Systemic acquired resistance (SAR), a highly desirable form of plant defense, provides broad-spectrum immunity against diverse pathogens. The recent identification of seemingly unrelated chemical inducers of SAR warrants an investigation of their mutual interrelationships. We show that SAR induced by the dicarboxylic acid azelaic acid (AA) requires the phosphorylated sugar derivative glycerol-3-phosphate (G3P). Pathogen inoculation induced the release of free unsaturated fatty acids (FAs) and thereby triggered AA accumulation, because these FAs serve as precursors for AA. AA accumulation in turn increased the levels of G3P, which is required for AA-conferred SAR. The lipid transfer proteins DIR1 and AZI1, both of which are required for G3P- and AA-induced SAR, were essential for G3P accumulation. Conversely, reduced G3P resulted in decreased AZI1 and DIR1 transcription. Our results demonstrate that an intricate feedback regulatory loop among G3P, DIR1, and AZI1 regulates SAR and that AA functions upstream of G3P in this pathway.

This study aimed to evaluate the effect of silicon (Si) rates on some components of sorghum resistance to anthracnose. Two 2×5 factorial experiments, consisting of two sorghum lines (BR005 and BR009, resistant and susceptible, respectively) and five Si application rates (0, 0.06, 0.12, 0.24 and 0.30 g Si kg −1 of soil) were arranged in a completely randomised design with three replications. Plants from both lines were inoculated with a conidial suspension of Colletotrichum sublineolum (1×10 6 conidia ml −1 ) 30 days after emergence. The incubation period (IP), latent period (LP 60 ), area under relative infection efficiency progress curve (AURIEPC), area under anthracnose index progress curve (AUAIPC), final disease severity (FDS), percentage of pigmented leaf area (PLA), and percentage of necrotic leaf area (NLA) were evaluated. Silicon and calcium (Ca) content in leaf tissue of both lines was also determined. The content of Si in leaf tissue increased relative to the control by 55 and 58%, respectively, for the susceptible and resistant lines. There was no significant change in Ca content in leaf tissue for either of the lines; therefore the variations in Si accounted for differences in the level of disease response. The IP for the resistant line was not affected by Si application rates. The LP 60 was not evaluated in the resistant line due to the absence of acervuli. For the resistant line, Si application rates had no significant effect on AUAIPC, FDS, percentage of PLA, and percentage of NLA. On the susceptible line, a quadratic regression model best described the effect of Si application rates on IP, LP 60 , AURIEPC, AUAIPC, FDS, percentage of PLA, and percentage of NLA. The correlation between Si content in leaf tissue of the susceptible line and the AURIEPC, AUAIPC, FDS, PLA, and NLA was negatively significant (r = −0.57, −0.37, −0.40, −0.67, and −0.77, respectively). There was no correlation between Si content and IP or LP 60 . The correlation between the percentage of PLA with the percentage of NLA was negatively significant (r = −0.74). In conclusion, the results from this study underscore the importance of Si in sorghum resistance to anthracnose particularly for the susceptible line.

Huanglongbing (HLB), a bacterial disease caused by Candidatus Liberibacter asiaticus (CLas), is a major threat to the citrus industry. In a previous study conducted by our laboratory, several citrus transgenic trees expressing the Arabidopsis thaliana NPR1 (AtNPR1) gene remained HLB-free when grown in a field site under high HLB disease pressure. To determine the molecular mechanisms behind AtNPR1-mediated tolerance to HLB, a transcriptome analysis was performed using AtNPR1 overexpressing transgenic trees and non-transgenic trees as control, from which we identified 57 differentially expressed genes (DEGs). Data mining revealed the enhanced transcription of genes encoding pathogen-associated molecular patterns (PAMPs), transcription factors, leucine-rich repeat receptor kinases (LRR-RKs), and putative ankyrin repeat-containing proteins. These proteins were highly upregulated in the AtNPR1 transgenic line compared to the control plant. Furthermore, analysis of protein–protein interactions indicated that AtNPR1 interacts with CsNPR3 and CsTGA5 in the nucleus. Our results suggest that AtNPR1 positively regulates the innate defense mechanisms in citrus thereby boosting resistance and effectively protecting the plant against HLB.

Shortening the juvenile stage in citrus and inducing early flowering has been the focus of several citrus genetic improvement programs. FLOWERING LOCUS T (FT) is a small phloem-translocated protein that regulates precocious flowering. In this study, two populations of transgenic Carrizo citrange rootstocks expressing either Citrus clementina FT1 or FT3 genes under the control of the Arabidopsis thaliana phloem specific SUCROSE SYNTHASE 2 (AtSUC2) promoter were developed. The transgenic plants were morphologically similar to the non-transgenic controls (non-transgenic Carrizo citrange), however, only AtSUC2-CcFT3 was capable of inducing precocious flowers. The transgenic lines produced flowers 16 months after transformation and flower buds appeared 30–40 days on juvenile immature scions grafted onto transgenic rootstock. Gene expression analysis revealed that the expression of SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) and APETALA1 (AP1) were enhanced in the transgenics. Transcriptome profiling of a selected transgenic line showed the induction of genes in different groups including: genes from the flowering induction pathway, APETALA2/ETHYLENE RESPONSE FACTOR (AP2/ERF) family genes, and jasmonic acid (JA) pathway genes. Altogether, our results suggested that ectopic expression of CcFT3 in phloem tissues of Carrizo citrange triggered the expression of several genes to mediate early flowering.

