Degradation of glyoxalase I in Brassica napus stigma leads to self-incompatibility response

Abstract

Self-incompatibility (rejection of 'self'-pollen) is a reproductive barrier that allows hermaphroditic flowering plants to prevent inbreeding, to promote outcrossing and hybrid vigour. The self-incompatibility response in Brassica involves allele-specific interaction between the pollen small cysteine-rich, secreted protein ligand (SCR/SP11) and the stigmatic S-receptor kinase (SRK), which leads to the activation of the E3 ubiquitin ligase ARC1 (Armadillo repeat-containing 1), resulting in proteasomal degradation of compatibility factors needed for successful pollination. Despite this, targets of ARC1 and the intracellular signalling network that is regulated by these targets, have remained elusive. Here we show that glyoxalase I (GLO1), an enzyme that is required for the detoxification of methylglyoxal (MG, a cytotoxic by-product of glycolysis), is a stigmatic compatibility factor required for pollination to occur and is targeted by the self-incompatibility system. Suppression of GLO1 was sufficient to reduce compatibility, and overexpression of GLO1 in self-incompatible Brassica napus stigmas resulted in partial breakdown of the self-incompatibility response. ARC1-mediated destruction of GLO1 after self-pollination results in increased MG levels and a concomitant increase in MG-modified proteins (including GLO1), which are efficiently targeted for destruction in the papillary cells, leading to pollen rejection. Our findings demonstrate the elegant nature of plants to use a metabolic by-product to regulate the self-incompatibility response.

Glyoxalase Goes Green: The Expanding Roles of Glyoxalase in Plants

“…Stigma-specific RNAi suppression of GLO1 in compatible B. napus Westar lines resulted in a significant reduction in pollen attachment and seed set, indicating the role of GLO1 as a compatibility factor to mediate successful pollination [60]. Interestingly, in the strong RNAi lines, during pod growth, we consistently observed development of a zone of tissue death, on the top half of the pod that radiated from the stigma (Figure 3A).…”

“…Using a DIGE (difference gel electrophoresis)-based proteomics assay, we had previously identified glyoxalase I (GLO1) as one of the major candidates downregulated following SI response in Brassica napus [68,69]. Functional analysis of the role of GLO1 during pollination using isogenic Brassica napus lines (self-compatible Westar and the self-incompatible W1) revealed that GLO1 activity is essential for compatible pollination to occur [60]. Biochemical assays to measure GLO1 activity and levels revealed a significant increase in GLO1 activity and levels following compatible pollinations (CP) within 60 min and a significant reduction in GLO1 activity and levels within 60 min after SI pollinations.…”

Characteristic Variations and Similarities in Biochemical, Molecular, and Functional Properties of Glyoxalases across Prokaryotes and Eukaryotes

“…In Arabidopsis , loss of the AtGLYI-2 gene, encoding a Zn 2+ -stimulated GLYI enzyme, has been shown to confer MG and NaCl sensitivity to the plants, with even lower doses of MG causing severe growth retardation in Arabidopsis [15]. Recently, the role of GLYI as a stigmatic compatibility factor has also been demonstrated in Brassica napus [95]. GLYI has been shown to be required for pollination to occur and is a target of the self-incompatibility system for preventing inbreeding or promoting hybrid vigor [95].…”

“…Recently, the role of GLYI as a stigmatic compatibility factor has also been demonstrated in Brassica napus [95]. GLYI has been shown to be required for pollination to occur and is a target of the self-incompatibility system for preventing inbreeding or promoting hybrid vigor [95]. Suppression of GLYI resulted in reduced compatibility, and its overexpression in self-incompatible B. napus stigmas led to a partial breakdown of the self-incompatibility response.…”

“…Suppression of GLYI resulted in reduced compatibility, and its overexpression in self-incompatible B. napus stigmas led to a partial breakdown of the self-incompatibility response. It is proposed that the degradation of GLYI after self-pollination concurrently increases MG levels and subsequent MG-mediated protein modifications (including that of GLYI), in turn triggering the MG-modified proteins to be efficiently targeted for destruction, leading to pollen rejection [95]. Furthermore, a GLYI protein, BnGLYI3 from B. napus , has been shown to be involved in seed thermotolerance, as its protein levels were significantly increased in response to heat stress and overexpression of BnGLYI-3 gene imparted heat and cold tolerance in yeast [96].…”