Didier Pellet

Organic acid exudation as an aluminum-tolerance mechanism in maize (Zea mays L.)

In this study, the role of root organic acid synthesis and exudation in the mechanism of aluminum tolerance was examined in Al-tolerant (South American 3) and Al-sensitive (Tuxpeño and South American 5) maize genotypes. In a growth solution containing 6 μM Al3+, Tuxpeño and South American 5 were found to be two- and threefold more sensitive to Al than South American 3. Root organic acid content and organic acid exudation from the entire root system into the bulk solution were investigated via high-performance liquid chromatographic analysis while exudates collected separately from the root apex or a mature root region (using a dividedroot-chamber technique) were analyzed with a more-sensitive ion chromatography system. In both the Al-tolerant and Al-sensitive lines, Al treatment significantly increased the total root content of organic acids, which was likely the result of Al stress and not the cause of the observed differential Al tolerance. In the absence of Al, small amounts of citrate were exuded into the solution bathing the roots. Aluminum exposure triggered a stimulation of citrate release in the Al-tolerant but not in the Al-sensitive genotypes; this response was localized to the root apex of the Al-tolerant genotype. Additionally, Al exposure triggered the release of phosphate from the root apex of the Al-tolerant genotype. The same solution Al3+ activity that elicited the maximum difference in Al sensitivity between Al-tolerant and Al-sensitive genotypes also triggered maximal citrate release from the root apex of the Al-tolerant line. The significance of citrate as a potential detoxifier for aluminum is discussed. It is concluded that organic acid release by the root apex could be an important aspect of Al tolerance in maize.

Multiple Aluminum-Resistance Mechanisms in Wheat (Roles of Root Apical Phosphate and Malate Exudation)

Although it is well known that aluminum (Al) resistance in wheat (Triticum aestivum) is multigenic, physiological evidence for multiple mechanisms of Al resistance has not yet been documented. The role of root apical phosphate and malate exudation in Al resistance was investigated in two wheat cultivars (Al-resistant Atlas and Al-sensitive Scout) and two near-isogenic lines (Al-resistant ET3 and Al-sensitive ES3). In Atlas Al resistance is multigenic, whereas in ET3 resistance is conditioned by the single Alt1 locus. Based on root- growth experiments, Atlas was found to be 3-fold more resistant in 20 [mu]M Al than ET3. Root-exudation experiments were conducted under sterile conditions; a large malate efflux localized to the root apex was observed only in Atlas and in ET3 and only in the presence of Al (5 and 20 [mu]M). Furthermore, the more Al-resistant Atlas exhibited a constitutive phosphate release localized to the root apex. As predicted from the formation constants for the Al-malate and Al-phosphate complexes, the addition of either ligand to the root bathing solution alleviated Al inhibition of root growth in Al-sensitive Scout. These results provide physiological evidence that Al resistance in Atlas is conditioned by at least two genes. In addition to the alt locus that controls Al-induced malate release from the root apex, other genetic loci appear to control constitutive phosphate release from the apex. We suggest that both exudation processes act in concert to enhance Al exclusion and Al resistance in Atlas.

Aluminum Interactions with Voltage-Dependent Calcium Transport in Plasma Membrane Vesicles Isolated from Roots of Aluminum-Sensitive and -Resistant Wheat Cultivars

The role of Al interactions with root-cell plasma membrane (PM) Ca2+ channels in Al toxicity and resistance was studied. The experimental approach involved the imposition of a transmembrane electrical potential (via K+ diffusion) in right-side-out PM vesicles derived from roots of two wheat (Triticum aestivum L.) cultivars (Al-sensitive Scout 66 and Al-resistant Atlas 66). We previously used this technique to characterize a voltage-dependent Ca2+ channel in the wheat root PM (J.W. Huang, D.L. Grunes, L.V. Kochian [1994] Proc Natl Acad Sci USA 91: 3473–3477). We found that Al3+ effectively blocked this PM Ca2+ channel; however, Al3+ blocked this Ca2+ channel equally well in both the Al-sensitive and -resistant cultivars. It was found that the differential genotypic sensitivity of this Ca2+ transport system to Al in intact roots versus isolated PM vesicles was due to Al-induced malate exudation localized to the root apex in Al-resistant Atlas but not in Al-sensitive Scout. Because malate can effectively chelate Al3+ in the rhizosphere and exclude it from the root apex, the differential sensitivity of Ca2+ influx to Al in intact roots of Al-resistant versus Al-sensitive wheat cultivars is probably due to the maintenance of lower Al3+ activities in the root apical rhizosphere of the resistant cultivar.

