Auxin regulates adventitious root formation in tomato cuttings

Background Adventitious root (AR) formation is a critical developmental process in cutting propagation for the horticultural industry. While auxin has been shown to regulate this process, the exact mechanism and details preceding AR formation remain unclear. Even though AR and lateral root (LR) formation share common developmental processes, there are exist some differences that need to be closely examined at the cytological level. Tomato stem cuttings, which readily form adventitious roots, represent the perfect system to study the influence of auxin on AR formation and to compare AR and LR organogenesis. Results Here we show the progression by which AR form from founder cells in the basal pericycle cell layers in tomato stem cuttings. The first disordered clumps of cells assumed a dome shape that later differentiated into functional AR cell layers. Further growth resulted in emergence of mature AR through the epidermis following programmed cell death of epidermal cells. Auxin and ethylene levels increased in the basal stem cutting within 1 h. Tomato lines expressing the auxin response element DR5pro:YFP showed an increase in auxin distribution during the AR initiation phase, and was mainly concentrated in the meristematic cells of the developing AR. Treatment of stem cuttings with auxin, increased the number of AR primordia and the length of AR, while stem cuttings treated with the pre-emergent herbicide/auxin transport inhibitor N-1-naphthylphthalamic acid (NPA) occasionally developed thick, agravitropic AR. Hormone profile analyses showed that auxin positively regulated AR formation, whereas perturbations to zeatin, salicylic acid, and abscisic acid homeostasis suggested minor roles during tomato stem rooting. The gene expression of specific auxin transporters increased during specific developmental phases of AR formation. Conclusion These data show that AR formation in tomato stems is a complex process. Upon perception of a wounding stimulus, expression of auxin transporter genes and accumulation of auxin at founder cell initiation sites in pericycle cell layers and later in the meristematic cells of the AR primordia were observed. A clear understanding and documentation of these events in tomato is critical to resolve AR formation in recalcitrant species like hardwoods and improve stem cutting propagation efficiency and effectiveness.

Clonal analysis reveals gradual recruitment of lateral root founder cells and a link between root initiation and cambium formation in Arabidopsis thaliana

The pericycle gives rise to lateral roots (LRs) and lateral meristems (LMs; cambium and phellogen), however, a thorough clonal analysis of pericycle cell lineage has not been investigated. This study fills in this gap and addresses pericycle impact in LR and LM development.Heath-shock inducible DS1 transposition in 35S-DS1-H2B:YFP; HS-Ac seedlings results in production of YFP-labelled cell clones. These clones in pericycle cell derivatives were identified with a confocal microscopy and subjected to 3D reconstructions and analysis.Participation of pericycle founder cells (FC) in LR formation is more variable than previously considered. LR initiation was found most commonly involved the specification of just one FC in the longitudinal and one or two cells in transverse direction. After LR initiation, FCs continue to be recruited in both directions from pre-existing cells. Anticlinal divisions in the pericycle resulting in LMs start already in the young differentiation zone where only the protoxylem is differentiated.The clonal analysis demonstrated that pericycle cell activity related to LR formation is not separated in time and space from that related to LM formation and that LR FC recruitment is a gradual process. The analysis demonstrated that immediate pericycle progeny lack self-renewal capacity.

The Glucocorticoid-Inducible GVG System Causes Severe Growth Defects in Both Root and Shoot of the Model Legume Lotus japonicus

During the past decade, the legume Lotus japonicus has emerged as an important model system for study of symbiotic nitrogen fixation. Controlled expression of genes involved in symbiosis from an inducible promoter at specific time points would be a valuable tool for investigating gene function in L. japonicus. We have attempted to study the function of the putative transcription factors LjNDX and LjCPP1 by expression from the GVG inducible system. This study showed that the GVG system itself causes growth disturbances in L. japonicus. Shoot internode elongation and root pericycle cell division are affected when the chimeric GVG transcription factor is activated. We suggest that deficient auxin signaling could cause the phenotype observed and conclude that the GVG inducible system is not well suited for use in the model legume L. japonicus.