Quantitative dissection of variations in root growth rate: a matter of cell proliferation or of cell expansion?

Root plasticity versus elasticity - when are responses acclimative?

Spatiotemporal soil heterogeneity and the resulting edaphic stress cycles can be decisive for crop growth. However, our understanding of the acclimative value of root responses to heterogeneous soil conditions remains limited. We outline a framework to evaluate the acclimative value of root responses that distinguishes between stress responses that are persistent and reversible upon stress release, termed 'plasticity' and 'elasticity', respectively. Using energy balances, we provide theoretical evidence that the advantage of plasticity over elasticity increases with the number of edaphic stress cycles and if responses lead to comparatively high energy gains. Our framework provides a conceptual basis for assessing the acclimative value of root responses to soil heterogeneity and can catalyse research on crop adaptations to heterogeneous belowground environments.

Genotypic differences in systemic root responses to mechanical obstacles

As roots grow through the soil to forage for water and nutrients, they encounter mechanical obstacles such as patches of dense soil and stones that locally impede root growth. Here, we investigated hitherto poorly understood systemic responses of roots to localised root impedance. Seedlings of two wheat genotypes were grown in hydroponics and exposed to impenetrable obstacles constraining the vertical growth of the primary or a single seminal root. We deployed high-resolution in vivo imaging to quantify temporal dynamics of root elongation rate, helical root movement, and root growth direction. The two genotypes exhibited distinctly different patterns of systemic responses to localised root impedance, suggesting different strategies to cope with obstacles, namely stress avoidance and stress tolerance. Shallower growth of unconstrained seminal roots and more pronounced helical movement of unconstrained primary and seminal roots upon localised root impedance characterised the avoidance strategy shown by one genotype. Stress tolerance to localised root impedance, as exhibited by the other genotype, was indicated by relatively fast elongation of primary roots and steeper seminal root growth. These different strategies highlight that the effects of mechanical obstacles on spatiotemporal root growth patterns can differ within species, which may have major implications for resource acquisition and whole-plant growth.

Rapid growth of Moso bamboo (Phyllostachys edulis): Cellular roadmaps, transcriptome dynamics, and environmental factors

Moso bamboo (Phyllostachys edulis) shows remarkably rapid growth (114.5 cm/day), but the underlying biological mechanisms remain unclear. After examining more than 12,750 internodes from more than 510 culms from 17 Moso populations, we identified internode 18 as a representative internode for rapid growth. This internode includes a 2-cm cell division zone (DZ), a cell elongation zone up to 12 cm, and a secondary cell wall (SCW) thickening zone. These zones elongated 11.8 cm, produced approximately 570,000,000 cells, and deposited ∼28 mg g-1 dry weight (DW) lignin and ∼44 mg g-1 DW cellulose daily, far exceeding vegetative growth observed in other plants. We used anatomical, mathematical, physiological, and genomic data to characterize development and transcriptional networks during rapid growth in internode 18. Our results suggest that (1) gibberellin may directly trigger the rapid growth of Moso shoots, (2) decreased cytokinin and increased auxin accumulation may trigger cell DZ elongation, and (3) abscisic acid and mechanical pressure may stimulate rapid SCW thickening via MYB83L. We conclude that internode length involves a possible tradeoff mediated by mechanical pressure caused by rapid growth, possibly influenced by environmental temperature and regulated by genes related to cell division and elongation. Our results provide insight into the rapid growth of Moso bamboo.

