Abstract Silicon is widely recognized as a beneficial element for plant growth. Numerous studies have shown the beneficial effects of silicon, particularly under stress conditions. For the efficient exploration of silicon derived benefits, understanding silicon uptake mechanism, subsequent transport and accumulation in different tissues is essential. Here, a thorough review of reports describing how plants benefit from silicon supplementation was performed to provide comprehensive and clear insights. The molecular mechanism involved in silicon transport has been discussed and highlighted the knowledge gap, particularly the xylem unloading and transport in heavily silicified cells. Silicification of plant tissues like sclerenchyma, fibers, storage tissues, epidermal, and vascular tissues have been described. Silicon deposition in different cell-types, tissues, and intercellular spaces impacting morphological and physiological status found to be associated with enhanced plant resilience under various stresses. The beneficial impact of silicon deposition under various biotic and abiotic stresses has been addressed in detail. Among the mechanisms discussed here to explain silicon derived benefits, most profoundly observed includes interference in physiological processes, modulation of stress responses, and biochemical interactions. Understanding different mechanisms specific to silicon deposition in tissues, developmental stages, and environmental factors will be helpful to elucidate the versatile role of silicon in plants.

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Summary• Physcomitrella patens is a bryophyte belonging to early diverging lineages of land plants following colonization of land in the Ordovician period. Mosses are typically found in refugial habitats and can experience rapidly fluctuating environmental conditions. The acquisition of dehydration tolerance by bryophytes is of fundamental importance as they lack waterconducting tissues and are generally one cell layer thick.• Here, we show that dehydration induced oscillations in the steady-state transcript abundances of two group 3 late embryogenesis abundant (LEA) protein genes in P. patens protonemata, and that the amplitudes of these oscillations are reflective of the severity of dehydration stress.• Dehydration stress also induced elevations in the concentrations of abscisic acid (ABA), and ABA alone can also induce dosage-dependent oscillatory increases in the steadystate abundance of LEA protein transcripts. Additionally, removal of ABA resulted in rapid attenuation of these oscillatory increases.• Our data demonstrate that dehydration stress-regulated expression of LEA protein genes is temporally dynamic and highlight the importance of oscillations as a robust mechanism for optimal responses. Our results suggest that dehydration stress-induced oscillations in the steady-state abundance of LEA protein transcripts may constitute an important cellular strategy for adaptation to life in a constantly changing environment.

BackgroundIn plants, proline synthesis occurs by two enzymatic steps starting from glutamate as a precursor. Some bacteria, including bacteria such as Agrobacterium rhizogenes have an Ornithine Cyclodeaminase (OCD) which can synthesize proline in a single step by deamination of ornithine. In A. rhizogenes, OCD is one of the genes transferred to the plant genome during the transformation process and plants expressing A. rhizogenes OCD have developmental phenotypes. One nuclear encoded gene of Arabidopsis thaliana has recently been annotated as an OCD (OCD-like; referred to here as AtOCD) but nothing is known of its function. As proline metabolism contributes to tolerance of low water potential during drought, it is of interest to determine if AtOCD affects proline accumulation or low water potential tolerance.ResultsExpression of AtOCD was induced by low water potential stress and by exogenous proline, but not by the putative substrate ornithine. The AtOCD protein was plastid localized. T-DNA mutants of atocd and AtOCD RNAi plants had approximately 15% higher proline accumulation at low water potential while p5cs1-4/atocd double mutants had 40% higher proline than p5cs1 at low water potential but no change in proline metabolism gene expression which could directly explain the higher proline level. AtOCD overexpression did not affect proline accumulation. Enzymatic assays with bacterially expressed AtOCD or AtOCD purified from AtOCD:Flag transgenic plants did not detect any activity using ornithine, proline or several other amino acids as substrates. Moreover, AtOCD mutant or over-expression lines had normal morphology and no difference in root elongation or flowering time, in contrast to previous report of transgenic plants expressing A. rhizogenes OCD. Metabolite analysis found few differences between AtOCD mutants and overexpression lines.ConclusionsThe Arabidopsis OCD-like protein (AtOCD) may not catalyze ornithine to proline conversion and this is consistent with observation that three residues critical for activity of bacterial OCDs are not conserved in AtOCD. AtOCD was, however, stress and proline induced and lack of AtOCD expression increased proline accumulation by an unknown mechanism which did not require expression of P5CS1, the main enzyme responsible for stress-induced proline synthesis from glutamate. The results suggest that AtOCD may have function distinct from bacterial OCDs.

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