Interaction of Fusarium Wilt Race 4 with Root-Knot Nematode Increases Disease Severity in Cotton.

Fusarium wilt, caused by Fusarium oxysporum f. sp. vasinfectum, is a severe disease of cotton (Gossypium spp.). Strains of the wilt pathogen in the United States, such as race 1, require the presence of nematodes such as southern root-knot nematode (Meloidogyne incognita) to cause appreciable disease. The exception is the race 4 strain of the wilt pathogen, which can attack cotton without concomitant infection by plant-parasitic nematodes and was first identified in California in 2001 and in Texas and New Mexico since 2017. The effects of the interaction between M. incognita and race 1 or race 4 on wilt severity and nematode reproduction on two Gossypium hirsutum cultivars, Acala 44 and FM 966, and a G. barbadense cultivar, Pima S-4, were directly compared in growth chamber assays. All three cultivars were susceptible to M. incognita. Suppression of nematode reproduction by the wilt pathogen was detected only for race 4 on all three cultivars on a per plant basis but not on a per gram root tissue basis. The control, M. incognita alone, and race 1 alone treatments caused no symptoms. Inoculation with race 1 and M. incognita caused moderate wilt symptoms in 'Acala 44' and 'FM 966' and mild symptoms in 'Pima S-4'. However, race 4 treatment caused severe wilt in 'Pima S-4' and moderate wilt severity in 'Acala 44' and 'FM 966'. The symptom severity of 'Acala 44' and 'FM 966' further increased in the presence of M. incognita. Thus, race 4 is not only capable of causing wilt in the absence of M. incognita but can also interact with the nematode to further increase disease severity. Though control of wilt caused by race 1 can be achieved mainly through breeding for nematode resistance, it will be imperative to incorporate both southern root-knot nematode and race 4 resistance to effectively control the disease should race 4 expand into southern root-knot nematode-infested fields.

lncRNA7 and lncRNA2 modulate cell wall defense genes to regulate cotton resistance to Verticillium wilt.

In plants, long noncoding RNAs (lncRNAs) regulate disease resistance against fungi and other pathogens. However, the specific mechanism behind this regulation remains unclear. In this study, we identified disease resistance-related lncRNAs as well as their regulating genes and assessed their functions by infection of cotton (Gossypium) chromosome segment substitution lines with Verticillium dahliae. Our results demonstrated that lncRNA7 and its regulating gene Pectin methylesterase inhibitor 13 (GbPMEI13) positively regulated disease resistance via the silencing approach, while ectopic overexpression of GbPMEI13 in Arabidopsis (Arabidopsis thaliana) promoted growth and enhanced resistance to V. dahliae. In contrast, lncRNA2 and its regulating gene Polygalacturonase 12 (GbPG12) negatively regulated resistance to V. dahliae. We further found that fungal disease-related agents, including the pectin-derived oligogalacturonide (OG), could downregulate the expression of lncRNA2 and GbPG12, leading to pectin accumulation. Conversely, OG upregulated the expression of lncRNA7, which encodes a plant peptide phytosulfokine (PSK-α), which was confirmed by lncRNA7 overexpression and Ultra Performance Liquid Chromatography Tandem Mass Spectrometry (UPLC-MS) experiments. We showed that PSK-α promoted 3-Indoleacetic acid (IAA) accumulation and activated GbPMEI13 expression through Auxin Response Factor 5. Since it is an inhibitor of pectin methylesterase (PME), GbPMEI13 promotes pectin methylation and therefore increases the resistance to V. dahliae. Consistently, we also demonstrated that GbPMEI13 inhibits the mycelial growth and spore germination of V. dahliae in vitro. In this study, we demonstrated that lncRNA7, lncRNA2, and their regulating genes modulate cell wall defense against V. dahliae via auxin-mediated signaling, providing a strategy for cotton breeding.

Detection and Genotyping of Fov4 (Race 4, VCG0114), the Fusarium Wilt Pathogen of Cotton.

