Localized induction of the ATP-binding cassette B19 auxin transporter enhances adventitious root formation in Arabidopsis

Transcriptomic analysis reveals ethylene as stimulator and auxin as regulator of adventitious root formation in petunia cuttings

Adventitious root (AR) formation in the stem base (SB) of cuttings is the basis for propagation of many plant species and petunia is used as model to study this developmental process. Following AR formation from 2 to 192 hours post-excision (hpe) of cuttings, transcriptome analysis by microarray revealed a change of the character of the rooting zone from SB to root identity. The greatest shift in the number of differentially expressed genes was observed between 24 and 72 hpe, when the categories storage, mineral nutrient acquisition, anti-oxidative and secondary metabolism, and biotic stimuli showed a notable high number of induced genes. Analyses of phytohormone-related genes disclosed multifaceted changes of the auxin transport system, auxin conjugation and the auxin signal perception machinery indicating a reduction in auxin sensitivity and phase-specific responses of particular auxin-regulated genes. Genes involved in ethylene biosynthesis and action showed a more uniform pattern as a high number of respective genes were generally induced during the whole process of AR formation. The important role of ethylene for stimulating AR formation was demonstrated by the application of inhibitors of ethylene biosynthesis and perception as well as of the precursor aminocyclopropane-1-carboxylic acid, all changing the number and length of AR. A model is proposed showing the putative role of polar auxin transport and resulting auxin accumulation in initiation of subsequent changes in auxin homeostasis and signal perception with a particular role of Aux/IAA expression. These changes might in turn guide the entrance into the different phases of AR formation. Ethylene biosynthesis, which is stimulated by wounding and does probably also respond to other stresses and auxin, acts as important stimulator of AR formation probably via the expression of ethylene responsive transcription factor genes, whereas the timing of different phases seems to be controlled by auxin.

CONSTANS-LIKE 7 (COL7) is involved in phytochrome B (phyB)-mediated light-quality regulation of auxin homeostasis

Arabidopsis phytochrome B (phyB) is the major photoreceptor that senses the ratio of red to far-red light (R:FR) to regulate the shade-avoidance response (SAR). It has been hypothesized that altered homeostasis of phytohormones such as auxin and strigolactone is at least partially responsible for SAR, but the mechanism underlying phyB regulation of the hormonal change is not fully understood. Previously we reported that CONSTANS-LIKE 7 (COL7) enhances branching number under high R:FR but not under low R:FR, implying that COL7 may be involved in the phyB-mediated SAR. In this study, we provide evidence that COL7 reduces auxin levels in a high R:FR-dependent manner. We found that the phyB mutation suppresses the COL7-induced branching proliferation. Moreover, COL7 promotes mRNA expression of SUPERROOT 2 (SUR2), which encodes a suppressor of auxin biosynthesis, in high R:FR but not in low R:FR. Consistently with these results, deficiency of phyB suppresses the elevated transcription of SUR2 in COL7 overexpression plants grown in high R:FR. Taking these results together with data suggesting that photo-excited phyB is required for stabilization of the COL7 protein, we argue that COL7 is a critical factor linking light perception to changes in auxin level in Arabidopsis.

The far side of auxin signaling: fundamental cellular activities and their contribution to a defined growth response in plants

Recent years have provided us with spectacular insights into the biology of the plant hormone auxin, leaving the impression of a highly versatile molecule involved in virtually every aspect of plant development. A combination of genetics, biochemistry, and cell biology has established auxin signaling pathways, leading to the identification of two distinct modes of auxin perception and downstream regulatory cascades. Major targets of these signaling modules are components of the polar auxin transport machinery, mediating directional distribution of the phytohormone throughout the plant body, and decisively affecting plant development. Alterations in auxin transport, metabolism, or signaling that occur as a result of intrinsic as well as environmental stimuli, control adjustments in morphogenetic programs, giving rise to defined growth responses attributed to the activity of the phytohormone. Some of the results obtained from the analysis of auxin, however, do not fit coherently into a picture of highly specific signaling events, but rather suggest mutual interactions between auxin and fundamental cellular pathways, like the control of intracellular protein sorting or translation. Crosstalk between auxin and these basic determinants of cellular activity and how they might shape auxin effects in the control of morphogenesis are the subject of this review.

