The plant hypersensitive response: concepts, control and consequences

Abstract

The hypersensitive defence response is found in all higher plants and is characterized by a rapid cell death at the point of pathogen ingress. It is usually associated with pathogen resistance, though, in specific situations, it may have other consequences such as pathogen susceptibility, growth retardation and, over evolutionary timescales, speciation. Due to the potentially severe costs of inappropriate activation, plants employ multiple mechanisms to suppress inappropriate activation of HR and to constrain it after activation. The ubiquity of this response among higher plants despite its costs suggests that it is an extremely effective component of the plant immune system.

Keywords: hypersensitive response.

Figure 1

(A) The phenotypes caused by various LES genes in the background of the maize inbred line Mo20W. (B) Examples of inbred lines showing a mild ‘flecking’ phenotype. Source: P. Balint‐Kurti/B. Olukolu. Adapted from Fig. 1 in Vontimitta et al. (2015).

Figure 2

(A) Incompatible interaction between Colletotrichum lindemuthianum, causal agent of bean anthracnose and bean (Phaseolus vulgaris). HR is observed only in the cell which is directly invaded by the infection vesicle. Picture source: G. Johal (Purdue University). (B) Macroscopic HR symptoms during the interaction of Puccinia sorghi with a maize line carrying the Rp1D resistance gene. Source: Saet‐Byul Kim (NC State University). (C) Hyphae of the oomycete downy mildew Hyaloperonospora parasitica growing on resistant Arabidopsis thaliana with the presence of trailing necrosis (staining is trypan blue). Picture source: Emmanuel Boutetvia, WikiMedia Commons.

Figure 3

Transient expression of 50 kDa helicase domain (TMV‐p50) in Nicotiana benthamiana carrying the N gene which confers a HR in the presence of TMV‐p50. In panel B the NbBECLIN gene, which is required for autophagy, has been silenced by viral‐induced gene silencing (VIGS), causing HR to spread from the initial site of HR activation. Source: Dr. S.P. Dinesh‐Kumar (UC Davis).

Figure 4

(A) Macroscopic lesions caused by the autoactive NLR Rp1‐D21 in maize. Upper image shows the phenotype conferred in a repressive background and the lower shows the phenotype in a permissive background. Source: P. Balint‐Kurti. (B) Leaf of a chimeric maize plant. The left sector carries the Rp1‐D21 autoactive NLR. The Rp1‐D21 gene has been mutated in the right sector so it is no longer functional. HR neither initiates nor propagates to the right sector. Source: P. Balint‐Kurti/S. Karre.

Figure 5

A summary of the processes controlling HR and the consequences of HR discussed in this review. The process of NLR transcription, translation and activation is shown on the right while the table on the left indicates the levels of control and consequences of activation. ‘Appropriate activation’ refers to the mode of activation that was selected for during evolution and ‘inappropriate activation’ refers to activation that has detrimental effects on plant growth and survival.