Fusarium wilt, caused by Fusarium oxysporum f. sp. vasinfectum (Fov), is an important disease of cotton. More than 14 different genotypes as determined by VCG and sequence analyses are known to occur in the United States. Fov4 (race 4, VCG0114), originally found in India, was first detected in the United States in 2001 in California and recently in 2017 and 2019 in Texas and New Mexico, respectively. Four sub-genotypes of Fov4 have been identified, with Fov4 N, T, and MiT genotypes occurring in California, and Fov4 T and MT genotypes occurring in Texas. Unlike other genotypes of Fov in the United States, Fov4 does not require the presence of root-knot nematodes (Meloidogyne incognita) to cause severe wilt in cotton and is a major concern to US cotton growers. Fov4 can be spread through a variety of mechanisms including through infected seed. Once a field is infested, the fungus becomes endemic since there are no economically viable means to eradicate the pathogen from infested fields. Therefore, a rapid and accurate detection method is essential for early identification of infested fields and seed lots to prevent further spread of Fov4. This chapter describes multiplex and singleplex PCR diagnostics for detection of Fov4, and for detection and genotyping N, T, MiT, and MT genotypes of Fov4 from wilted cotton plants.

Genetic Diversity and Pathogenicity of Verticillium dahliae Isolates and Their Co-occurrence with Fusarium oxysporum f. sp. vasinfectum Causing Cotton Wilt in Xinjiang, China.

Cotton production in Xinjiang, the largest cotton-producing area in China, has an increasingly serious disease threat from Verticillium dahliae. Eighty-five V. dahliae isolates were obtained from wilted cotton plants collected from eight counties in Xinjiang. The isolates were assessed for genotypic diversity by DNA sequence analysis and PCR molecular genotyping with specific markers for race 1, race 2, defoliating (D) pathotype, nondefoliating (ND) pathotype, and mating type idiomorph Mat1-2. Isolates belonged to lineages 1A or 2B, with three subgenotypes found in each lineage. All isolates tested positive for race 2 and Mat1-2 markers. All isolates in lineage 2B tested positive for the ND pathotype marker but only isolates in the major subgenotype in lineage 1A tested positive for the D pathotype marker. Pathogenicity assays on Gossypium hirsutum 'Acala 44' demonstrated no significant difference among subgenotypes within each lineage. Isolates in lineage 1A caused greater shoot weight reductions, percent leaf drop, and percent diseased leaves than isolates in lineage 2B. One isolate in each lineage for 1A and 2B was avirulent. Isolates in lineage 1A caused greater than 50% leaf drop and a 17-g shoot weight reduction compared with a 9% leaf drop and a 6-g shoot weight reduction by isolates in lineage 2B. Overall, 42% of the V. dahliae isolates from Xinjiang were D pathotype but the percentage varied widely among locations. Two plants had both pathotypes. Nineteen isolates of Fusarium oxysporum f. sp. vasinfectum VCG0114 (race 4) also were recovered from wilted plants in Xinjiang. Two plants had both Verticillium wilt and Fusarium wilt pathogens. Both pathogens should be considered when using or developing wilt resistant or tolerant materials for Xinjiang.

Detection and Characterization of Fusarium oxysporum f. sp. vasinfectum VCG0114 (Race 4) Isolates of Diverse Geographic Origins.

A highly virulent cotton wilt pathogen, Fusarium oxysporum f. sp. vasinfectum VCG0114 (race 4) was found in West Texas in 2017, after being known in California since 2001. Isolates obtained from wilted plants collected in 2017 from Texas, in 2015 from China, and during 2001 to 2014 from California and isolates from historical collections including the race 4 reference isolate were characterized by soil-infestation pathogenicity assays, DNA sequence analysis, and vegetative compatibility analysis. All obtained F. oxysporum f. sp. vasinfectum isolates belonged to VCG0114. All of these isolates, except one isolate from China, caused disease in a soil-infestation assay without nematodes. Thus, they belong to the nematode-independent pathotype. Texas isolates were significantly more virulent than were isolates from China or California on Gossypium barbadense 'Pima S-7'. Four different genotypes (N, T, MT, and MiT) were identified based on the transposable element Tfo1 insertion into the PHO gene and independent MULE or MITE insertions into the Tfo1 transposon. Some significant differences in virulence were detected among the genotypes in some locations. No differences in pathogenicity were observed between the California and China collection isolates on Pima S-7, and the virulence of the major genotypes was similar on the Gossypium hirsutum cultivar 'Stoneville 474' or the Barbren 713 germplasm line. Simple polymerase chain reaction (PCR) methods were developed to specifically determine and detect the four genotypes within VCG0114. A specific PCR method to detect all VCG0114 isolates was also developed. These methods will facilitate the timely identification of infested fields and seed lots and the elucidation of evolutionary relationships among the isolates. This should help to closely monitor the movement of the pathogen and reduce dissemination of these devastating pathogens.

