Editorial: Root adaptations to multiple stress factors

Editorial article of an e-book on Root adaptations to multiple stress factor

Soil carbon storage potential of acid soils of Colombia's Eastern High Plains

Improving soil organic carbon (SOC) storage enhances soil quality and mitigates climate change. Agricultural and livestock specialists increasingly view tropical grasslands as a potential target for storing more soil carbon while boosting productivity. Earlier research in the 1990s showed the promise of improving SOC storage in the Eastern High Plains of Colombia. But these studies were limited to two experimental stations, without focusing on conditions on farms or under variable management. This research examined whether those early studies did indeed reflect possibilities for improving SOC storage and livestock productivity. We measured SOC stocks at one of the experiment stations from previous research and on farms throughout the study area in Colombia's Eastern High Plains. Complementarily our team sampled other predominant land uses to map SOC storage across the nearly 1 million ha study area. Using that information, we also constructed scenarios suggesting changes in SOC and productivity based on land-use changes. The high SOC accumulation found at experimental sites in the 1990s declined 24 years later. However, SOC storage was over 27 Mg ha−1 yr−1 higher than reference native savanna sites, with an accumulation rate of 0.96 Mg ha−1 yr−1. On farms under variable management, improved pastures stored 10 Mg ha−1 more SOC than degraded pastures or native savanna. For the whole region, we estimate that carbon storage observed across soils and land use of the 1 million ha study area could store 0.08 Gt of carbon down to 1 m depth, with wide variation across the region. While the SOC measured in grasslands in the early 1990s did not persist under inadequate management over the period of two decades, the potential to accumulate SOC of Colombia's Eastern High Plains through appropriate management is high, pointing to a sustainable livestock strategy that boosts productivity and reduces emissions

Proteomic and phosphoproteomic analysis of polyethylene glycol-induced osmotic stress in root tips of common bean (Phaseolus vulgaris L.)

Previous studies have shown that polyethylene glycol (PEG)-induced osmotic stress (OS) reduces cell-wall (CW) porosity and limits aluminium (Al) uptake by root tips of common bean (Phaseolus vulgaris L.). A subsequent transcriptomic study suggested that genes related to CW processes are involved in adjustment to OS. In this study, a proteomic and phosphoproteomic approach was applied to identify OS-induced protein regulation to further improve our understanding of how OS affects Al accumulation. Analysis of total soluble proteins in root tips indicated that, in total, 22 proteins were differentially regulated by OS; these proteins were functionally categorized. Seventy-seven per- cent of the total expressed proteins were involved in metabolic pathways, particularly of carbohydrate and amino acid metabolism. An analysis of the apoplastic proteome revealed that OS reduced the level of five proteins and increased that of seven proteins. Investigation of the total soluble phosphoproteome suggested that dehydrin responded to OS with an enhanced phosphorylation state without a change in abundance. A cellular immunolocalization analysis indicated that dehydrin was localized mainly in the CW. This suggests that dehydrin may play a major protective role in the OS-induced physical breakdown of the CW structure and thus maintenance of the reversibility of CW extensibility during recovery from OS. The proteomic and phosphoproteomic analyses provided novel insights into the complex mechanisms of OS-induced reduction of Al accumulation in the root tips of common bean and highlight a key role for modification of CW structure.BMZ/GTZ/05.7860.9-001.00BMZ/GTZ/05.7860.9-001.0

Alteration of cell-wall porosity is involved in osmotic stress-induced enhancement of aluminium resistance in common bean (Phaseolus vulgaris L.)

