Alex Levine

Regulation of stomata movements is crucial for plants ability to cope with their changing environment. Guard cell’s (GC) water potential directs water flux inside/outside this cell, which eventually is causing the stoma to open or close, respectively. Some of the osmolytes which accumulates in the GC cytoplasm and are known to play a role in stomata opening are sugars, arising from chloroplast starch degradation. During stomata closure, the accumulated osmolytes are removed from the GC cytoplasm. Surprisingly little is known about prevention of starch degradation and forming additional sugars which may interfere with osmotic changes that are necessary for correct closure of stomata. One of the early events leading to stomata closure is production of reactive oxygen species (ROS) in various sub-cellular sites and organelles of the stoma. Here we report that ROS production during abscisic acid (ABA) and methyl jasmonate (MJ) stimuli in Arabidopsis GC chloroplasts were more than tripled. Moreover, ROS were detected on the sub-organelle level in compartments that are typically occupied by starch grains. This observation leads us to suspect that ROS function in that particular location is necessary for stomata closure. We therefore hypothesize that these ROS are involved in redox control that lead to the inactivation of starch degradation that takes place in these compartments, thus contributing to the stoma closure in an additional way.

Leguminous plants have exclusive ability to form symbiotic relationship with soil bacteria of the genus Rhizobium. Symbiosis is a complex process that involves multiple molecular signaling activiti...

Plants activate a number of defense reactions in response to pathogen attack. One of the major pathways involves biosynthesis of Salicylic Acid (SA), which acts as a signaling molecule that regulat...

Legumes are unique in their ability to establish symbiotic interaction with rhizobacteria from Rhizobium genus, which provide them with available nitrogen. Nodulation factors (NFs) produced by Rhizobium initiate legume root hair deformation and curling that entrap the bacteria, and allow it to grow inside the plant. In contrast, legumes and non-legumes activate defense responses when inoculated with pathogenic bacteria. One major defense pathway is mediated by salicylic acid (SA). SA is sensed and transduced to downstream defense components by a redox-regulated protein called NPR1. We used Arabidopsis mutants in SA defense pathway to test the role of NPR1 in symbiotic interactions. Inoculation of Sinorhizobium meliloti or purified NF on Medicago truncatula or nim1/npr1 A. thaliana mutants induced root hair deformation and transcription of early and late nodulins. Application of S. meliloti or NF on M. truncatula or A. thaliana roots also induced a strong oxidative burst that lasted much longer than in plants inoculated with pathogenic or mutualistic bacteria. Transient overexpression of NPR1 in M. truncatula suppressed root hair curling, while inhibition of NPR1 expression by RNAi accelerated curling. We show that, while NPR1 has a positive effect on pathogen resistance, it has a negative effect on symbiotic interactions, by inhibiting root hair deformation and nodulin expression. Our results also show that basic plant responses to Rhizobium inoculation are conserved in legumes and non-legumes.

Intracellular localization of stress induced reactive oxygen species (ROS) has emerged as an important aspect towards understanding of cellular responses to environmental stimuli. Our recent study published in the PNAS (103:18008–13)1 shows that NaCl-induced ROS appear within endosomes on the way to tonoplast as part of the vacuolar vesicle trafficking. In addition to showing ROS damage to the tonoplast, this finding may shed light upon recently reported aspects of root water relations during salt stress, suggesting a new signaling role for intracellular ROS in Arabidopsis root cells, during salt stress: ROS that are compartmentalized in endosomes are delivered by the vacuolar vesicle trafficking pathway to the tonoplast, resulting in oxidative gating of TIPs water channels. The closure of the tonoplast aquaporins contributes to the observed reduction in root hydraulic conductivity during salt stress.

Activation of aspartate‐specific cysteine proteases (caspases) plays a crucial role in programmed cell death (PCD) in animals. Although to date caspases have not been identified in plants, caspase‐like activity was described in tobacco during a hypersensitive response to pathogens and in Arabidopsis and tomato cell cultures during chemical‐induced PCD. Caspase‐like activity was also detected in the course of plant development during petal senescence and endosperm PCD. It is shown here that caspase‐like proteases play a crucial role in the developmental cell death of secondary shoots of pea seedlings that emerge after removal of the epicotyl. Caspase‐like activity was induced in senescing secondary shoots, but not in dominant growing shoots, in contrast to the papain‐like cysteine protease activity that was stronger in the dominant shoot. Revitalization of the senescing shoot by cutting of the dominant shoot reduced the caspase‐like activity. Injection of caspase or cysteine protease inhibitors into the remaining epicotyl tissue suppressed the death of the secondary shoots, producing seedlings with two equal shoots. These results suggest that shoot selection in pea seedlings is controlled by PCD, through the activation of caspase‐like proteases.