A novel viable allele of Arabidopsis CULLIN1 identified in a screen for superroot2 suppressors by next generation sequencing-assisted mapping

Map-based cloning (MBC) is the conventional approach for linking phenotypes to genotypes, and has been successfully used to identify causal mutations in diverse organisms. Next-generation sequencing (NGS) technologies offer unprecedented possibilities to sequence the entire genomes of organisms, thereby in principle enabling direct identification of causal mutations without mapping. However, although mapping-by-sequencing has proven to be a cost effective alternative to classical MBC in particular situations, methods based solely on NGS still have limitations and need to be refined. Aiming to identify the causal mutations in suppressors of Arabidopsis thaliana superroot2 phenotype, generated by ethyl methane sulfonate (EMS) treatment, we combined NGS and classical mapping, to rapidly identify the point mutations and restrict the number of testable candidates by defining the chromosomal intervals containing the causal mutations, respectively. The NGS-assisted mapping approach we describe here facilitates unbiased identification of virtually any causal EMS-generated mutation by overlapping the identification (deep sequencing) and validation (mapping) steps. To exemplify the useful marriage of the two approaches we discuss the strategy used to identify a new viable recessive allele of the Arabidopsis CULLIN1 gene in the non-reference Wassilewskija (Ws-4) accession.

Auxin is a central player in the hormone cross-talks that control adventitious rooting

Vegetative propagation of economically important woody, horticultural and agricultural species rely on an efficient adventitious root (AR) formation. The formation of ARs is a complex genetic trait regulated by the interaction of environmental and endogenous factors among which the phytohormone auxin plays an essential role. This article summarizes the current knowledge related to the intricate network through which auxin controls adventitious rooting. How auxin and recently identified auxin-related compounds affect AR formation in different plant species is discussed. Particular attention is addressed to illustrate how auxin has a central role in the hormone cross-talk leading to AR development. In parallel, we describe the molecular players involved in the control of auxin homeostasis, transport and signaling, for a better understanding of the auxin action during adventitious rooting.

Adventitious roots and lateral roots: similarities and differences

In addition to its role in water and nutrient uptake, the root system is fundamentally important because it anchors a plant to its substrate. Although a wide variety of root systems exist across different species, all plants have a primary root (derived from an embryonic radicle) and different types of lateral roots. Adventitious roots, by comparison, display the same functions as lateral roots but develop from aerial tissues. In addition, they not only develop as an adaptive response to various stresses, such as wounding or flooding, but also are a key limiting component of vegetative propagation. Lateral and adventitious roots share key elements of the genetic and hormonal regulatory networks but are subject to different regulatory mechanisms. In this review, we discuss the developmental processes that give rise to lateral and adventitious roots and highlight knowledge acquired over the past few years about the mechanisms that regulate adventitious root formation.

Proteomic changes in the base of chrysanthemum cuttings during adventitious root formation

Background

A lack of competence to form adventitious roots by cuttings of Chrysanthemum (Chrysanthemum morifolium) is an obstacle for the rapid fixation of elite genotypes. We performed a proteomic analysis of cutting bases of chrysanthemum cultivar ‘Jinba’ during adventitious root formation (ARF) in order to identify rooting ability associated protein and/or to get further insight into the molecular mechanisms controlling adventitious rooting.

Results

The protein profiles during ARF were analyzed by comparing the 2-DE gels between 0-day-old (just severed from the stock plant) and 5-day-old cutting bases of chrysanthemum. A total of 69 differentially accumulated protein spots (two-fold change; t-test: 95% significance) were excised and analyzed using MALDI-TOF/TOF, among which 42 protein spots (assigned as 24 types of proteins and 7 unknown proteins) were confidently identified using the NCBI database. The results demonstrated that 19% proteins were related to carbohydrate and energy metabolism, 16% to photosynthesis, 10% to protein fate, 7% to plant defense, 6% to cell structure, 7% to hormone related, 3% to nitrate metabolism, 3% to lipid metabolism, 3% to ascorbate biosynthesis and 3% to RNA binding, 23% were unknown proteins. Twenty types of differentially accumulated proteins including ACC oxidase (CmACO) were further analyzed at the transcription level, most of which were in accordance with the results of 2-DE. Moreover, the protein abundance changes of CmACO are supported by western blot experiments. Ethylene evolution was higher during the ARF compared with day 0 after cutting, while silver nitrate, an inhibitor of ethylene synthesis, pretreatment delayed the ARF. It suggested that ACC oxidase plays an important role in ARF of chrysanthemum.

Conclusions

The proteomic analysis of cutting bases of chrysanthemum allowed us to identify proteins whose expression was related to ARF. We identified auxin-induced protein PCNT115 and ACC oxidase positively or negatively correlated to ARF, respectively. Several other proteins related to carbohydrate and energy metabolism, protein degradation, photosynthetic and cell structure were also correlated to ARF. The induction of protein CmACO provide a strong case for ethylene as the immediate signal for ARF. This strongly suggests that the proteins we have identified will be valuable for further insight into the molecular mechanisms controlling ARF.