Citrus cultivation is challenging due to the plethora of abiotic and biotic stresses faced by the crop. In recent years, production has been severely affected by diseases such as citrus canker and huanglongbing (HLB). Disease management is hampered as there is no field resistance to these diseases in any of the important commercially planted varieties. Traditionally, conventional breeding approaches have been applied for the improvement of the susceptible cultivars; however, this technique is laborious and time consuming. Genetic transformation of citrus allows for the rapid integration of novel genes into the plant's genome to develop disease-resistant transgenic plants. Therefore, efforts have been made to utilize genetic engineering tools to develop genetically modified citrus that are resistant to citrus canker and HLB. This review summarizes the major achievements made in the development of citrus canker and HLB tolerance using transgenic technologies.

Blast, caused by the fungus Pyricularia oryzae, is an important disease affecting rice and wheat yield worldwide. This study investigated the cytological aspects of incompatible (non-host resistance) and compatible (host resistance) rice-(R_Po) and wheat-(W_Po) Pyricularia oryzae isolate interactions. Inoculations of rice and wheat with the R_Po and W_Po isolates of P. oryzae, respectively, were expected to be compatible interactions (host resistance), whereas inoculations of rice and wheat with the W_Po and R_Po isolates of P. oryzae, respectively, were considered to be incompatible interactions (non-host resistance). For the compatible interactions (rice-R_Po and wheat-W_Po), fungal hyphae penetrated and colonized the epidermal cells and also invaded many neighboring cells. By contrast, in the case of the incompatible interactions (rice-W_Po and wheat-R_Po), fungal hyphae were not able to penetrate nor colonize the epidermal cells, but when penetration did occur, the hyphae were restricted to the first-invaded epidermal cell. The frequency of appressorial sites exhibiting infection hyphae within the epidermal cell underlying an appressorium was greater in the case of the compatible interactions. By contrast, unsuccessful penetrations with cytoplasmic granulation occurred with high frequency in the incompatible wheat-R_Po and rice-W_Po interactions and the number of necrotic epidermal cells underlying the appressorium was low for the rice-W_Po interaction as well as for the wheat-R_Po interaction, where no symptoms of necrosis were exhibited. However, the opposite was observed for the compatible interactions. The present study presents cytological features associated with incompatible and compatible rice-and wheat-P. oryzae interactions that may be useful to studies involving variability, coevolution, diagnosis, and regulation of quarantine or even in a rice or wheat breeding program whose aim is to transfer genes involved in non-host resistance to host resistance due to similarities in downstream mechanisms.

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Impatiens walleriana, busy lizzy or balsam (local names in Brazil maria-sem-vergonha or beijo-de-frade), is an African member of the Balsaminaceae that has long ago been introduced and established in Brazil. It is now widely cultivated commercially as a potted plant and a popular garden plant (3). It also is a common weed along the coast and is particularly troublesome in some banana plantation areas. There are only two records of fungal pathogens attacking this plant in Brazil: Cercospora fukushiana (leaf spot) and Oidiopsis haplophylli (powdery mildew). In January 2009, a population of diseased plants of I. walleriana was found in a private garden in the city of Rio de Janeiro, Brazil. Plants had rotted and girdled stem bases, leading to a collapse of stems Necrotic areas were covered with fans of white mycelium as well as abundant spherical sclerotia. The fungus was isolated in pure culture by direct aseptic transfer of mycelial fragments and sclerotia to potato dextrose agar (PDA) plates. Colonies were white, cottony, often forming fans, primary hyphae 3.0 to 6.0 μm in diameter, and bearing clamp connections; sclerotia formed after 7 days, initially white becoming dark brown with age, and 0.8 to 1.85 mm in diameter. These are typical features of Sclerotium rolfsii. A specimen was deposited in the local herbarium (Herbarium VIC) under Accession No. VIC 30732. Koch's postulates were performed by inoculating three healthy potted I. walleriana plants (10 × 40 cm high) with approximately 100 freshly collected sclerotia placed in close vicinity with the stem bases. Noninoculated plants kept in a separate pot served as controls. Plants were incubated in a dew chamber for 48 h at 25 ± 2°C. All inoculated plants showed symptoms of stem rot 72 h after inoculation, whereas controls remained healthy. S. rolfsii is a highly polyphagous species that has been recorded to be causing rots (also known as Southern blights) in Brazil on numerous hosts but there are no records of it attacking any members of the Balsaminaceae in Brazil. The only other published records of S. rolfsii on Impatiens spp. are from the United States (Hawaii and Illinois) (2) and the Philippines (1). In South America, there is a single report from Argentina (4) where the disease is regarded as a major threat to the potted plant industry because I. walleriana is one of the most popular potted plants in that country. The potential for losses is also significant for Brazil. To our knowledge, this is the first report of S. rolfsii as a pathogen of I. walleriana in Brazil. Although very damaging to I. walleriana, it is unlikely that this fungus can be used as a natural enemy of this plant species in weed situations because of its wide host range. References: (1) T. O. Dizon and R. B. Pimentel. Philipp. Phytopathol. 29:101, 1993. (2) D. F. Farr and A. Y. Rossman. Fungal Databases, Systematic Mycology and Microbiology Laboratory. Online publication. ARS, USDA, 2009. (3) H. Lorenzi and H. M. Souza. Plantas Ornamentais no Brasil – Arbustivas, Herbáceas e Trepadeiras. Nova Odessa: Instituto Plantarum, 1995. (4). S. M. Wolcan and P. J. Grego. Australas. Plant Dis. Notes 4:54, 2009.