Al Imran Malik

Changes in physiological and morphological traits of roots and shoots of wheat in response to different depths of waterlogging

The growth reduction of wheat (Triticum aestivum L.) during and after waterlogging stress depends on the depth of water from the soil surface. In a pot experiment with 3-week-old plants, soil was waterlogged for 14 d at the surface, or at 100 or 200 mm below the surface, and pots were then drained to assess recovery. A fully drained treatment kept at field capacity served as control. During waterlogging, the relative growth rate of roots decreased more than that of shoots (by 6-27% for shoots, by 15-74% for roots), and plant growth was reduced proportionally as the water level was increased. Light-saturated net photosynthesis was reduced by 70-80% for the two most severe waterlogging treatments, but was little affected for plants in soil waterlogged at 200 mm below the surface. The number of adventitious roots formed per stem in plants grown in waterlogged soil increased up to 1.5 times, but the number of tillers per plant was reduced by 24-62%. The adventitious roots only penetrated 85-116 mm below the water level in all waterlogging treatments. Adventitious root porosity was enhanced up to 10-fold for plants grown in waterlogged soil, depending on water level and position along the roots. Porosity also increased in basal zones of roots above the water level when the younger tissues had penetrated the waterlogged zone. Fourteen days after draining the pots, growth rates of plants where the soil had been waterlogged at 200 mm below the surface had recovered, while those of plants in the more severely waterlogged treatments had only partially recovered. These findings show that the depth of waterlogging has a large impact on the response of wheat both during and after a waterlogging event so that assessment of recovery is essential in evaluating waterlogging tolerance in crops.

Enhanced formation of aerenchyma and induction of a barrier to radial oxygen loss in adventitious roots of Zea nicaraguensis contribute to its waterlogging tolerance as compared with maize (Zea mays ssp. mays)

Enhancement of oxygen transport from shoot to root tip by the formation of aerenchyma and also a barrier to radial oxygen loss (ROL) in roots is common in waterlogging-tolerant plants. Zea nicaraguensis (teosinte), a wild relative of maize (Zea mays ssp. mays), grows in waterlogged soils. We investigated the formation of aerenchyma and ROL barrier induction in roots of Z. nicaraguensis, in comparison with roots of maize (inbred line Mi29), in a pot soil system and in hydroponics. Furthermore, depositions of suberin in the exodermis/hypodermis and lignin in the epidermis of adventitious roots of Z. nicaraguensis and maize grown in aerated or stagnant deoxygenated nutrient solution were studied. Growth of maize was more adversely affected by low oxygen in the root zone (waterlogged soil or stagnant deoxygenated nutrient solution) compared with Z. nicaraguensis. In stagnant deoxygenated solution, Z. nicaraguensis was superior to maize in transporting oxygen from shoot base to root tip due to formation of larger aerenchyma and a stronger barrier to ROL in adventitious roots. The relationships between the ROL barrier formation and suberin and lignin depositions in roots are discussed. The ROL barrier, in addition to aerenchyma, would contribute to the waterlogging tolerance of Z. nicaraguensis.

Transfer of the barrier to radial oxygen loss in roots of Hordeum marinum to wheat (Triticum aestivum): evaluation of four H. marinum–wheat amphiploids

• Wide hybridization of waterlogging-tolerant Hordeum marinum with wheat (Triticum aestivum) to produce an amphiploid might be one approach to improve waterlogging tolerance in wheat. • Growth, root aerenchyma and porosity, and radial oxygen loss (ROL) along roots were measured in four H. marinum-wheat amphiploids and their parents (four accessions of H. marinum and Chinese Spring wheat) in aerated or stagnant nutrient solution. A soil experiment was also conducted. • Hordeum marinum maintained shoot dry mass in stagnant nutrient solution, whereas the growth of wheat was markedly reduced (40% of aerated control). Two of the four amphiploids were more tolerant than wheat (shoot dry masses of 59-72% of aerated controls). The porosity of adventitious roots when in stagnant solution was higher in H. marinum (19-25%) and the four amphiploids (20-24%) than in wheat (16%). In stagnant solution, adventitious roots of H. marinum formed a strong ROL barrier in basal zones, whereas, in wheat, the barrier was weak. Two amphiploids formed a strong ROL barrier and two formed a moderate barrier when in stagnant solution. • This study demonstrates the transfer of higher root porosity and a barrier to ROL from H. marinum to wheat through wide hybridization and the production of H. marinum-wheat amphiploids.

