therefore, in the line of assessing the described plant growth repressing role of IAM, it was highly relevant to characterize the impact of endogenous IAM accumulation, aiming at deepening on the molecular and physiological mechanisms for IAM signal perception and integration. As a starting point to tackle this question, the consequences derived from the simultaneous genetic interruption of the indole glucosinolate and IAM pathways in A.thaliana were recently reported [71]. Strikingly, it was found that, normally germinating homozygous ami1 rty parentals, set a reduced number of siliques producing non-viable seeds.…”

“…To further dissect the observed IAM dwarfish effect on ami1 rty embryos and seeds, additional IAM and IAA mass spectrometric analysis of these homozygous seeds were performed, finding higher IAM:IAA ratios in ami1 rty relative to those of wt [71] and the rty allelic sur1-1 mutant [62]. Alternatively, RNAseq transcriptional profiling of these double mutant seeds, found not only any differentially expressed genes involved neither in auxin metabolism nor camalexin biosynthesis pathway, but also, the induction of WRKY33, a transcriptional repressor of this camalexin anabolic route [71]. So, the initially hypothesized IAOx or IAM metabolic redirection into this pathway had to be finally discarded.…”