Differences in Active Defense Responses of Two Gossypium barbadense L. Cultivars Resistant to Fusarium oxysporum f. sp. vasinfectum Race 4.

A highly virulent race 4 genotype of Fusarium oxysporum f. sp. vasinfectum (Fov) was identified for the first time in the western hemisphere in 2002 in cotton fields in the San Joaquin Valley of California. The Gossypium barbadense L. cotton cultivars 'Seabrook Sea Island 12B2' ('SBSI') and 'Pima S-6' are resistant to Fov race 4. Active defense responses were quantitated by monitoring the accumulation of antimicrobial terpenoids (i.e., phytoalexins) in inoculated stem stele tissue in these cultivars. The increase in the concentration of the most toxic phytoalexins was statistically faster after 24 h in 'SBSI' compared to 'Pima S-6'. The sesquiterpenoid hemigossylic acid lactone, which was observed for the first time in nature, also accumulated in diseased plants. Neither hemigossylic acid lactone nor the disesquiterpenoids gossypol, gossypol-6-methyl ether, and gossypol-6,6'-dimethyl ether showed toxicity to Fov. Segregation of F2 progeny from 'SBSI' × 'Pima S-6' crosses gave a few highly susceptible plants and a few highly resistant plants, indicating separate genes for resistance in the two cultivars.

Microbial Resistance Mechanisms to the Antibiotic and Phytotoxin Fusaric Acid.

Fusaric acid (FA) produced by Fusarium oxysporum plays an important role in disease development in plants, including cotton. This non-specific toxin also has antibiotic effects on microorganisms. Thus, one expects a potential pool of diverse detoxification mechanisms of FA in nature. Bacteria and fungi from soils infested with Fusarium and from laboratory sources were evaluated for their ability to grow in the presence of FA and to alter the structure of FA into less toxic compounds. None of the bacterial strains were able to chemically modify FA. Highly FA-resistant strains were found only in Gram-negative bacteria, mainly in the genus of Pseudomonas. The FA resistance of the Gram-negative bacteria was positively correlated with the number of predicted genes for FA efflux pumps present in the genome. Phylogenetic analysis of predicted FA resistance proteins (FUSC, an inner membrane transporter component of the efflux pump) revealed that FUSC proteins having high sequence identities with the functionally characterized FA resistance protein FusC or Fdt might be the major contributors of FA resistance. In contrast, most fungi converted FA to less toxic compounds regardless of the level of FA resistance they exhibited. Five derivatives were detected, and the detoxification of FA involved either oxidative reactions on the butyl side chain or reductive reactions on the carboxylic acid group. The production of these metabolites from widely different phyla indicates that resistance to FA by altering its structure is highly conserved. A few FA resistant saprophytic or biocontrol strains of fungi were incapable of altering FA, indicating a possible involvement of efflux transporters. Deployment of both efflux and derivatization mechanisms may be a common feature of fungal FA resistance.

Detoxification of Fusaric Acid by the Soil Microbe Mucor rouxii.

Fusarium oxysporum f. sp. vasinfectum race 4 (VCG0114), which causes root rot and wilt of cotton (Gossypium hirsutum and G. barbadense), has been identified recently for the first time in the western hemisphere in certain fields in the San Joaquin Valley of California. This pathotype produces copious quantities of the plant toxin fusaric acid (5-butyl-2-pyridinecarboxylic acid) compared to other isolates of F. oxysporum f. sp. vasinfectum (Fov) that are indigenous to the United States. Fusaric acid is toxic to cotton plants and may help the pathogen compete with other microbes in the soil. We found that a laboratory strain of the fungus Mucor rouxii converts fusaric acid into a newly identified compound, 8-hydroxyfusaric acid. The latter compound is significantly less phytotoxic to cotton than the parent compound. On the basis of bioassays of hydroxylated analogues of fusaric acid, hydroxylation of the butyl side chain of fusaric acid may affect a general detoxification of fusaric acid. Genes that control this hydroxylation may be useful in developing biocontrol agents to manage Fov.

Genome-wide comparative analysis of NBS-encoding genes in four Gossypium species.

