Leaf shedding as an anti-bacterial defense in Arabidopsis cauline leaves

Abstract

Plants utilize an innate immune system to protect themselves from disease. While many molecular components of plant innate immunity resemble the innate immunity of animals, plants also have evolved a number of truly unique defense mechanisms, particularly at the physiological level. Plant’s flexible developmental program allows them the unique ability to simply produce new organs as needed, affording them the ability to replace damaged organs. Here we develop a system to study pathogen-triggered leaf abscission in Arabidopsis. Cauline leaves infected with the bacterial pathogen Pseudomonas syringae abscise as part of the defense mechanism. Pseudomonas syringae lacking a functional type III secretion system fail to elicit an abscission response, suggesting that the abscission response is a novel form of immunity triggered by effectors. HAESA/HAESA-like 2, INFLORESCENCE DEFICIENT IN ABSCISSION, and NEVERSHED are all required for pathogen-triggered abscission to occur. Additionally phytoalexin deficient 4, enhanced disease susceptibility 1, salicylic acid induction-deficient 2, and senescence-associated gene 101 plants with mutations in genes necessary for bacterial defense and salicylic acid signaling, and NahG transgenic plants with low levels of salicylic acid fail to abscise cauline leaves normally. Bacteria that physically contact abscission zones trigger a strong abscission response; however, long-distance signals are also sent from distal infected tissue to the abscission zone, alerting the abscission zone of looming danger. We propose a threshold model regulating cauline leaf defense where minor infections are handled by limiting bacterial growth, but when an infection is deemed out of control, cauline leaves are shed. Together with previous results, our findings suggest that salicylic acid may regulate both pathogen- and drought-triggered leaf abscission.

A dual function of the IDA peptide in regulating cell separation and modulating plant immunity at the molecular level

“…Several plant species can abscise infected organs to limit colonization of pathogenic microorganisms thereby adding an additional layer of defense to the innate immune system 18,19 . It has also been shown that plant pathogens can stimulate the expression of genes encoding peptide ligands suggested, or shown to regulate cell separation and promote abscission 11,19 . However, little is known as to how molecular components regulating abscission can modulate plant defense.…”

“…The HAE and HSL2 receptors have been shown to be important for Pseudomonas syringae induced cauline leaf abscission in that hae hsl2 mutant plants retain their leaves when exposed to the bacterial pathogen 19 . However, during drought induced leaf abscission the presence of a functional HAE, but not HSL2 receptor is necessary; indicating that HAE and HSL2 can diverge in signaling and that HSL2 could be exerting a different function when plants are exposed to stress 24 .…”

“…IDA and the IDL peptides control cell separation processes during plant development and in response to abiotic and biotic stress 7,14,19,24 . The regulation of IDA expression is under developmental control but can also be activated by abiotic and biotic stimuli.…”

“…As such, mediators of cell-wall alteration during plant development could have an effect on plant pathogenicity and substantiating this, it has been shown that pathogenic bacteria utilize components of the abscission signaling pathway to induce pectin degrading enzymes and ease Arabidopsis rosette leaf colonization 17 . Furthermore, plants can contain bacterial infection by shedding diseased organs 18,19 . and in Arabidopsis, cauline leaves infected with Pseudomonas syringae are abscised by utilizing the same signaling pathway that regulates floral organ abscission 19 .…”

“…Furthermore, plants can contain bacterial infection by shedding diseased organs 18,19 . and in Arabidopsis, cauline leaves infected with Pseudomonas syringae are abscised by utilizing the same signaling pathway that regulates floral organ abscission 19 .…”