Microarray analysis of laser-microdissected tissues indicates the biosynthesis of suberin in the outer part of roots during formation of a barrier to radial oxygen loss in rice (Oryza sativa)

Internal aeration is crucial for root growth in waterlogged soil. A barrier to radial oxygen loss (ROL) can enhance long-distance oxygen transport via the aerenchyma to the root tip; a higher oxygen concentration at the apex enables root growth into anoxic soil. The ROL barrier is formed within the outer part of roots (OPR). Suberin and/or lignin deposited in cell walls are thought to contribute to the barrier, but it is unclear which compound is the main constituent. This study describes gene expression profiles during ROL barrier formation in rice roots to determine the relative responses of suberin and/or lignin biosyntheses for the barrier. OPR tissues were isolated by laser microdissection and their transcripts were analysed by microarray. A total of 128 genes were significantly up- or downregulated in the OPR during the barrier formation. Genes associated with suberin biosynthesis were strongly upregulated, whereas genes associated with lignin biosynthesis were not. By an ab initio analysis of the promoters of the upregulated genes, the putative cis-elements that could be associated with transcription factors, WRKY, AP2/ERF, NAC, bZIP, MYB, CBT/DREB, and MADS, were elucidated. They were particularly associated with the expression of transcription factor genes containing WRKY, AP2, and MYB domains. A semiquantitative reverse-transcription PCR analysis of genes associated with suberin biosynthesis (WRKY, CYP, and GPAT) confirmed that they were highly expressed during ROL barrier formation. Overall, these results suggest that suberin is a major constituent of the ROL barrier in roots of rice.

Submergence tolerance in Hordeum marinum: dissolved CO2 determines underwater photosynthesis and growth

Floodwaters differ markedly in dissolved CO2, yet the effects of CO2 on submergence responses of terrestrial plants have rarely been examined. The influence of dissolved CO2 on underwater photosynthesis and growth was evaluated for three accessions of the wetland plant Hordeum marinum Huds. All three accessions tolerated complete submergence, but only when in CO2 enriched floodwater. Plants submerged for 7 days in water at air equilibrium (18 m MC O 2) suffered loss of biomass, whereas those with 200 m MC O 2 continued to grow. Higher underwater net photosynthesis at 200 m MC O2 increased by 2.7- to 3.2-fold sugar concentrations in roots of submerged plants, compared with at air equilibrium CO2. Leaf gas films enhancing gas exchange with floodwater, lack of a shoot elongation response conserving tissue sugars and high tissue porosity (24-31% in roots) facilitating internal O2 movement, would all contribute to submergence tolerance in H. marinum. The present study demonstrates that dissolved CO2 levels can determine submergence tolerance of terrestrial plants. So, submergence experiments should be conducted with defined CO2 concentrations and enrichment might be needed to simulate natural environments and, thus, provide relevant plant responses.

Relay sowing of lentil ( Lens culinaris subsp. culinaris) to intensify rice-based cropping

SUMMARY The cropping systems of the Eastern Gangetic Plains of Bangladesh, India and Nepal are based on rice. There is a scope to intensify such systems through diversification with lentil, the most popular food legume. Two strategies were evaluated to fit lentil into the short fallow between successive monsoonal (i.e., T. aman) and pre-monsoonal (aus) or irrigated rice (boro) crop. These were early-flowering sole-cropped lentil and relay-sown lentil into rice. Firstly, 18 early-flowering lentil lines at three contrasting sowing dates were tested over two seasons on a research station at Ishurdi in Bangladesh. Secondly, relay sowing was evaluated at the same location with six early-flowering lines and two control cultivars in two seasons. It was also assessed on ten farms in Western Bangladesh, comparing relay with sole cropping over 3 years. Flowering in the early-flowering lentil lines was consistently 9–17 days earlier, than the control cultivars, but they did not achieve an economic yield (<1·0 t/ha). Relay sowing with an existing cultivar produced an economic yield of lentil, which was comparable or higher than sole-cropped lentil in all situations. The relay-sown lentil matured in sufficient time to allow the land to be prepared for the succeeding rain-fed rice crop. It was concluded that the substitution of relay-sown lentil for fallow in the monsoonal rice–fallow–rain-fed rice cropping pattern is a useful option to intensify and diversify cropping in the Eastern Gangetic Plain.

Tolerance of three grain legume species to transient waterlogging

Crop production can be limited by soil waterlogging. Tolerance to waterlogging can vary between and within species. This study quantified tolerance to soil waterlogging in two divergent genotypes of pea (Pisum sativum), two of lentil (Lens culinaris) and a grasspea (Lathyrus sativus) control at germination and during vegetative growth. Soil waterlogging at 10 mm depth had no significant effect on shoot and root dry mass after 14 days. Significant genetic variation in both pea and lentil in tolerance to waterlogging after germination and subsequent recovery was evident. Screening of additional pea and lentil germplasm for waterlogging conditions is clearly warranted.

