Drought and low soil N cause significant yield reductions in maize (Zea mays L.) grown in the tropics. Understanding the genetic basis of hybrid performance under these stresses is crucial to designing appropriate breeding strategies. This study evaluates under optimal, drought and low N stress conditions (i) the performance, combining abilities and stability of a group of tropical white inbred lines; (ii) the genetic control and modes of gene action for grain yield; and (iii) the relationship between line per se and hybrid performance. Seventeen lowland white‐grained tropical maize inbred lines were used in a diallel study. Lines and their hybrids were evaluated separately in trials under drought stress, low N, and optimal conditions in a total of 12 environments. The differences in grain yield between hybrids and inbreds (i.e., heterosis) increased with the intensity of drought stress. Significant interactions were observed for combining abilities under low and high N. The type of gene action appeared to be different under drought than under low N, with additive effects more important under drought and dominance effects more important under low N. The importance of additive effects increased with intensity of drought stress. This suggests the need for drought tolerance in both parental lines to achieve acceptable hybrid performance under severe drought. Inbreds derived from the population ‘La Posta Sequía’ exhibited the highest GCA effects, stability coefficients, and frequency of dominant alleles for grain yield. Good performance across stress levels can be achieved in tropical maize hybrids.

The objectives of this study were to evaluate direct and correlated responses to recurrent selection for drought tolerance in two CIMMYT maize (Zea mays L.) source germplasm populations, 'DTP1' and 'DTP2', adapted to the lowland and mid-altitude tropics. Selection was primarily based on grain yield, ears per plant, anthesis-silking interval, and leaf senescence under drought. Cycles C 0 , C 3 , and C 6 of DTP1 and C 0 , C 3 , C 5 and C 9 of DTP2 were evaluated under drought, low N, and optimal conditions. In both populations, significant yield gains were observed under drought conditions, associated with a significant increase in numbers of ears per plant and grains per ear, and significant reductions in anthesis-silking interval, ovule number and abortion rate during grain filling. Abortion rate was positively correlated with the number of ovules at silking and with anthesis-silking interval. In DTP1, recurrent selection under drought was associated with a derease of tassel and stem dry weight and with an increase of ear dry weight and harvest index. This study confirms the effectiveness of recurrent selection under drought as a means of improving tropical maize source populations for performance under water deficits and to a lesser extent under low N. The primary mechanism underlying these changes appears to be improved partitioning of assimilates to the ear at flowering, at the expense of tassel and stem growth. P.

The entire genome of tobacco mosaic virus (TMV) was copied into a series of subgenomic cDNA clones. cDNA sequences of the 5' and 3' ends of TMV were cloned separately. A synthetic oligonucleotide primer was used to generate a Pst I site at the 5' terminus, whereas a different primer was used to generate an Nde I site at the 3' terminus.This strategy permitted removal of non-TMV sequences from doned cDNA inserts by treatment with exonuclease VII following restriction endonuclease cleavage. Pst I linkers were added to TMV 3' terminal cDNAs. Subgenomic cDNA fragments were ligated together into several independent fullgenomic constructions from which TMV cDNA sequences could be cleanly excised as a single fragment by Pst I digestion.Full-genomic TMV cDNA was ligated immediately downstream from the X phage promoter from pPM1 and transcribed in vitro with Escherichia coli RNA polymerase. RNA transcripts from three of four full-genomic cDNA constructions were infectious, even though they contained 6 non-TMV nucleotides at the 3' end. Transcripts from a construction with 6 extra nucleotides at the 5' end also were infectious. Progeny virus from plants infected with cDNA transcripts appeared identical to the parental vinrus. Restriction maps of independent cDNA clones of the same regions of the genome were identical to each other and as predicted from the reported nucleotide sequence of TMV. Also, sequences of the 200 nucleotides proximal to the 5' termini of four independent cDNA clones were identical to each other and to published sequences, suggesting that independent isolates of TMV may have remarkably similar sequences.Tobacco mosaic virus (TMV) is a virus with worldwide distribution that infects -200 plant species (1). Numerous strains of TMV cause serious losses in tobacco, tomato, and other crop plants. The virion is a rod-shaped particle 18 x 300 nm. The genome of TMV resides in a single strand of messenger-sense RNA encoding at least four proteins, two of which are translated from the same reading frame and two from processed mRNA fragments.Historically, TMV has contributed to the understanding of the genetics of RNA viruses, including the first studies on biological variability, mutability and protein sequence variation of viral genomes (2-4). TMV continues to offer numerous advantages as a genetic system. Variants of this virus express a large number of easily scored phenotypic characters useful in studying specific viral functions. Isolates of TMV that vary in symptomology, host range, elicitation of host defense mechanisms, cross-protection, encapsidation, cell-to-cell and long-distance movement within the host, and several functions of RNA replication have been described. TMV strains infecting tobacco and tomato have been most intensely investigated. Of these, two viral strains and a variant have been fully sequenced (5-7). However, in spite of our understanding of the general organization and replication of this virus, only a few viral functions have been assigned to specific sequences. The inabil...

