Immunity at Cauliflower Hydathodes Controls Systemic Infection by Xanthomonas campestris pv campestris

Abstract

Hydathodes are water pores found on leaves of a wide range of vascular plants and are the sites of guttation. We report here on the detailed anatomy of cauliflower () and Arabidopsis () hydathodes. Hydathode surface presents pores resembling stomata giving access to large cavities. Beneath, the epithem is composed of a lacunar and highly vascularized parenchyma offering a direct connection between leaf surface and xylem vessels. Arabidopsis hydathode pores were responsive to ABA and light similar to stomata. The flg22 flagellin peptide, a well-characterized elicitor of plant basal immunity, did not induce closure of hydathode pores in contrast to stomata. Because hydathodes are natural infection routes for several pathogens, we investigated hydathode infection by the adapted vascular phytopathogenic bacterium pv (), the causal agent of black rot disease of Brassicaceae. Microscopic observations of hydathodes six days postinoculation indicated a digestion of the epithem cells and a high bacterial multiplication. Postinvasive immunity was shown to limit pathogen growth in the epithem and is actively suppressed by the type III secretion system and its effector proteins. Altogether, these results give a detailed anatomic description of Brassicaceae hydathodes and highlight the efficient use of this tissue as an initial niche for subsequent vascular systemic dissemination of in distant plant tissues.

Plant responses underlying nonhost resistance of Citrus limon against Xanthomonas campestris pv. campestris

“…AvrGf2 belongs to the XopAG effector family which is present in several xanthomonads, including Xcc (Gochez et al ., ). To further determine whether the NHR triggered by Xcc involves type III effector proteins, the hrcV ‐deficient mutant strain of Xcc 8004 ( Xcc Δ hrcV ), defective in the T3S system (Cerutti et al ., ), was assayed. Regardless of the method used for bacterial inoculation in C. limon , Xcc Δ hrcV developed an asymptomatic phenotype and bacterial population growth similar to wild‐type Xcc (data not shown).…”

“…(), and were transformed by electroporation with plasmid pMP2444 expressing GFP (Rigano et al ., ). The Xcc strain 8004 mutant in the hrcV gene ( Xcc Δ hrcV ), defective in the T3S system, has been described previously (Cerutti et al ., ).…”

“…Xcc is the major bacterial pathogen of plants of the Brassicaceae family, including Arabidopsis, and is an economically important phytopathogen of cruciferous plants worldwide (Vicente and Holub, ). In Arabidopsis, Xcc can enter into the leaf through hydathodes, wounds and stomata (Cerutti et al ., ; Gudesblat et al ., ). Similarly to X. citri , Xcc requires a functional type III secretion (T3S) system to deliver effector proteins into their host cells and to cause disease.…”

Anatomy of epithemal hydathodes in four monocotyledon plants of economic and academic relevance

“…Mutations affecting stomatal development often similarly affect water pore development [24]. Several markers for stomatal identity or differentiation cannot be used to differentiate water pores for stomata either [9], thus suggesting a common origin of both cell types. Yet, several morphological differences can distinguish water pores from stomata.…”

“…Hydathodes are also relevant to plant health because they represent natural entry points for several vascular pathogens in both monocot and dicot plants. Hydathode infection is visible by chlorotic and necrotic symptoms starting at leaf tips or leaf margins leading to systemic infections as observed in black rot of Brassicaceae [9], in bacterial blight of aroids caused by Xanthomonas axonopodis pv. dieffenbachiae [10, 11], in bacterial canker of tomato caused by Clavibacter michiganensis subsp.…”