Viswanathan Mahalakshmi

Opportunities for marker-assisted selection (MAS) to improve the feed quality of crop residues in pearl millet and sorghum

Cereal crop residues (straw, chaff, etc.) are important components of maintenance rations for ruminant livestock in many parts of the world. They are especially important in small-holder crop-livestock production systems in the sub-humid, semi-arid, and arid tropics and subtropics where most of the world’s poorest livestock producers and consumers are found. Taking as examples tropically adapted cereals in the crop improvement mandate of the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), namely pearl millet [Pennisetum glaucum (L.) R. Br.] and sorghum [Sorghum bicolor (L.) Moench], this paper explores opportunities for using marker-assisted crop breeding methods to improve the quality and quantity of cereal crop residues for use as ruminant livestock feedstuffs. In the case of pearl millet, ICRISAT has been heavily involved with several UK-based collaborating research institutes, in development and initial application of the molecular genetic tools for marker-assisted breeding. We have obtained some useful experience in quantitative trait loci (QTL) mapping and marker-assisted selection (MAS) for stover yield, foliar disease resistance, and in vitro estimates of the nutritive value of various stover fractions for ruminants. In sorghum, ICRISAT has focused on initiating a large-scale high-throughput marker-assisted backcrossing program for the stay-green component of terminal drought tolerance—a trait that is likely to be associated not only with more stable grain and stover yield, but which is also expected to contribute to maintenance of ruminant nutritional value of stover produced under drought stress conditions. Conventional and marker-assisted breeding for foliar disease resistance is recommended for dual-purpose cereal improvement, or indeed for improvement of the nutritional value of residues for any crop in which these are used as feedstuffs for ruminant livestock. Practical problems faced and proposed ways of dealing with these are discussed.

Effect of timing of water deficit on pearl millet (Pennisetum americanum)

The susceptibility of pearl millet to soil water deficits at different times of growth was determined in field experiments conducted over 2 years. Grain yields and yield components were expressed as ratios of stressed to non-stressed treatments. Where water deficit was imposed earlier in crop growth and then relieved, the time of termination of stress from flowering determined the extent of grain yield loss. Water stress relieved at anthesis or early grain filling had little adverse effect on the grain yield because additional tillers produced panicles. This response by tillers was less evident and grain yields were more severely reduced when water stress was not relieved until after flowering. When stress was imposed late in crop growth and not relieved, the time of initiation of stress was directly related to the extent of loss in grain yield. Thus susceptibility to mid-season drought stress was related to the time stress was terminated and in late-season drought to the time stress was initiated.

Plant genomics and agriculture: From model organisms to crops, the role of data mining for gene discovery

Sequencing and analysing of the Arabidopsis thaliana genome, the first plant kingdom genome to be unraveled, will always remain a scientific landmark. International initiatives to sequence rice, the most important cereal in Asia, are underway. However as functional information piles up in Arabidopsis and rice, researchers working in other crops will benefit from this new knowledge and apply it to their studied plants or crop species. The increasing role of public databases of model organisms and bio-informatics in data mining, presents a new opportunity as well as a challenge to researchers to develop more focused molecular tools for gene discovery and deployment. The work presented in here describes how such an approach has benefited sorghum, a rainfed semi-arid troprical cereal.

Genomic sequence derived simple sequence repeats markers. A case study with Medicago spp

Simple sequence repeats (SSR) or micro-satellites are becoming standard DNA markers for plant genome analysis and are being used as markers in marker assisted breeding. De novo generation of micro-satellite markers through laboratory-based screening of SSR-enriched genomic libraries is highly time consuming and expensive. An alternative is to screen the public databases of related model species where abundant sequence data is already available. All the genomic sequences of Medicago from the public domain database were searched and analysed of di, tri, and tetra nucleotide repeats. Of the total of about 156,000 sequences which were searched, 7325 sequences were found to contain repeat motif and may yield SSR which will yield product sizes of around 200 bp. Of these the most abundantly found repeats were the tri-nucleotide (5210) group. Except for a very small proportion (436), these link to the gene annotation database at TIGR ( http://www.tigr.org ). To facilitate further exploration of this resource, a dynamic database with options to search and link to other resources is available at ( http://www.icrisat.org/text/research/grep/homepage/genomics/medssrs1.asp ) and on CDs from V.Mahalakshmi@cgiar.org .

Use of the senescing agent potassium iodide to simulate water deficit during flowering and grainfilling in pearl millet

Abstract Grain growth in cereal crops is largely dependent on the availability of current photosynthates. Water deficits during flowering and grainfilling limit grain yield partly by reducing the availability of current assimilates. Drought response of breeders lines is often evaluated by screening material in drought prone environments or during the rain-free season by withholding irrigation during the desired treatment periods. The former approach is time consuming due to the erratic nature of drought patterns. In the latter approach, temperatures and daylength during the dry periods often affect crop growth and limits its use. It was therefore hypothesized that the plant photosynthetic source could be progressively destroyed in a controlled manner using a chemical desiccant or senescing agent to simulate water deficit effects. The effects of spraying the senescing agent potassium iodide (KI), at different stages before and after anthesis, on grain yield and yield components were compared with water deficit during grainfilling using four millet hybrids grown under field conditions in 1989 and 1991. The senescing agent KI reduced the chlorophyll content of the leaves more severely than did the water deficit treatment. Spraying with KI reduced grain yield, grain number and grain size. Effects of KI spraying at anthesis on grain yield and yield components were similar in both years and were similar to the effects of water deficits during grainfilling in 1989. Spraying with KI at the anthesis stage for each hybrid eliminated the confounding effects of phenology often encountered in water deficit treatment. We conclude that in pearl millet, spraying a senescing agent at anthesis is effective in simulating the reduction of current photosynthesis that occurs during posts anthesis water deficit and can be used as a screening method to evaluate genetic response to water deficit during grainfilling in the normal season.

Effect of drought stress during grain filling in near-isogenic tall and dwarf hybrids of pearl millet ( Pennisetum glaucum )

The susceptibility to drought stress during flowering and grain filling of dwarf hybrids of pearl millet carrying the dwarfing gene d2 was investigated in 1987 at Patancheru, India, under field conditions during the dry season and, in the rainy season, under a rain shelter, using four pairs of near-isogenic tall and dwarf hybrids. Drought stress during grain filling reduced the number of grains per unit area and individual grain mass. Grain yields of the dwarf hybrids were lower than those of the corresponding tall hybrids in the unstressed control and under drought stress and were associated with a lower individual grain mass in the dwarf lines. In the dwarf hybrids, harvest index was similar to or better than that of the tall versions but a reduced biomass resulted in lower grain yields. Dwarf hybrids were not more adversely affected by water stress, however, than their tall counterparts, indicating that susceptibility to drought stress would not be likely to limit acceptance of new dwarf varieties.