Estimation of genetic diversity and distance among tropical maize (Zea mays L.) lines and the correlation between genetic distance (GD) and hybrid performance would determine breeding strategies, classify inbred lines, define heterotic groups, and predict future hybrid performance. The objectives of this study were to estimate (i) heterosis and specific combining ability (SCA) for grain yield under stress and non‐stress environments; (ii) genetic diversity for restriction fragment length polymorphisms (RFLPs) within a set of tropical lines; (iii) GD and classify the lines according to their GD; and (iv) correlation between the GD and hybrid performance, heterosis, and SCA. Seventeen lowland, white tropical inbred lines were represented in a diallel study. Inbred lines and hybrids were evaluated in 12 stress and nonstress environments. The expression of heterosis was greater under drought stress and smaller under low N environments than under nonstress environments. A set of DNA markers identifying 81 loci was used to fingerprint the 17 lines. The level of genetic diversity was high, with 4.65 alleles/locus and polymorphism information content (PIC) values ranging from 0.11 to 0.82. Genomic regions with quantitative trait loci (QTL) for drought tolerance previously identified showed lower genetic diversity. Genetic distance based on RFLP marker data classified the inbred lines in accordance with their pedigree. Positive correlation was found between GD and F1 performance (F1), SCA, midparent heterosis (MPH) and high‐parent heterosis (HPH). Specific combining ability had the strongest correlation with GD. Environment significantly affected the correlations between F1, SCA, MPH, and HPH, with lower values of GD revealed in the more stressed conditions.

Heme proteins play pivotal roles in a wealth of biological processes. Despite this, the molecular mechanisms by which heme traverses bilayer membranes for use in biosynthetic reactions are unknown. The biosynthesis of c-type cytochromes requires that heme is transported to the bacterial periplasm or mitochondrial intermembrane space where it is covalently ligated to two reduced cysteinyl residues of the apocytochrome. Results herein suggest that a family of integral membrane proteins in prokaryotes, protozoans, and plants act as transmembrane heme delivery systems for the biogenesis of c-type cytochromes. The complete topology of a representative from each of the three subfamilies was experimentally determined. Key histidinyl residues and a conserved tryptophan-rich region (designated the WWD domain) are positioned at the site of cytochrome c assembly for all three subfamilies. These histidinyl residues were shown to be essential for function in one of the subfamilies, an ABC transporter encoded by helABCD. We believe that a directed heme delivery pathway is vital for the synthesis of cytochromes c, whereby heme iron is protected from oxidation via ligation to histidinyl residues within the delivery proteins.