Aluminium (Al) toxicity and drought are the two major abiotic stress factors limiting common bean production in the tropics. Using hydroponics, the short-term effects of combined Al toxicity and drought stress on root growth and Al uptake into the root apex were investigated. In the presence of Al stress, PEG 6000 (polyethylene glycol)-induced osmotic (drought) stress led to the amelioration of Al-induced inhibition of root elongation in the Al-sensitive genotype VAX 1. PEG 6000 (>> PEG 1000) treatment greatly decreased Al accumulation in the 1 cm root apices even when the roots were physically separated from the PEG solution using dialysis membrane tubes. Upon removal of PEG from the treatment solution, the root tips recovered from osmotic stress and the Al accumulation capacity was quickly restored. The PEG-induced reduction of Al accumulation was not due to a lower phytotoxic Al concentration in the treatment solution, reduced negativity of the root apoplast, or to enhanced citrate exudation. Also cell-wall (CW) material isolated from PEG-treated roots showed a low Al-binding capacity which, however, was restored after destroying the physical structure of the CW. The comparison of the Al3+, La3+, Sr2+, and Rb+ binding capacity of the intact root tips and the isolated CW revealed the specificity of the PEG 6000 effect for Al. This could be due to the higher hydrated ionic radius of Al3+ compared with other cations (Al3+ >> La3+ > Sr2+ > Rb+). In conclusion, the results provide circumstantial evidence that the osmotic stress-inhibited Al accumulation in root apices and thus reduced Al-induced inhibition of root elongation in the Al-sensitive genotype VAX 1 is related to the alteration of CW porosity resulting from PEG 6000-induced dehydration of the root apoplast

Physiological and molecular analysis of the interaction between aluminium toxicity and drought stress in common bean (Phaseolus vulgaris)

Aluminium (Al) toxicity and drought are two major factors limiting common bean (Phaseolus vulgaris) production in the tropics. Short-term effects of Al toxicity and drought stress on root growth in acid, Al-toxic soil were studied, with special emphasis on Al–drought interaction in the root apex. Root elongation was inhibited by both Al and drought. Combined stresses resulted in a more severe inhibition of root elongation than either stress alone. This result was different from the alleviation of Al toxicity by osmotic stress (–0.60 MPa polyethylene glycol) in hydroponics. However, drought reduced the impact of Al on the root tip, as indicated by the reduction of Al-induced callose formation and MATE expression. Combined Al and drought stress enhanced up-regulation of ACCO expression and synthesis of zeatin riboside, reduced drought-enhanced abscisic acid (ABA) concentration, and expression of NCED involved in ABA biosynthesis and the transcription factors bZIP and MYB, thus affecting the regulation of ABA-dependent genes (SUS, PvLEA18, KS-DHN, and LTP) in root tips. The results provide circumstantial evidence that in soil, drought alleviates Al injury, but Al renders the root apex more drought-sensitive, particularly by impacting the gene regulatory network involved in ABA signal transduction and cross-talk with other phytohormones necessary for maintaining root growth under drought

Differences in root distribution, nutrient acquisition and nutrient utilization by tropical forage species grown in degraded hillside soil conditions

