Plant Science

Sida is a genus of flowering herbs in the family Malvaceae, which includes several species that are weeds in Brazil. Plants of a Sida sp. exhibiting symptoms characterized by stunting, chlorosis, small leaves, and witches'-broom, indicative of infection by phytoplasmas, were found in a field previously cultivated with tomato, located in the region of Campinas, State of São Paulo, in December 2008. To demonstrate the presence of phytoplasmas in diseased tissues, DNA was extracted from shoots and leaves from three symptomatic and eight asymptomatic plants. Nested PCR was performed using primers P1/Tint followed by primer pair R16F2n/R16R2 (1). DNA fragments of 1.2 kb, corresponding to 16S rDNA, were amplified only for DNA from two symptomatic samples. Phytoplasma identification was initially carried out by restriction fragment length polymorphism (RFLP) analysis through digesting the PCR products with the restriction enzymes AluI, HhaI, HaeIII, HpaII, MseI, and RsaI. The two phytoplasma isolates found to be infecting a Sida sp. showed identical RFLP patterns, which were indistinguishable from the phytoplasma previously reported in association with hibiscus (Hibiscus rosa-sinensis) witches'-broom in Brazil (2). Nucleotide sequence alignment revealed that 16S rDNA of both phytoplasma isolates found in a Sida sp. (GenBank Accession No. HQ230579) shared 99.9% sequence similarity with 16S rDNA from hibiscus witches'-broom phytoplasma (HibWB) (GenBank Accession No. AF147708). HibWB is the representative of the 16SrXV group and it was proposed as a putative species nominated “Candidatus Phytoplasma brasiliense” (2). The disease is frequently observed in hibiscus plants used as ornamentals in the states of São Paulo (4) and Rio de Janeiro (2). “Ca. Phytoplasma brasiliense” has only been reported in Brazil to be infecting hibiscus (2,4) and periwinkle (Catharanthus roseus) (3). The presence of a phytoplasma belonging to group 16SrXV in a Sida sp. expands its natural host range. The role of this weed as a potential source of inoculum for crops should be investigated. References: (1) I. M. Lee et al. Int. J. Syst. Bacteriol. 48:1153, 1998. (2) H. G. Montano et al. Int. J. Syst. Evol. Microbiol. 51:1109, 2001. (3) H. G. Montano et al. Plant Dis. 85:1209, 2001. (4) E. G. Silva et al. Summa Phytopathol. 35:234, 2009.

Changes in carbohydrate metabolism in sweet orange ( Citrus sinensis ) infected with ‘ Candidatus Liberibacter asiaticus’, a purported cause of citrus Huanglongbing (HLB), were investigated. Starch levels in HLB‐infected leaves with and without symptoms increased 3·1‐ and 7·9‐fold, respectively, compared to healthy controls. In symptomless leaves, sucrose and fructose accumulated significantly ( P  <   0·05) in both midribs and lobes, and glucose only in the midribs (greater than fivefold); whereas maltose levels were reduced to 64·6% and 86·8% in the midribs and foliar lobes, respectively, of the values in healthy leaves. In leaves with symptoms, sucrose and glucose remained at high levels compared to healthy leaves, whilst no accumulation of fructose was observed; by contrast, the maltose content decreased to as low as 49·6% of that in healthy leaves. Fourfold induction of cell‐wall‐bound invertase activity was detected in both types of leaves on diseased plants. Additionally, the expression profiles of starch breakdown genes suggested that the transcription of DPE2 and MEX1 was down‐regulated. Together with the reduction of maltose, it is suggested that the impairment of starch breakdown may contribute to the starch accumulation in infected leaves. The imbalance of carbohydrate partitioning and its relation to disease physiology are discussed.

Pelargonium capitatum (rose pelargonium) is a plant indigenous to southern Africa, originally brought to Western Australia for its ornamental qualities. It has since become naturalized in the Southwest Australian Floristic Region, recognized for its high level of species endemism, where it is a serious invasive weed in bushlands and coastal dunes. Since P. capitatum outcompetes native species it is listed among the top 10 most important coastal weeds of the region (3). In 2008, large patches of stunted, dying, and dead P. capitatum plants were observed within a population covering coastal dunes at Woodman Point, Western Australia (GPS coordinates 32°07′40.51″S, 115°45′28.39″E). Diseased plants had small misshapen leaves in clumps that were often chlorotic or pink, shortened internodes, and exhibited phylloidy typical of infection by a phytoplasma. From August 2009 to January 2010, samples from symptomatic and asymptomatic plants were collected from the site and from plants of an asymptomatic population at another site located on the Murdoch University campus nearby. DNA was extracted from 15 samples collected from symptomatic and asymptomatic plants at the dune site and from five at the campus site. Briefly, 2 to 5 g of leaf and stem tissue was cut into 5-mm pieces and shaken overnight in 30 ml of phosphate-buffered saline buffer. Supernatant was filtered and a pellet was collected by centrifugation. After resuspension in 500 μl of extraction buffer (200 mM Tris-HCl [pH 7.5] 250mM NaCl, 25mM ethylenediaminetetraacetic acid, 0.5% sodium dodecyl sulfate, and 2% polyvinylpyrrolidone), DNA was precipitated in 500 μl of cold isopropanol. Samples were tested for the presence of phytoplasma ribosomal 16S DNA by nested PCR using phytoplasma universal primers P1/P7 followed by amplification with primers Tint, R16mF2, and R16mR1 (1,2,4). Phytoplasma-specific DNA sequences were synthesized directly from amplicons using the above primers. Phytoplasma was detected from both symptomatic and asymptomatic plant samples collected from the dune site but not from the campus site. Analysis of the nine sequences obtained (GenBank Accession Nos. HM583339, HM583340, HM583341, HM583342, HM583343, HM583344, HM583345, HM583346, and HM583347) revealed high sequence identity between isolates (~99%) and with the ‘Candidatus Phytoplasma aurantifolia’ (16SrII) group of phytoplasmas (1,4). Presence of phytoplasma in symptomatic plants was confirmed by histological examination of stem sections stained with Dienes' stain. This finding is significant because there is potential for utilizing this phytoplasma to control P. capitatum where it has invaded ecologically significant sites, although its effect on indigenous plants must be determined first. Although phytoplasmas within the 16SrII group have been identified in Australia previously (1,4), to our knowledge, this is the first report of it infecting P. capitatum. References: (1) K. S. Gibb et al. Phytopathology 85:169, 1995. (2) D. E. Gundersen and I.-M. Lee. Phytopathol. Mediterr. 35:144, 1996. (3) B. M. J. Hussey et al. Western Weeds. A Guide to the Weeds of Western Australia. 2nd ed. Plant Protection Society of Western Australia, Victoria Park, 2007. (4) M. Saqib et al. J. R. Soc. West. Aust. 90:175, 2007.