Involvement of multiple aluminium exclusion mechanisms in aluminium tolerance in wheat

The goal of this study was to determine if Al-chelators other than malate are released from root apices and are involved in Al-tolerance in different wheat (Triticum aestivum L.) genotypes. Also we wanted to establish if root exudates contribute to increases in rhizosphere pH around the root tip. In seedlings of Al-tolerant Atlas, we have documented a constitutive phosphate exudation from the root apex. Because phosphate can complex Al and bind protons, it could play an important role in Al tolerance, both via complexation of Al3+ and by contributing to the alkalinization of rhizosphere pH observed at the apex of Atlas. This study suggests that in wheat, Al-tolerance can be mediated by multiple exclusion mechanisms controlled by different genes.

Cassava varietal response to phosphorus fertilization. I. Yield, biomass and gas exchange

Abstract Cassava ( Manihot esculenta Crantz) yield can be limited by phosphorus (P) supply. Previous trials conducted at CIAT indicated a wide range of response to P fertilization in cassava germplasm. The purpose of this study was to elucidate possible mechanisms and morphological traits underlying cassavas differential response to P fertilization. Four contrasting varieties were grown under rainfed conditions for 10 months in the field for two consecutive seasons under four fertilizer treatments. There were three dosages of P fertilizer at 0, 50 and 100 kg P ha −1 with 100 kg NK ha −1 , plus a control with no fertilizer. Sequential harvests that included biomass determination were conducted. Single-leaf gas exchange was monitored by portable infrared gas analysis systems. All varieties showed a significant positive response in total and aerial biomass as well as mean leaf area index (LAI) to P fertilization, at least in the second year. The variety M Col 1684 was the only one that did not significantly respond in terms of dry root yield and number of storage roots to P fertilization over the years. This variety was among the most responsive for peak LAI, number of aerial apices and reproductive organs (flowers and fruits). Yield and storage root number correlated positively with net photosynthetic rates and negatively with reproductive organs. Variety M Col 1684 could not maintain a high harvest index with increasing aerial growth. In contrast, CM 489-1 balanced aerial growth enhancement with intensive root formation, augmenting root sink strength and yield. It is concluded that yield response to P fertilization depended upon the balance between aerial growth potential of varieties and storage root sink strength.

Cassava varietal response to phosphorus fertilization. II. Phosphorus uptake and use efficiency

Abstract In Latin America, where cassava is commonly grown, yield is partly limited by low-P soil. Previous trials conducted at Centro Internacional de Agricultura Tropical (CIAT) indicated a wide range of response for adaptation to low-P soils in cassava germplasm. The objectives of this study were to identify plant characteristics and possible mechanisms involved in varietal response to P and to assess the relative importance of P uptake and internal use. Two-year trials with four varieties were conducted in a field low in P with three levels (0, 50 and 100 kg ha −1 ) of P application. Five sequential harvests (with determination of P uptake) were carried out during each of two growth cycles, and VAM infection rates of fine roots and root length density in the surface soil layer were measured. In all varieties P uptake increased with P fertilization, and cultivar variation in P uptake was related to differences in fine root length density more than to VAM infection rates. Uptake efficiency of roots (as uptake rate per unit root length) did not differ among varieties. Phosphorus-use efficiency (as root yield per accumulated P in the whole plant) and patterns of dry matter allocation to roots and shoots were significantly different among varieties. It was concluded that cultivar differences in response to applied P were mainly due to contrasting patterns of dry matter partitioning and P-use efficiency. Adaptation to low-P soils could be enhanced by selection of varieties with high fine-root length density, moderate top growth and stable harvest index.

Experimental determination of genetic and environmental influences on the viscosity of triticale

Low viscosity in cereals is important for monogastric livestock feeding. With respect to triticale, knowledge on the variability of its viscosity and its environmental dependence is deplorably low. Six winter varieties with similar earliness at maturity were chosen that covered a large range of potential applied viscosity (PAV) (individual values ranging from 1.8 to 4.9 ml/g). These were cultivated in four locations in Switzerland, at altitudes ranging between 430 and 700 m a.s.l., in 2008 and 2009. The effect of genotype on the PAV was significant and clearly influenced by the location factor. Although variety × location and variety × year interactions were rather low, they were still very important for the PAV compared with other variables such as grain yield and specific grain weight. The PAV expression of one variety seemed not to be susceptible to environmental conditions. The varietal range in viscosity demonstrates a high potential for breeding to raise quality, especially as the viscosity and the grain yield were not correlated. The favourable relationship between the PAV and protein content found in the present study may provide a further incentive to improve this trait to yield high-quality triticale. Existing variability might be used to guide the choice of favourable varieties.