Low nutrient availability, especially phosphorus (P) and nitrogen (N) supply is the major limitation to forage production in acid infertile soils of the tropics. A field study was conducted at the farm 'La Esperanza' located in Mondomo, Department of Cauca, in the coffee growing zone of Colombia. The main objective was to determine differences in root distribution, nutrient (N, P, K, Ca, Mg and S) acquisition and nutrient utilization of one C4 forage grass (Brachiaria dictyoneura) and two C3 forage legumes (Arachis pintoi and Centrosema macrocarpum) grown under two fertilization levels, cultivated either in monoculture or in association and harvested at four different ages.There were no significant differences in root biomass among the grass and legumes and their combinations. The native vegetation had the lowest root biomass; while the introduced grass (B. dictyoneura) had the highest root length density among all materials at all depths and ages and the native vegetation had the highest specific root length. As expected, nutrient uptake increased with age and with high fertilization in all species. Centrosema macrocarpun had the highest N and Ca uptake among all plant materials tested. Uptake of P, K and Mg was greater in the grass B. dictyoneura than in the other plant species and combination planting at all ages. On the other hand, the grass had the lowest Ca uptake. The grass and its mixture with the legumes A. pintoi and C. macrocarpun had the highest S uptake. A highly significant (pLa baja disponibilidad de nutrientes, especialmente fósforo (P) y nitrógeno (N) es el mayor limitante para la producción de forrajes en los suelos ácidos de baja fertilidad del trópico. En la finca La Esperanza. localizada en Mondomo, departamento del Cauca, zona cafetera de Colombia, se llevo a cabo un estudio con el objeto de determinar las diferencias en la distribución de raíces, absorción y utilización de nutrientes (N, P, K, Ca, Mg, y S) de una gramínea forrajera C4 (Brachiaria dictyoneura) y dos leguminosas C3 (Arachis pintoi y Centrosema macrocarpum) creciendo bajo dos niveles de fertilización y cultivadas en monocultivo y en asociación y cosechadas a cuatro diferentes edades. No se encontraron diferencias significativas en biomasa de raíces entre la gramínea y las leguminosas y sus asociaciones. La vegetación nativa presentó la menor cantidad de biomasa de raíces; mientras que la gramínea introducida (B. dictyoneura) presentó la menor cantidad entre todos los materiales en todas las profundidades y edades, y la vegetación nativa presentó la más alta longitud especifica de raíces. Como era de esperar, la absorción de nutrientes se incremento con la edad y con el mayor nivel de fertilización en todas las especies. Centrosema macrocarpun tuvo la más alta absorción de N y Ca entre todos los materiales evaluados. La absorción de P, K y Mg fue mayor en la gramínea que en las otras especies solas o asociadas. Por otro lado, la gramínea presentó la más baja absorción de Ca. La gramínea sola y las asociaciones con las leguminosas A. pintoi y C. macrocarpun presentaron la mayor alta absorción de S. Se encontró una alta correlación (P 0.001) entre la densidad de longitud de raíces (profundidades 0 - 10 y 10 - 20cm) y la absorción de N y P. La eficiencia de uso de nutrientes (forraje producido/nutriente absorbido -g/g)) incrementó con la edad hasta las 38 semanas. A 55 semanas se observó una fuerte caída en la eficiencia de uso de nutrientes. Los valores de la eficiencia de uso de N, Ca y P fueron más altos con las gramíneas que con las leguminosas evaluadas. La eficiencia de uso K fue similar entre las tres especies. La gramínea mostró los mas altos valores de Mg y S, y la leguminosa A. pintoi el menor

Effective use of water and increased dry matter partitioned to grain contribute to yield of common bean improved for drought resistance

Common bean (Phaseolus vulgaris L.) is the most important food legume in the diet of poor people in the tropics. Drought causes severe yield loss in this crop. Identification of traits associated with drought resistance contributes to improving the process of generating bean genotypes adapted to these conditions. Field studies were conducted at the International Center for Tropical Agriculture (CIAT), Palmira, Colombia, to determine the relationship between grain yield and different parameters such as effective use of water (EUW), canopy biomass, and dry partitioning indices (pod partitioning index, harvest index, and pod harvest index) in elite lines selected for drought resistance over the past decade. Carbon isotope discrimination (CID) was used for estimation of water use efficiency (WUE). The main objectives were: (i) to identify specific morpho-physiological traits that contribute to improved resistance to drought in lines developed over several cycles of breeding and that could be useful as selection criteria in breeding; and (ii) to identify genotypes with desirable traits that could serve as parents in the corresponding breeding programs. A set of 36 bean genotypes belonging to the Middle American gene pool were evaluated under field conditions with two levels of water supply (irrigated and drought) over two seasons. Eight bean lines (NCB 280, NCB 226, SEN 56, SCR 2, SCR 16, SMC 141, RCB 593, and BFS 67) were identified as resistant to drought stress. Resistance to terminal drought stress was positively associated with EUW combined with increased dry matter partitioned to pod and seed production and negatively associated with days to flowering and days to physiological maturity. Differences in genotypic response were observed between grain CID and grain yield under irrigated and drought stress. Based on phenotypic differences in CID, leaf stomatal conductance, canopy biomass, and grain yield under drought stress, the lines tested were classified into two groups, water savers and water spenders. Pod harvest index could be a useful selection criterion in breeding programs to select for drought resistance in common bean