Abstract
Lesion mimic mutants (LMMs) are ideal materials for studying programmed cell death and defense response in plants. Here we report investigations on two LMMs (msl-1 and msl-2) from the indica rice cultivar JG30 treated by ethyl methyl sulfone. Both of the mutants showed similar mosaic spot lesions at seedling stage, but they displayed different phenotypes along with development of the plants. At tillering stage, larger orange spots appeared on leaves of msl-2, while only small reddish-brown spots exhibit on leaves of msl-1. At heading stage, the msl-2 plants were completely dead, while the msl-1 plants were still alive even if showed apparent premature senility. For both the mutants, the mosaic spot lesion formation was induced by light; DAB and trypan blue staining showed a large amount of hydrogen peroxide accumulated at the lesion sites, accompanied by a large number of cell death. Consequently, reactive oxygen species were enriched in leaves of the mutants; SOD and CAT activities in the scavenging enzyme system were decreased compared with the wild type. In addition, degraded chloroplasts, decreased photosynthetic pigment content, down-regulated expression of genes associated with chloroplast synthesis/photosynthesis and up-regulated expression of genes related to senescence were detected in the mutants, but the abnormality of msl-2 was more serious than that of msl-1 in general. Genetic analysis and map-based cloning revealed that the lesion mimic and premature senescence traits of both the mutants were controlled by recessive mutated alleles of the SL (Sekiguchi lesion) gene, which encodes the CYP71P1 protein belonging to cytochrome P450 monooxygenase family. The difference of mutation sites and mutation types (SNP-caused single amino acid change and SNP-caused early termination of translation) led to the different phenotypes in severity between msl-1 and msl-2. Taken together, this work revealed that the CYP71P1 is involved in regulation of both premature senescence and cell death in rice, and its different mutation sites and mutation types could cause different phenotypes in terms of severity.
Highlights
Plant lesion mimic mutants (LMMs) spontaneously forms necrotic spots on leaves, leaf sheaths and stems under the external conditions without damage or pathogen infection (Hu et al, 1996)
We identified two LMMs, designated as mosaic spot lesion mutants msl-1 and msl-2, from indica rice cultivar JG30 treated by ethyl methyl sulfone (EMS)
This study demonstrated that mutation of SL mediated programmed cell death of rice, and led to premature senescence, and mutation sites and types could cause different phenotypes in terms of severity
Summary
Plant lesion mimic mutants (LMMs) spontaneously forms necrotic spots on leaves, leaf sheaths and stems under the external conditions without damage or pathogen infection (Hu et al, 1996). A single base substitution in the first exon of the mutant allele spl results in the early termination of translation, which confers broad-spectrum resistance to rice blast and bacterial blight (Yin et al, 2000). A single base mutation in the mutant spl results in the early termination of translation, resulting in loss of protein function and activation of defense response to rice blast and bacterial blight (Wang et al, 2017). NLS1 encodes a typical CC-NB-LRR type protein, its mutant nls exhibits constitutive defense responses, including cell death, excessive accumulation of hydrogen peroxide and salicylic acid (SA), and enhanced resistance to bacterial pathogens Xanthomonas oryzae pv. Most of the rice LMMs show improved resistance to pathogens (Xu et al, 2018; Tian et al, 2020)
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