Root knot and cyst nematodes induce large multinucleated cells, designated giant cells and syncytia, respectively, in plant roots. We have used molecular markers to study cell cycle progression in these specialized feeding cells. In situ hybridization with two cyclin-dependent kinases and two cyclins showed that these genes were induced very early in galls and syncytia and that the feeding cells progressed through the G2 phase. By using cell cycle blockers, DNA synthesis and progression through the G2 phase, or mitosis, were shown to be essential for gall and syncytium establishment. When mitosis was blocked, further gall development was arrested. This result demonstrates that cycles of endoreduplication or other methods of DNA amplification are insufficient to drive giant cell expansion. On the other hand, syncytium development was much less affected by a mitotic block; however, syncytium expansion was inhibited.

Molecular markers and cell cycle inhibitors show the importance of cell cycle progression in nematode-induced galls and syncytia.

Sedentary nematodes are important pests of crop plants. They are biotrophic parasites that can induce the (re)differentiation of either differentiated or undifferentiated plant cells into specialized feeding cells. This (re)differentiation includes the reactivation of the cell cycle in specific plant cells finally resulting in a transfer cell-like feeding site. For growth and development the nematodes fully depend on these cells. The mechanisms underlying the ability of these nematodes to manipulate a plant for its own benefit are unknown. Nematode secretions are thought to play a key role both in plant penetration and feeding cell induction. Research on plant-nematode interactions is hampered by the minute size of cyst and root knot nematodes, their obligatory biotrophic nature and their relatively long life cycle. Recently, insights into cell cycle control in Arabidopsis thaliana in combination with reporter gene technologies showed the differential activation of cell cycle gene promoters upon infection with cyst or root knot nematodes. In this review, we integrate the current views of plant cell fate manipulation by these sedentary nematodes and made an inventory of possible links between cell cycle activation and local, nematode-induced changes in auxin levels.

Cell cycle activation by plant parasitic nematodes.

In bread wheat, QTL interval mapping was conducted for nine important drought responsive agronomic traits. For this purpose, a doubled haploid (DH) mapping population derived from Kukri/Excalibur was grown over three years at four separate locations in India, both under irrigated and rain-fed environments. Single locus analysis using composite interval mapping (CIM) allowed detection of 98 QTL, which included 66 QTL for nine individual agronomic traits and 32 QTL, which affected drought sensitivity index (DSI) for the same nine traits. Two-locus analysis allowed detection of 19 main effect QTL (M-QTL) for four traits (days to anthesis, days to maturity, grain filling duration and thousand grain weight) and 19 pairs of epistatic QTL (E-QTL) for two traits (days to anthesis and thousand grain weight). Eight QTL were common in single locus analysis and two locus analysis. These QTL (identified both in single- and two-locus analysis) were distributed on 20 different chromosomes (except 4D). Important genomic regions on chromosomes 5A and 7A were also identified (5A carried QTL for seven traits and 7A carried QTL for six traits). Marker-assisted recurrent selection (MARS) involving pyramiding of important QTL reported in the present study, together with important QTL reported earlier, may be used for improvement of drought tolerance in wheat. In future, more closely linked markers for the QTL reported here may be developed through fine mapping, and the candidate genes may be identified and used for developing a better understanding of the genetic basis of drought tolerance in wheat.

/pdf/qtl-mapping-for-nine-drought-responsive-agronomic-traits-in-14y7al9uod.pdf

QTL mapping for nine drought-responsive agronomic traits in bread wheat under irrigated and rain-fed environments

Trade-off between grain weight and grain number in wheat depends on GxE interaction: A case study of an elite CIMMYT panel (CIMCOG)

Phenotyping, Genetic Dissection, and Breeding for Drought and Heat Tolerance in Common Wheat: Status and Prospects

To understand the genetic control of grain filling duration (GFD), i.e., the number of days from anthesis to physiological maturity, we studied the F1, F2, BC1 and BC2 generations of six spring wheat crosses from nine varieties/genotypes. Generation mean analysis for gene effects indicated that one or more types of epistasis were significant in all crosses. In each pairing, the F1 and F2 means were either intermediate or closer to the mean of the parent having the longer GFD. Our narrow-sense heritability estimate was reasonably high, at 47.67 (based on diallel analysis). This demonstrated that progress could be made from the selection in these crosses for either long or short GFD. The two early varieties that had identical maturity durations differed in their GFD values, indicating that maturity dates are not good criteria when choosing parents for modifying GFD. To utilize favorable additive × additive effects during this selection, we suggest that a single seed descent (SSD) or bulk popula-tion approach be adopted. In comparison, dominance effects would prove quite useful in hybrid wheat breeding programs.