Plant root growth against a mechanical obstacle: the early growth response of a maize root facing an axial resistance is consistent with the Lockhart model

Plant root growth is dramatically reduced in compacted soils, affecting the growth of the whole plant. Through a model experiment coupling force and kinematics measurements, we probed the force-growth relationship of a primary root contacting a stiff resisting obstacle, which mimics the strongest soil impedance variation encountered by a growing root. The growth of maize roots just emerging from a corseting agarose gel and contacting a force sensor (acting as an obstacle) was monitored by time-lapse imaging simultaneously to the force. The evolution of the velocity field along the root was obtained from kinematics analysis of the root texture with a particle image velocimetry derived technique. A triangular fit was introduced to retrieve the elemental elongation rate or strain rate. A parameter-free model based on the Lockhart law quantitatively predicts how the force at the obstacle modifies several features of the growth distribution (length of the growth zone, maximal elemental elongation rate and velocity) during the first 10 min. These results suggest a strong similarity of the early growth responses elicited either by a directional stress (contact) or by an isotropic perturbation (hyperosmotic bath).

Ammonium regulates the development of pine roots through hormonal crosstalk and differential expression of transcription factors in the apex

Ammonium is a prominent source of inorganic nitrogen for plant nutrition, but excessive amounts can be toxic for many species. However, most conifers are tolerant to ammonium, a relevant physiological feature of this ancient evolutionary lineage. For a better understanding of the molecular basis of this trait, ammonium-induced changes in the transcriptome of maritime pine (Pinus pinaster Ait.) root apex have been determined by laser capture microdissection and RNA sequencing. Ammonium promoted changes in the transcriptional profiles of multiple transcription factors, such as SHORT-ROOT, and phytohormone-related transcripts, such as ACO, involved in the development of the root meristem. Nano-PALDI-MSI and transcriptomic analyses showed that the distributions of IAA and CKs were altered in the root apex in response to ammonium nutrition. Taken together, the data suggest that this early response is involved in the increased lateral root branching and principal root growth, which characterize the long-term response to ammonium supply in pine. All these results suggest that ammonium induces changes in the root system architecture through the IAA-CK-ET phytohormone crosstalk and transcriptional regulation.

Exploiting Differential Gene Expression to Discover Ionic and Osmotic-Associated Transcripts in the Halophyte Grass Aeluropus littoralis

BACKGROUND

Salinity as a most significant environmental challenges affects the growth and productivity of plants worldwide. In this study, the ionic and iso-osmotic effects of salt stress were investigated in Aeluropus littoralis L., a halophyte grass species from Poaceae family, by cDNA-amplified fragment length polymorphism (cDNA-AFLP) technique. To dissect the two different effects (ionic and osmotic) exerted by salt stress, various ionic agents including 200 and 400 mM sodium chloride (NaCl), 200 and 400 mM potassium chloride (KCl) as well as 280 and 406 gl- 1 (- 0.9 and - 1.4 MPa) polyethylene glycol 6000 (PEG) as their iso-osmotic concentrations were applied.

RESULTS

Application of KCl and PEG significantly reduced the fresh weight (FW) of A. littoralis seedlings compared to control while NaCl treatment markedly enhanced the FW. At the transcriptome level, different observations of changes in gene expression have been made in response of A. littoralis to ionic and osmotic stresses. Out of 69 transcript derived fragments (TDFs), 42 TDFs belong to 9 different groups of genes involved in metabolism (11.6%), transcription (10.2%), ribosomal protein (8.7%), protein binding (8.7%) transporter (5.8%), translation (5.8%), signal transduction (4.3%), nucleosome assembly protein (2.9%) and catabolism (2.9%). The 44 and 28 percent of transcripts were expressed under ionic stress (NaCl-specific and KCl-specific) and osmotic stress (common with NaCl, KCl and PEG), respectively which indicating a greater response of plants to ionic stress than osmotic stress. Expression pattern of eight candidate TDFs including; SYP81, CAND1, KATN, ISB1, SAMDC, GLY1, HAK18 and ZF30 was evaluated by RT-qPCR at high salinity levels and recovery condition.

CONCLUSION

Differential regulation of these TDFs was observed in root and shoot which confirm their role in salt stress tolerance and provide initial insights into the transcriptome of A. littoralis. Expression pattern of ionic and osmotic-related TDFs at A. littoralis can be taken as an indication of their functional relevance at different salt and drought stresses.