BACKGROUND: Nucleotide binding site (NBS) genes encode a large family of disease resistance (R) proteins in plants. The availability of genomic data of the two diploid cotton species, Gossypium arboreum and Gossypium raimondii, and the two allotetraploid cotton species, Gossypium hirsutum (TM-1) and Gossypium barbadense allow for a more comprehensive and systematic comparative study of NBS-encoding genes to elucidate the mechanisms of cotton disease resistance. RESULTS: Based on the genome assembly data, 246, 365, 588 and 682 NBS-encoding genes were identified in G. arboreum, G. raimondii, G. hirsutum and G. barbadense, respectively. The distribution of NBS-encoding genes among the chromosomes was nonrandom and uneven, and was tended to form clusters. Gene structure analysis showed that G. arboreum and G. hirsutum possessed a greater proportion of CN, CNL, and N genes and a lower proportion of NL, TN and TNL genes compared to that of G. raimondii and G. barbadense, while the percentages of RN and RNL genes remained relatively unchanged. The percentage changes among them were largest for TNL genes, about 7 times. Exon statistics showed that the average exon numbers per NBS gene in G. raimondii and G. barbadense were all greater than that in G. arboretum and G. hirsutum. Phylogenetic analysis revealed that the TIR-NBS genes of G. barbadense were closely related with that of G. raimondii. Sequence similarity analysis showed that diploid cotton G. arboreum possessed a larger proportion of NBS-encoding genes similar to that of allotetraploid cotton G. hirsutum, while diploid G. raimondii possessed a larger proportion of NBS-encoding genes similar to that of allotetraploid cotton G. barbadense. The synteny analysis showed that more NBS genes in G. raimondii and G. arboreum were syntenic with that in G. barbadense and G. hirsutum, respectively. CONCLUSIONS: The structural architectures, amino acid sequence similarities and synteny of NBS-encoding genes between G. arboreum and G. hirsutum, and between G. raimondii and G. barbadense were the highest among comparisons between the diploid and allotetraploid genomes, indicating that G. hirsutum inherited more NBS-encoding genes from G. arboreum, while G. barbadense inherited more NBS-encoding genes from G. raimondii. This asymmetric evolution of NBS-encoding genes may help to explain why G. raimondii and G. barbadense are more resistant to Verticillium wilt, whereas G. arboreum and G. hirsutum are more susceptible to Verticillium wilt. The disease resistances of the allotetraploid cotton were related to their NBS-encoding genes especially in regard from which diploid progenitor they were derived, and the TNL genes may have a significant role in disease resistance to Verticillium wilt in G. raimondii and G. barbadense.

Specific PCR Detection of Fusarium oxysporum f. sp. vasinfectum California Race 4 Based on a Unique Tfo1 Insertion Event in the PHO Gene.

A highly virulent race 4 (Cal race 4) of Fusarium oxysporum f. sp. vasinfectum was identified in California cotton fields in 2001, and has since been found in increasing numbers of fields. Cal race 4 isolates contain a unique Tfo1 transposon insertion in the PHO gene that was not found in other F. oxysporum f. sp. vasinfectum genotypes. Based on this insertion, a multiplex polymerase chain reaction method was developed to detect the Cal race 4 pathogen. A panel of F. oxysporum f. sp. vasinfectum isolates representing different vegetative compatibility groups (VCG) and DNA sequence types was assembled to test the specificity of the detection method. In all, 16 of 17 Cal race 4 isolates produced a 583-bp amplicon; the other isolate produced a 396-bp amplicon reflecting the absence of the Tfo1 insertion. This isolate was a moderately virulent pathogen among Cal race 4 isolates. In total, 80 other F. oxysporum isolates associated with cotton and 11 other formae speciales of F. oxysporum produced only the 396-bp amplicon. The method also distinguished Cal race 4 isolates from India race 4 isolates and China race 7 isolates, which did not possess the unique Tfo1 insertion but otherwise had identical DNA sequences, and all belong to VCG0114. The method is capable of detecting the pathogen directly from infected stem tissues even before external symptom appears and, thus, provides an effective tool for timely identification of infested fields and seed lots, and should help reduce dissemination of Cal race 4 in the U.S. Cotton Belt.

Genetic Diversity, Virulence, and Meloidogyne incognita Interactions of Fusarium oxysporum Isolates Causing Cotton Wilt in Georgia.