Inheritance of grain filling duration in spring wheat (Triticum aestivum L. em thell)

Interphase nuclear organization was studied in six species of Cicer and all the species showed chromocentric nuclear organization in both meristematic and differentiated cells instead of reticulate organization. The number of chromocentres and treatment duration with acid or alkali were found to be species specific character. Percentage heterochromatin values determined by two different techniques were somewhat high in meristematic cells than those in differentiated cells. On the basis of heterochromatin values both in meristematic and differentiated cells C. reticulaturn can be considered as primitive of the six Cicer species. Nuclear organization was found to be governed by small size of chromosomes and low DNA content, but the relationship between heterochromatin values and DNA content was not clear.

https://www.jstage.jst.go.jp/article/cytologia1929/54/1/54_1_27/_pdf

Interphase nuclear structure and heterochromatin in Cicer species

Studies on the genetics of leaf blight caused byAlternaria triticina using generation mean analysis revealed that additive components played a major role, but that dominance components also contributed significantly in controlling the variability for leaf blight resistance in wheat crosses. Furthermore, the additive x additive type of epistasis was predominant in the first three crosses, whereas in the fourth cross additive x dominance (j) and dominance x dominance (1) components of epistasis were most significant. Because of this it may be desirable to follow a simple recurrent selection scheme for higher tolerance, to isolate resistant plants from the segregating populations derived from crosses of parents of diverse origin following the pedigree method of breeding. CPAN-1887 was very tolerant to leaf blight in the present study and should be utilized in hybridization programs to develop leaf-blight-resistant varieties.

Genetics of leaf blight resistance in wheat.

Transmission rates of extra chromosomes found in the full set of trisomics of pearl millet (Pennisetum americanum) (2n = 14) were estimated by examining the progeny of selfed trisomics and the progeny of trisomics crossed to disomics. When the trisomics were selfed, ‘dark green’ and ‘tiny’ had the highest transmission rate (23.8% and 23.3%, respectively) and ‘pseudonormal’ the lowest (13.8%). Other trisomics had an intermediate rate of transmission. When the trisomics were used as females in crosses with disomics, both ‘dark green’ and ‘tiny’ again had the highest transmission rate and ‘pseudonormal’ the lowest. When the trisomics were used as males in crosses to disomics, no trisomic was transmitted to the progeny except for ‘spindle’, and this occurred with a very low frequency (2.0%). A variation in transmission rate was observed from plant to plant and season to season for the same trisomic type. A study of the transmission rate of the extra chromosomes indicated that the following factors were probably contributing to the lower rate of transmission: small- or light-weight seeds tended to have a higher proportion of trisomics than heavier seeds; lighter seeds had a lower percentage germination; a positive and significant correlation was noticed between trivalent frequency and transmission rate. Plants with reduced vigour produced a higher frequency of trisomics. Though trisomics involving longer extra chromosomes showed a high degree of pollen and ovule sterility, they were highly transmissible. This has resulted in a close relationship between gametic sterility and transmission rate of extra chromosome.

Transmission of primary trisomics in pearl millet.

Nineteen tertiary trisomics were isolated from some translocation heterozygotes and interchange trisomics of pearl millet. Cytological analysis of these trisomics indicates that chromosome association of trivalents, univalents and pentavalents were frequent in all the trisomics. But their ratio varied from one trisomic to the next. Other associations were relatively infrequent. The relative frequencies of 6 pentavalent configurations observed in different trisomics were studied and their probable association with mode of fertility and transmission rates have been discussed. Furthermore, the techniques for isolation of balanced tertiary trisomics and their further use in production of hybrid seeds in pearl millet have been discussed.

/pdf/tertiary-trisomics-and-their-use-in-pearl-millet-improvement-4ww52wiqdw.pdf

R. M. Singh

Papers

Inheritance of grain filling duration in spring wheat (Triticum aestivum L. em thell)

Interphase nuclear structure and heterochromatin in Cicer species

Genetics of leaf blight resistance in wheat.

Transmission of primary trisomics in pearl millet.

Tertiary trisomics and their use in pearl millet improvement.