Mueen Alam Khan

Breeding approaches for bacterial leaf blight resistance in rice (Oryza sativa L.), current status and future directions

Bacterial leaf blight (BLB) is one of the most serious threats to the rice crop in irrigated and rainfed areas of the world. It is caused by Xanthomonas oryzae pv. oryzae and has been known for more than a century. Through rigorous screening and selection, a number of resistant cultivars have been produced and utilized, but resistance was overcome by the development of mutant strains of pathogen and by the dynamic change in Xoo populations. About 38 resistance genes have been reported in rice against the disease and a few have been cloned. The pyramiding of several resistance genes through marker assisted selection has been a quite effective strategy for combating the disease. However, new powerful tools such as transgenics have been introduced to make a significant impact. The purpose of this mini-review is to consolidate the existing knowledge about bacterial leaf blight in rice and the progress made both in conventional as well as in molecular dimensions of breeding together with potential findings and constraints.

Paclobutrazol Soil Drenching Suppresses Vegetative Growth, Reduces Malformation, and Increases Production in Mango

Emergence of unproductive vegetative shoots/flushes is considered to be a main cause of low yield, irregular bearing, and malformation of inflorescences in mango. Selected mango trees of the cultivars ‘Chaunsa’, ‘Dushehare’, and ‘Anwar Ratool’ growing in the subtropical region of Khanewal (30°18′0N, 71°56′0E), a district of Pakistan, were characterized as having excessive vegetative growth, erratic flowering, and fruiting with declining productivity due to malformation of inflorescences. Paclobutrazol soil drenching was evaluated as a method to suppress excessive vegetative growth and to increase the number of reproductive shoots even during the ‘off’ season. Different rates of paclobutrazol was applied at rates of (0, 2, 4, 6, 8, 10, and 12 g a.i.) in September through soil drenching. Results showed that soil drenching with paclobutrazol at the highest rates (12, 10, and 8 g a.i.) was significantly effective in suppressing vegetative growth, reducing the increase in canopy volume, and flush length as compared to control trees in all treated cultivars of mango. Statistically significant differences were recorded in treated trees as compared to control regarding the emergence of reproductive shoots, fruit setting, panicle length, fruit drop, intensity of emergence of malformed panicles, and yield. Response of selected cultivars of mango to paclobutrazol was statistically different regarding various vegetative and reproductive growth characteristics. Paclobutrazol was more useful in ‘Seasonal Chaunsa’ as compared to ‘Dushehare’ and ‘Anwar Ratool’ for improving various vegetative and reproductive parameters studied in this research.

Nitrogen application improves gas exchange characteristics and chlorophyll fluorescence in maize hybrids under salinity conditions

The understanding of crop physiological responses to salinity stress is of paramount importance for selection of genotypes with improved tolerance to this stress. Maize (Zea mays L.) hybrids Pioneer 32B33 and Dekalb 979 were grown in pots and subjected to three levels of salinity under four nitrogen levels to determine the role of nitrogen under saline conditions. Salinity stress effects on gas exchange characteristics and chlorophyll fluorescence of maize hybrids were evaluated under semi-controlled conditions. Under salinity stress, the changes in the net photosynthetic rate (PN), stomatal conductance (gs), and transpiration rate (E) were similarly directed: all decreased and were lower than in control at the higher salinity level (10 dS/m). Water use efficiency was increased with increasing salinity since transpiration was stronger depressed by salt than photosynthesis. Plants subjected to the lower level of salinity did not differ from control in tested characteristics. Nitrogen application ameliorated the effects of salinity.

Rice molecular markers and genetic mapping: Current status and prospects

Abstract Dramatic changes in climatic conditions that supplement the biotic and abiotic stresses pose severe threat to the sustainable rice production and have made it a difficult task for rice molecular breeders to enhance production and productivity under these stress factors. The main focus of rice molecular breeders is to understand the fundamentals of molecular pathways involved in complex agronomic traits to increase the yield. The availability of complete rice genome sequence and recent improvements in rice genomics research has made it possible to detect and map accurately a large number of genes by using linkage to DNA markers. Linkage mapping is an effective approach to identify the genetic markers which are co-segregating with target traits within the family. The ideas of genetic diversity, quantitative trait locus (QTL) mapping, and marker-assisted selection (MAS) are evolving into more efficient concepts of linkage disequilibrium (LD) also called association mapping and genomic selection (GS), respectively. The use of cost-effective DNA markers derived from the fine mapped position of the genes for important agronomic traits will provide opportunities for breeders to develop high-yielding, stress-resistant, and better quality rice cultivars. Here we focus on the progress of molecular marker technologies, their application in genetic mapping and evolution of association mapping techniques in rice.

Molecular breeding of rice for improved disease resistance, a review

Diseases are considered to be the major limiting factors in rice production around the globe. Considering rice as an important cereal crop, developing disease resistant cultivars is a prime objective of breeders. Compared to conventional breeding, molecular breeding especially using marker assisted selection appears to be more effective and precise. The best management of disease is to bring durable wide spectrum resistance in rice cultivars. This can be accomplished by accumulating both qualitative and quantitative resistance genes in to rice cultivars. Introducing these resistance genes from the wild relatives of rice in to commercial cultivars has greatly helped the breeders to accomplish this task. Marker assisted selection is extremely valuable in resolving the issues the breeders face with traditional breeding. A number of transgenic rice lines harboring the so called ‘foreign’ resistant genes have been produced. However such promising lines must be extensively replicated in fields to evaluate stable integration and continued expression of genes. None the less identification of disease resistance genes and quantitative trait loci (QTLs), use of marker assisted selection along with gene pyramiding and transgenic approaches all provide breeders a hope to build high yielding disease resistant cultivars. Based on such premises, we can truly envision broadening of our understanding the genetic and molecular basis of disease resistance in rice.