Locally severe outbreaks of Fusarium wilt of cotton (Gossypium spp.) in South Georgia raised concerns about the genotypes of the causal pathogen, Fusarium oxysporum f. sp. vasinfectum. Vegetative complementation tests and DNA sequence analysis were used to determine genetic diversity among 492 F. oxysporum f. sp. vasinfectum isolates obtained from 107 wilted plants collected from seven fields in five counties. Eight vegetative complementation groups (VCG) were found, with VCG 01117B and VCG 01121 occurring in 66% of the infected plants. The newly recognized VCG 01121 was the major VCG in Berrien County, the center of the outbreaks. All eight VCG resulted in significant increases in the percentages of wilted leaves (27 to 53%) and significant reductions in leaf weight (40 to 67%) and shoot weight (33 to 60%) after being stem punctured into Gossypium hirsutum 'Rowden'. They caused little or no significant reductions in shoot weight and height or increases in foliar symptoms and vascular browning in a soil-infestation assay. Soil infestation with Meloidogyne incognita race 3 (root-knot nematode) alone also failed to cause significant disease. When coinoculated with M. incognita race 3, all VCG caused moderate to severe wilt. Therefore, the VCG identified in this study belong to the vascular-competent pathotype, and should pose similar threats to cotton cultivars in the presence of the root-knot nematode. Use of nematode-resistant cultivars, therefore, is probably the best approach to control the disease in Georgia.

A paralog of the proteinaceous elicitor SM1 is involved in colonization of maize roots by Trichoderma virens.

The biocontrol agent, Trichoderma virens, has the ability to protect plants from pathogens by eliciting plant defense responses, involvement in mycoparasitism, or secreting antagonistic secondary metabolites. SM1, an elicitor of induced systemic resistance (ISR), was found to have three paralogs within the T. virens genome. The paralog sm2 is highly expressed in the presence of plant roots. Gene deletion mutants of sm2 were generated and the mutants were found to overproduce SM1. The ability to elicit ISR in maize against Colletotrichum graminicola was not compromised for the mutants compared to that of wild type isolate. However, the deletion strains had a significantly lowered ability to colonize maize roots. This appears to be the first report on the involvement of an effector-like protein in colonization of roots by Trichoderma.

RNAi construct of a cytochrome P450 gene CYP82D109 blocks an early step in the biosynthesis of hemigossypolone and gossypol in transgenic cotton plants.

Naturally occurring terpenoid aldehydes from cotton, such as hemigossypol, gossypol, hemigossypolone, and the heliocides, are important components of disease and herbivory resistance in cotton. These terpenoids are predominantly found in the glands. Differential screening identified a cytochrome P450 cDNA clone (CYP82D109) from a Gossypium hirsutum cultivar that hybridized to mRNA from glanded cotton but not glandless cotton. Both the D genome cotton Gossypium raimondii and A genome cotton Gossypium arboreum possessed three additional paralogs of the gene. G. hirsutum was transformed with a RNAi construct specific to this gene family and eight transgenic plants were generated stemming from at least five independent transformation events. HPLC analysis showed that RNAi plants, when compared to wild-type Coker 312 (WT) plants, had a 90% reduction in hemigossypolone and heliocides levels, and a 70% reduction in gossypol levels in the terminal leaves, respectively. Analysis of volatile terpenes by GC-MS established presence of an additional terpene (MW: 218) from the RNAi leaf extracts. The (1)H and (13)C NMR spectroscopic analyses showed this compound was δ-cadinen-2-one. Double bond rearrangement of this compound gives 7-hydroxycalamenene, a lacinilene C pathway intermediate. δ-Cadinen-2-one could be derived from δ-cadinene via a yet to be identified intermediate, δ-cadinen-2-ol. The RNAi construct of CYP82D109 blocks the synthesis of desoxyhemigossypol and increases the induction of lacinilene C pathway, showing that these pathways are interconnected. Lacinilene C precursors are not constitutively expressed in cotton leaves, and blocking the gossypol pathway by the RNAi construct resulted in a greater induction of the lacinilene C pathway compounds when challenged by pathogens.

FUBT, a putative MFS transporter, promotes secretion of fusaric acid in the cotton pathogen Fusarium oxysporum f. sp. vasinfectum.