A major locus involved in the formation of the radial oxygen loss barrier in adventitious roots of teosinte Zea nicaraguensis is located on the short-arm of chromosome 3

A radial oxygen loss (ROL) barrier in roots of waterlogging-tolerant plants promotes oxygen movement via aerenchyma to the root tip, and impedes soil phytotoxin entry. The molecular mechanism and genetic regulation of ROL barrier formation are largely unknown. Zea nicaraguensis, a waterlogging-tolerant wild relative of maize (Zea mays ssp. mays), forms a tight ROL barrier in its roots when waterlogged. We used Z. nicaraguensis chromosome segment introgression lines (ILs) in maize (inbred line Mi29) to elucidate the chromosomal region involved in regulating root ROL barrier formation. A segment of the short-arm of chromosome 3 of Z. nicaraguensis conferred ROL barrier formation in the genetic background of maize. This chromosome segment also decreased apoplastic solute permeability across the hypodermis/exodermis. However, the IL and maize were similar for suberin staining in the hypodermis/exodermis at 40 mm and further behind the root tip. Z. nicaraguensis contained suberin in the hypodermis/exodermis at 20 mm and lignin at the epidermis. The IL with ROL barrier, however, did not contain lignin in the epidermis. Discovery of the Z. nicaraguensis chromosomal region responsible for root ROL barrier formation has improved knowledge of this trait and is an important step towards improvement of waterlogging tolerance in maize.

Rice leaf hydrophobicity and gas films are conferred by a wax synthesis gene (LGF1) and contribute to flood tolerance

Floods impede gas (O2 and CO2 ) exchange between plants and the environment. A mechanism to enhance plant gas exchange under water comprises gas films on hydrophobic leaves, but the genetic regulation of this mechanism is unknown. We used a rice mutant (dripping wet leaf 7, drp7) which does not retain gas films on leaves, and its wild-type (Kinmaze), in gene discovery for this trait. Gene complementation was tested in transgenic lines. Functional properties of leaves as related to gas film retention and underwater photosynthesis were evaluated. Leaf Gas Film 1 (LGF1) was identified as the gene determining leaf gas films. LGF1 regulates C30 primary alcohol synthesis, which is necessary for abundant epicuticular wax platelets, leaf hydrophobicity and gas films on submerged leaves. This trait enhanced underwater photosynthesis 8.2-fold and contributes to submergence tolerance. Gene function was verified by a complementation test of LGF1 expressed in the drp7 mutant background, which restored C30 primary alcohol synthesis, wax platelet abundance, leaf hydrophobicity, gas film retention, and underwater photosynthesis. The discovery of LGF1 provides an opportunity to better understand variation amongst rice genotypes for gas film retention ability and to target various alleles in breeding for improved submergence tolerance for yield stability in flood-prone areas.

Differential nitrogen supply causes large variability in photosynthetic traits in wheat germplasm

Abstract. Increased food production and enhanced sustainability depend on improving nitrogen-use efficiency (NUE) of crops. Breeding for enhanced NUE can take advantage of doubled-haploid populations derived from parents differing in the trait. This study evaluated variation in photosynthetic parameters at various growth stages in 43 wheat genotypes (parents of the existing doubled-haploid mapping populations) under optimal and low (one-quarter of the optimal) N supply. For relative chlorophyll content, the genotype × N treatment interaction was significant at tillering, booting, pre-anthesis and anthesis. Genotypes with small differences in relative chlorophyll content between the two N supplies were CD87 at tillering and pre-anthesis, and Batavia at anthesis. Potential photochemical activity (Fv/Fm) was measured at tillering and anthesis. The genotype × N treatment interaction was significant in both growth stages. Based on net photosynthesis, stomatal conductance and intrinsic water usez efficiency, there was variable potential of the genotypes to cope with low N supply; significant differences were found among genotypes at ambient CO2 and between N treatments at elevated CO2 concentration (2000 µmol mol–1) for all three parameters. Based on all studied parameters, a dissimilarity matrix was constructed, separating the 43 genotypes into four groups. Group 2 comprised 15 of the genotypes (Batavia, Beaver, Calingiri, CD87, Frame, Krichauff, Neepawa, Soissons, Spear, Stiletto, WAWHT2036, WAWHT2074, Westonia, Wilgoyne, Yitpi), characterised by small differences in relative chlorophyll content and Fv/Fm caused by different N supply at tillering and anthesis. These genotypes therefore appear to have relative tolerance to low N supply and a potential to be used in discerning the molecular basis of tolerance to low N supply.