Events in Plant Development

In the field, plant root systems and hydathodes appear most likely to internalize microorganisms. Plant roots are likely to internalize soil microbes because wounds form during root growth. The development of lateral roots in plants usually begins at the pericycle, which underlies the endodermis [18]. The endodermis is a tightly packed cell structure located several cell layers below the root surface. As the root tip forms and then emerges, it breaks through the endodermis and cortex creating an open wound, which is a frequent site for colonization by soilborne bacteria [1,12]. Even casual bacteria can grow in wounds created by the emergence of lateral roots. Escherichia coli O157:H7 internalized in lettuce apparently through the root system when the plants were fertilized with contaminated manure or irrigated with contaminated water [57]. In controlled studies with seedlings of several plant species grown in test tubes, strains of Salmonella enterica were able to colonize the lateral root emergence wounds and then colonize intercellular spaces in the interior of the root [12]. Certain bacterial types applied to these plantlets were observed in xylem vessels, whereas others were rarely found in such cells. All applied bacteria were observed in the cortex of the root. Populations of a known endophytic bacterium, Klebsiella pneumoniae, exceeding log 8.0 CFU/g fresh weight were found in the root system of seedling rice plants grown in test tubes [7]. Endophytic populations were successfully initiated by the inoculation of seedlings with as few as 1 CFU per seedling [12]. Whether populations multiplied on the rhizoplane prior to entering the plant or found sufficient nutrition to multiply totally inside plants could not be determined; however, endophytic populations were correlated with those on the rhizoplane.

The soilborne, bacterial wilt pathogen Ralstonia solanacearum (Pseudomonas solanacearum) was observed to penetrate tobacco roots through epidermal cells that were damaged by lateral root emergence [58]. After penetration, the bacteria moved intercellularly in the cortex. The entrance of bacteria into xylem vessels appeared to occur where the endodermis was not fully developed or as a consequence of hypertrophy of xylem parenchyma cells, which appeared to disrupt young xylem vessels.

Growth cracks in plant surfaces during maturation processes could, at least temporarily, provide microbe internalization sites. Wide temperature changes, rainfall, the planting of crack-susceptible cultivars, and fertilization programs featuring high nitrogen and low potash have been associated with growth cracks in tomatoes [59]. Growth cracks in tomato fruit surfaces are a frequent site of microbial attack and predispose the fruit to pre- and postharvest decay [60]. Such cracks could enable internalization of a wide range of microorganisms. Many other crops have cultivars designated as crack resistant. In any fresh fruit or vegetable, the development of cracks or punctures in surfaces leading up to harvest, at harvest, or after harvest could enable various microbes to internalize. While most plant organs with growth cracks are culled during the packing process, items with minor cracks or punctures could be shunted to fresh-cut processing and lead to a contaminated product. Alternatively, microbes could internalize in plant organs that naturally crack during development such as cantaloupes [18].

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