Fusaric acid (FA) is a key component in virulence and symptom development in cotton during infection by Fusarium oxysporum. A putative major facilitator superfamily (MFS) transporter gene was identified downstream of the polyketide synthase gene responsible for the biosynthesis of FA in a region previously believed to be unrelated to the known FA gene cluster. Disruption of the transporter gene, designated FUBT, resulted in loss of FA secretion, decrease in FA production and a decrease in resistance to high concentrations of FA. Uptake of exogenous FA was unaffected in the disruption transformants, suggesting that FA enters the cell in Fusarium by an independent mechanism. Thus, FUBT is involved both in the extracellular transport of FA and in resistance of F. oxysporum to this non-specific toxin. A potential secondary resistance mechanism, the production of FA derivatives, was observed in FUBT deletion mutants. Molecular analysis of key biochemical processes in the production of FA could lead to future host plant resistance to Fusarium pathogens.

Conversion of fusaric acid to Fusarinol by Aspergillus tubingensis: a detoxification reaction.

The fungus Fusarium oxysporum causes wilt diseases of plants and produces a potent phytotoxin fusaric acid (FA), which is also toxic to many microorganisms. An Aspergillus tubingensis strain with high tolerance to FA was isolated from soil and designated as CDRAt01. HPLC analysis of culture filtrates from A. tubingensis isolate CDRAt01 grown with the addition of FA indicated the formation of a metabolite over time that was associated with a decrease of FA. Spectral analysis and chemical synthesis confirmed the compound as 5-butyl-2-pyridinemethanol, referred to here as fusarinol. The phytotoxicity of fusarinol compared to FA was measured by comparing necrosis induced in cotton (Gossypium hirsutum L. cv. Coker 312) cotyledons. Fusarinol was significantly less phytotoxic than FA. Therefore, the A. tubingensis strain provides a novel detoxification mechanism against FA which may be utilized to control Fusarium wilt.

Desoxyhemigossypol 6-methyl ether.

THE TITLE SESQUITERPENE [SYSTEMATIC NAME: 6-methoxy-10-methyl-7-(propan-2-yl)-2-oxatricyclo[6.3.1.0(4,12)]dodeca-1(11),4,6,8(12),9-pentaen-5-ol], C(16)H(18)O(3), was isolated from pathogen-infected stele tissue of Gossypium barbadense. There are two mol-ecules in the asymmetric unit and the dihedral angle between their naphtho-furan systems is 86.48 (2)°. In the crystal, O-H⋯O hydrogen bonds between the hy-droxy groups and etheric O atoms link the mol-ecules into centrosymmetric tetra-mers. These tetra-mers are assembled into (010) layers via stacking inter-actions between the naphtho-furan systems [inter-planar distance 3.473 (3) Å] and short C-H⋯O contacts.

Hemigossypol, a constituent in developing glanded cottonseed (Gossypium hirsutum).

Gossypol is a dimeric sesquiterpenoid first identified in cottonseed, but found in various tissues in the cotton plant including the seed. From its first discovery, it was assumed that hemigossypol was the biosynthetic precursor of gossypol. Previous studies established that peroxidase (either from horseradish or from cottonseed) converts hemigossypol to gossypol. However, hemigossypol has never been identified in healthy cottonseed. In a temporal study using HPLC and LC-MS, hemigossypol was identified in the developing cotton embryo. It was shown to concomitantly accumulate until 40 days postanthesis (dpa) with gossypol and with transcripts of δ-cadinene synthase and 8-hydroxy-δ-cadinene synthase, genes involved in the biosynthesis of hemigossypol and gossypol. After 40 dpa, hemigossypol and its biosynthetic gene transcript levels declined, whereas the gossypol level remained almost unchanged until the bolls were open. These results provide further evidence to support the previous findings that establish hemigossypol as the biosynthetic precursor of gossypol.

Phylogeny and pathogenicity of Fusarium oxysporum isolates from cottonseed imported from Australia into California for dairy cattle feed.

