Abstract
Glasshouse experiments were conducted to study the colonization of seedpods (siliques) and seeds of rapid cycling Brassica oleracea plants after spraying inoculum on clusters of recently opened flowers with Xanthomonas campestris pv. campestris (Xcc) at densities of 107–108 cfu ml−1. A green fluorescent protein (GFP) tagged Xcc strain was used to allow visualization of the bacteria by epifluorescence stereo microscopy (ESM) and confocal laser scanning microscopy (CLSM). The GFP-tagged strain showed reduced virulence compared to the untagged parental strain, but was still able to cause black rot symptoms. Two to three days after spray-inoculation, sepals, stamen and petals were colonized by Xcc, as observed by ESM. In green siliques a GFP-signal was observed on valves, septa and seeds, despite the fact that a high percentage of Xcc cells had lost their ability to express GFP as found by dilution-plating. Densities of Xcc in infected silique tissues were up to 109 cfu g−1. A fluorescent signal using ESM was found in seeds harvested from symptomatic siliques after incubation of seeds on blotting paper wetted with broth to enhance the multiplication of Xcc. Xcc was found in association with the seed coat and in a single seed, also in the endosperm and embryo, indicating deep-seated seed infection. The estimated incidence of contaminated seeds in both years was ca. 7%. The estimated incidence of deep-seated infections, still detectable after warm water treatment of seeds, was also high (2–3.8%). It is concluded that spray-inoculation of flower clusters with Xcc can result in the infection of sepals and reproductive organs, and in deep-seated seed infections.
Highlights
The Brassicaceae is a large botanical family that includes 372 genera, under which several plant species of great economic and agronomic importance, such as oil seed rape and vegetables, and the model plant in botanical research Arabidopsis thaliana. (Bailey et al 2006)
Xanthomonas campestris pv. campestris (Xcc) strain IPO 3078 was maintained at 17 °C on yeast dextrose chalk (YDC) agar (Duchefa Biochemie, NL) and inoculum was prepared from cultures grown on tryptone soya agar (TSA; Oxoid, UK) at 25 °C for 48 h
No green fluorescent protein (GFP) signal developed on flowers of mock-inoculated plants (Fig. 3a3, b3), the anther lobe of the stamen displayed a high yellow green autofluorescence (Fig. 3b3)
Summary
The Brassicaceae is a large botanical family that includes 372 genera, under which several plant species of great economic and agronomic importance, such as oil seed rape and vegetables, and the model plant in botanical research Arabidopsis thaliana. (Bailey et al 2006). Despite considerable efforts to produce Xcc-free seed, seed infections still occur (Williams 1980; Gitaitis and Walcott 2007). Physical treatment such as with hot water is useful to eliminate Xcc from seeds (Walker 1923; Nega et al 2003). It is not always effective in the eradication of deep-seated seed infections, present in seed coat, endosperm or embryo (Schaad 1983; Van der Wolf et al 2013). Low rates of seed infection already may cause serious epidemics in transplants, wherein production yield favorable conditions for black rot to develop (Roberts et al 1999)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
