Ralstonia solanacearum preferential colonization in the shoot apical meristem explains its pathogenicity pattern in tomato seedlings

Abstract

Ralstonia solanacearum causes a lethal bacterial wilt disease in many plants by colonizing the vascular tissues of the hosts. Upon inoculation of tomato seedlings through either leaf or root, the wilting symptoms occur first at the apical region and then proceed downward along the shoot. The systemic order of the disease initiation and progression in the host, independent of the site of pathogen inoculation, is yet to be investigated. To understand the disease progression more clearly, we have carried out a systematic study of the pathogen localization by GUS staining of inoculated tomato seedlings, at 24‐hour intervals from 0 days post‐inoculation (dpi) to 5 dpi. In both inoculation methods, pathogen colonization was observed at 1 dpi at the apical meristem as well as the cotyledon leaves, where the disease initiates. As the disease progressed, colonization by the pathogen towards the lower region of the shoot was observed. Disease consistency and pathogenicity magnitude were observed to be higher using the leaf inoculation method than the root inoculation method. Several R. solanacearum transposon‐induced mutants that were reduced in virulence by root inoculation but virulent by leaf inoculation were obtained. Using GUS staining, it was observed that these mutants were unable to localize in the shoot region when inoculated in the root. Our study indicates that the apical meristem and the cotyledon leaves are the first regions to be colonized in inoculated tomato seedlings, which might explain the disease initiation from this region.

Ralstonia solanacearum Virulence in Cotyledon Stage Tomato Seedlings

“…Due to small size, the entire seedling could be stained at regular intervals after the infection. By gus staining, it is observed that R. solanacearum colonizes the shoot apical region of the seedlings within 24h of inoculation either in root or in the leaf [15]. This provides explanation for two important observations i.e.…”

“…(i) The initiation of the disease from the apical region, (ii) Why the disease magnitude is higher by leaf inoculation than by root inoculation [15]. This finding has given a new insight to the disease mechanism.…”

“…R. solanacearum mutants deficient in the same virulence factor exhibit significance difference with regard to their virulence between the two hosts. Apart from the above advantages, the small size and cost efficiency of the seedlings allows to perform largescale screening of virulence deficient mutants [15].…”

“…7(3).2020 whole experiment starting from seed germination, inoculation and disease scoring can be completed in two weeks period (Figure 1). The pathogen localization in the host can be studied by appropriate staining of the entire seedling [15]. Recently, bio control against R. solanacearum infection has been done using seedlings [16,17].…”

Shoot meristem culture eliminates bacterial and fungal infections from elite varieties of turmeric (Curcuma longa L.)

“…R. solanacearum is responsible for >30% losses in yield of C. longa (turmeric), C. aromatica (wild turmeric), and C. zedoaria (white turmeric) in Okinawa, Japan (Ajitomi et al 2015). Meristem tips (without vascular tissues) of plants infected with R. solanacearum have been identified as disease-free zone of the tissue (Kabyyashree et al 2020). Apical meristem cutting and meristem culture play an important role in eliminating the bacterial, fungal, or even viral particles from the host plants (Ramgareeb et al 2010).…”