A unique biotype of the Fusarium wilt pathogen, Fusarium oxysporum Schlecht. f.sp. vasinfectum (Atk) Sny. & Hans., found in Australia in 1993 is favored by neutral or alkaline heavy soils and does not require plant parasitic nematodes to cause disease. This makes it a threat to 4-6 million acres of USA Upland cotton ( Gossypium hirsutum L.) that is grown on heavy alkaline soil and currently is not affected by Fusarium wilt. In 2001-2002, several shiploads of live cottonseed were imported into California for dairy cattle feed. Thirteen F. oxysporum f.sp. vasinfectum isolates and four isolates of a Fusarium spp. that resembled F. oxysporum were isolated from the imported cottonseed. The isolates, designated by an AuSeed prefix, formed four vegetative compatibility groups (VCG) all of which were incompatible with tester isolates for 18 VCGs found in the USA. Isolate AuSeed14 was vegetatively compatible with the four reference isolates of Australian biotype VCG01111. Phylogenetic analyses based on EF-1α, PHO, BT, Mat1-1, and Mat1-2 gene sequences separated the 17 seed isolates into three lineages (race A, race 3, and Fusarium spp.) with AuSeed14 clustering into race 3 lineage or race A lineage depending on the genes analyzed. Indel analysis of the EF-1α gene sequences revealed a close evolutionary relationship among AuSeed14, Australian biotype reference isolates, and the four Fusarium spp. isolates. The Australian seed isolates and the four Australian biotype reference isolates caused disease with root-dip inoculation, but not with stem-puncture inoculation. Thus, they were a vascular incompetent pathotype. In contrast, USA race A lineage isolates readily colonized vascular tissue and formed a vascular competent pathotype when introduced directly into xylem vessels. The AuSeed14 isolate was as pathogenic as the Australian biotype, and it or related isolates could cause a severe Fusarium wilt problem in USA cotton fields if they become established.

Nuclear magnetic resonance (NMR) studies on the biosynthesis of fusaric acid from Fusarium oxysporum f. sp. vasinfectum.

Fusarium oxysporum is a fungal pathogen that attacks many important plants. Uniquely pathogenic strains of F. oxysporum f. sp. vasinfectum were inadvertently imported into the United States on live cottonseed for dairy cattle feed. These strains produce exceptionally high concentrations of the phytotoxin fusaric acid. Thus, fusaric acid may be a critical component in the pathogenicity of these biotypes. This study investigated the biosynthesis of fusaric acid using (13)C-labeled substrates including [1,2-(13)C(2)]acetate as well as (13)C- and (15)N-labeled aspartate and [(15)N]glutamine. The incorporation of labeled substrates is consistent with the biosynthesis of fusaric acid from three acetate units at C5-C6, C7-C8, and C9-C10, with the remaining carbons being derived from aspartate via oxaloacetate and the TCA cycle; the oxaloacetate originates in part by transamination of aspartate, but most of the oxaloacetate is derived by deamination of aspartate to fumarate by aspartase. The nitrogen from glutamine is more readily incorporated into fusaric acid than that from aspartate.

Total and percent atropisomers of gossypol and gossypol-6-methyl ether in seeds from pima cottons and accessions of Gossypium barbadense L.

Gossypol occurs naturally in the seed, foliage, and roots of the cotton plant ( Gossypium ) as atropisomers due to restricted rotation around the binaphthyl bond. The atropisomers differ in their biological activities. (-)-(R)-Gossypol is more toxic and exhibits significantly greater anticancer activity than the (+)-(S)-atropisomer. Most commercial Upland ( Gossypium hirsutum ) cottonseeds have an (R)- to (S)-gossypol ratio of approximately 2:3, but some Pima ( Gossypium barbadense ) seeds have an excess of (R)-gossypol. There is no known source of cottonseed with an (R)- to (S)-gossypol ratio of greater than approximately 70:30. Cottonseed with a high percentage of (R)-gossypol would be of value to the pharmaceutical industry. It was theorized that G. barbadense cotton might be a source of this desirable high (R)-gossypol seed trait. There are 671 different accessions of G. barbadense in the U.S. Cotton Germplasm Collection, few of which had been characterized with respect to their (R)- to (S)-gossypol ratio. This work completed that analysis and found considerable variation in the atropisomer ratio. Approximately half of the accessions have an excess of (R)-gossypol, and 52 accessions have essentially a 1:1 ratio. The highest percentage of (R)-gossypol was found in accessions GB26 (68.2%) and GB283 (67.3%). Surprisingly, five accessions had 5% or less of (R)-gossypol: GB516 (5.0%), GB761 (4.5%), GB577 (4.3%), GB719 (3.7%), and GB476 (2.3%). These accessions may be useful in a breeding program to reduce (R)-gossypol in Pima seed, which is a concern to the dairy industry because of the toxicity and male antifertility activity of this atropisomer. Also, GB710 was devoid of gossypol.