Data are presented which suggest the existence of a light-harvesting pigment-protein complex which is functionally and structurally associated with photosystem I (PSI) reaction centers. These observations are based on techniques which aflow isolation of PSI using minimal concentrations of Triton X-100. Properties of density and self aggregation allowed purification of a "native" PSI complex. The isolated PSI particles appear as 106 A spherical subunits when viewed by freeze fracture microscopy. When incorporated into phosphatidyl choline vesicles, the particles lose self-aggregation properties and disperse uniformly within the lipid membrane. The isolated PSI preparation contains 110 ± 10 chlorophylls/P700 (Chi a/b ratio greater than 18); this represents a recovery of 27% of the original chloroplast membrane Chi. These particles were enriched in Chi a forms absorbing at 701 to 710 nm. Chi fluorescence at room temperature exhibited a maximum at 690 nm with a pronounced shoulder at 710 nm. At 77 K, peak fluorescence emission was at 736 am; in the presence of dithionite an additional fluorescence maximum at 695 nm was obtained at 77 K. This dual fluorescence emission peak for the PSI particles is evidence for at least two Chl populations within the PSI membrane subunit. The fluorescence emission observed at 695 nm was identified as arising from the core of PSI which contains 40 Chl/P700 (PSI40). This core complex, derived from native PSI particles, was enriched in Chi a absorbing at 680 and 690 nm and fluorescing with maximal emission at 694 am at 77 K. PSI particles consisting of the PSI core complex plus 20 to 25 Chi antennae (65 Chl/P700) could also be derived from native PSI complexes. These preparations were enriched in Chl a forms absorbing at 697 nm and exhibited a 77 K fluorescence emission maximum at 722 am. A comparison of native PSI particles which contain 110 Chl/P700 (PSI-110) and PSI particles containing 65 Chl/P700 (PSI-65) provides evidence for the existence of a peripheral Chi-protein complex tightly associated in the native PSI complex. The native PSI subunits contain polypeptides of 22,500 to 24,500 daltons which are not found in the PSI-65 or PSI-40 subfractions. It is suggested that these polypeptides function to bind 40 to 45 Chi per structural complex, including the Chi which emits fluorescence at 736 am. A model for the organization of Chli forms is presented in which the native PSI membrane subunit consists of a reaction center core complex plus two regions of associated light-harvesting antennae. The presence of energy "sinks" within the antennae is discussed.

Transgenic tobacco plants that express a chi- The inhibition of photosynthesis that can occur when excess excitation energy reaches the reaction center is commonly referred to as photoinhibition. High light intensity, especially at extreme temperatures or water deficit, can cause increased electron flow to 02, resulting in greater production ofO2 and H202. Although oxygen radicals appear to be involved in photoinhibition (9)(10)(11), the role of SOD in limiting the oxidative damage associated with photoinhibition has not been directly demonstrated (12, 13).To investigate the possible protective functions of SOD in plant chloroplasts, we have developed transgenic tobacco plants that overexpress chloroplast-localized Cu/Zn SOD. These plants were analyzed for photosynthetic rate when exposed to light and temperature conditions that inhibit photosynthesis and for their ability to recover photosynthetic capacity after stress. Our results indicate that these transgenic plants have improved photosynthetic function at chilling temperatures and moderate light intensity, and they recover more effectively from severe stress than control plants. These changes correlate with increased resistance to oxidative damage caused by the herbicide methyl viologen (MV).

High-throughput genotyping arrays provide a standardized resource for plant breeding communities that are useful for a breadth of applications including high-density genetic mapping, genome-wide association studies (GWAS), genomic selection (GS), complex trait dissection, and studying patterns of genomic diversity among cultivars and wild accessions. We have developed the CottonSNP63K, an Illumina Infinium array containing assays for 45,104 putative intraspecific single nucleotide polymorphism (SNP) markers for use within the cultivated cotton species Gossypium hirsutum L. and 17,954 putative interspecific SNP markers for use with crosses of other cotton species with G. hirsutum. The SNPs on the array were developed from 13 different discovery sets that represent a diverse range of G. hirsutum germplasm and five other species: G. barbadense L., G. tomentosum Nuttal × Seemann, G. mustelinum Miers × Watt, G. armourianum Kearny, and G. longicalyx J.B. Hutchinson and Lee. The array was validated with 1,156 samples to generate cluster positions to facilitate automated analysis of 38,822 polymorphic markers. Two high-density genetic maps containing a total of 22,829 SNPs were generated for two F2 mapping populations, one intraspecific and one interspecific, and 3,533 SNP markers were co-occurring in both maps. The produced intraspecific genetic map is the first saturated map that associates into 26 linkage groups corresponding to the number of cotton chromosomes for a cross between two G. hirsutum lines. The linkage maps were shown to have high levels of collinearity to the JGI G. raimondii Ulbrich reference genome sequence. The CottonSNP63K array, cluster file and associated marker sequences constitute a major new resource for the global cotton research community.

Background: Sorghum [Sorghum bicolor (L.) Moench] is ranked as the fifth most important grain crop and serves as a major food staple and fodder resource for much of the world, especially in arid and semi-arid regions. The recent surge in sorghum research is driven by its tolerance to drought/heat stresses and its strong potential as a bioenergy feedstock. Completion of the sorghum genome sequence has opened new avenues for sorghum functional genomics. However, the availability of genetic resources, specifically mutant lines, is limited. Chemical mutagenesis of sorghum germplasm, followed by screening for mutants altered in important agronomic traits, represents a rapid and effective means of addressing this limitation. Induced mutations in novel genes of interest can be efficiently assessed using the technique known as Targeting Induced Local Lesion IN Genomes (TILLING).

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