The triple gene block (TGB; consisting of proteins TGB1–3) and coat protein (CP) of white clover mosaic potexvirus (WClMV) are required for cell-to-cell movement of viral RNA. Cell-to-cell spread of WClMV mutants in which the TGB open reading frames had been mutated was rescued in transgenic plants expressing specific TGB proteins (TGBPs). This indicated that there are no requirements for the synthesis in cis of viral TGBPs. These transgenic plants provided an experimental framework to explore the roles performed by the TGBPs and CP in cell-to-cell movement of WClMV RNA. Microinjection experiments established that TGB1 functions as the WClMV cell-to-cell movement protein (MP). Furthermore, combined microinjection and dual-channel confocal laser scanning microscopy provided direct evidence that infectious transcripts of WClMV move cell to cell as a ribonucleoprotein complex, consisting of single-stranded RNA, TGB1, and CP. Movement of this ribonucleoprotein complex displayed an absolute requirement for the presence of both TGB2 and TGB3. A model consistent with these findings is presented.

The triple gene block proteins (TGBp1-3) and coat protein (CP) of potexviruses are required for cell-to-cell movement. Separate models have been proposed for intercellular movement of two of these viruses, transport of intact virions, or a ribonucleoprotein complex (RNP) comprising genomic RNA, TGBp1, and the CP. At issue therefore, is the form(s) in which RNA transport occurs and the roles of TGBp1-3 and the CP in movement. Evidence is presented that, based on microprojectile bombardment studies, TGBp1 and the CP, but not TGBp2 or TGBp3, are co-translocated between cells with viral RNA. In addition, cell-to-cell movement and encapsidation functions of the CP were shown to be separable, and the rate-limiting factor of potexvirus movement was shown not to be virion accumulation, but rather, the presence of TGBp1-3 and the CP in the infected cell. These findings are consistent with a common mode of transport for potexviruses, involving a non-virion RNP, and show that TGBp1 is the movement protein, whereas TGBp2 and TGBp3 are either involved in intracellular transport or interact with the cellular machinery/docking sites at the plasmodesmata.

of maize-based farming systems is to be sustained or increased. Drought and low soil N cause significant yield reductions in maizeMaize population improvement for drought tolerance (Zea mays L.) grown in the tropics. Understanding the genetic basis (DT) at flowering has been accomplished in source popof hybrid performance under these stresses is crucial to designing ulations by recurrent selection using managed drought appropriate breeding strategies. This study evaluates under optimal, stress (Bolañ os and Edmeades, 1993a; Edmeades et al., drought and low N stress conditions (i) the performance, combining abilities and stability of a group of tropical white inbred lines; (ii) the 1999). Recurrent selection under drought was effective genetic control and modes of gene action for grain yield; and (iii) the at increasing yield across a range of drought stress levels relationship between line per se and hybrid performance. Seventeen in all populations under evaluation (Edmeades et al., lowland white-grained tropical maize inbred lines were used in a 1999). Gains under stressed conditions were signifidiallel study. Lines and their hybrids were evaluated separately in cantly greater than those observed in conventionally trials under drought stress, low N, and optimal conditions in a total selected counterpart populations without loss of yield of 12 environments. The differences in grain yield between hybrids potential (Byrne et al., 1995). Improvements were due and inbreds (i.e., heterosis) increased with the intensity of drought to a significant reduction in barrenness and increases stress. Significant interactions were observed for combining abilities in grain number per ear and harvest index, and were under low and high N. The type of gene action appeared to be different accompanied by a reduction in the anthesis-silking interunder drought than under low N, with additive effects more important val (ASI) and a delay in leaf senescence (Bolañ os and under drought and dominance effects more important under low N. , 1993b; Edmeades et al., 1999). Improvement Edmeades The importance of additive effects increased with intensity of drought stress. This suggests the need for drought tolerance in both parentalfor drought tolerance also brought specific adaptation lines to achieve acceptable hybrid performance under severe drought. and improved performance under low N conditions sug-Inbreds derived from the population 'La Posta Sequía' exhibited the gesting that tolerance to either stress involves common highest GCA effects, stability coefficients, and frequency of dominant adaptive mechanisms (Bä nziger et al., 1999). alleles for grain yield. Good performance across stress levels can be Edmeades et al. (1997) showed that population imachieved in tropical maize hybrids.