Cell autonomous and non-autonomous functions of plant intracellular immune receptors in stomatal defense and apoplastic defense.

Stomatal closure defense and apoplastic defense are two major immunity mechanisms restricting the entry and propagation of microbe pathogens in plants. Surprisingly, activation of plant intracellular immune receptor NLR genes, while enhancing whole plant disease resistance, was sometimes linked to a defective stomatal defense in autoimmune mutants. Here we report the use of high temperature and genetic chimera to investigate the inter-dependence of stomatal and apoplastic defenses in autoimmunity. High temperature inhibits both stomatal and apoplastic defenses in the wild type, suppresses constitutive apoplastic defense responses and rescues the deficiency of stomatal closure response in autoimmune mutants. Chimeric plants have been generated to activate NLR only in guard cells or the non-guard cells. NLR activation in guard cells inhibits stomatal closure defense response in a cell autonomous manner likely through repressing ABA responses. At the same time, it leads to increased whole plant resistance accompanied by a slight increase in apoplastic defense. In addition, NLR activation in both guard and non-guard cells affects stomatal aperture and water potential. This study thus reveals that NLR activation has a differential effect on immunity in a cell type specific matter, which adds another layer of immune regulation with spatial information.

Effects of elevated carbon dioxide on gas exchange and photochemical and nonphotochemical quenching at low temperature in tobacco plants varying in Rubisco activity.

Elevated (700 micromol mol-1) and ambient (350 micromol mol-1) CO2 effects on total ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity, photosynthesis (A), and photoinhibition during 6 d at low temperature were measured on wild type (WT), and rbcS antisense DNA mutants (T3) of tobacco (Nicotiana tabacum L.) with 60% of WT total Rubisco activity. Prior to the low temperature treatment, A and quantum yield of PSII photochemistry in the light adapted state (phiPSII) were significantly lower in T3 compared to WT at each CO2 level. At this time, total nonphotochemical quenching (NPQTotal) levels were near maximal (0.75-0.85) in T3 compared to WT (0.39-0.50). A was stimulated by 107% in T3 and 25% in WT at elevated compared to ambient CO2. Pre-treatment acclimation to elevated CO2 occurred in WT resulting in lower Rubisco activity per unit leaf area and reduced stimulation of A. At low temperature, A of WT was similar at elevated and ambient CO2 while stimulation of A by elevated CO2 in T3 was reduced. In addition, at low temperature we measured significantly lower photochemical quenching at elevated CO2 compared to ambient CO2 in both genotypes. NPQTotal was similar (0.80-0.85) among all treatments. However, a larger proportion of NPQTotal was composed of qI,d, the damage subcomponent of the more slowly relaxing NPQ component, qI, in both genotypes at elevated compared to ambient CO2. Greater qI,d, at elevated CO2 during and after the low temperature treatment was not related to pre-treatment differences in total Rubisco activity.

Chilling responses of maize (Zea mays L.) seedlings: root hydraulic conductance, abscisic acid, and stomatal conductance.

Maize seedling water relations and abscisic acid (ABA) levels were measured over 24 h of root chilling (5.5 degrees C). At 2.5 h into chilling, leaf ABA levels increased by 40x and stomatal conductance (g(s)) decreased to 20% compared with prechill levels. Despite a rapid g(s) response to root chilling, leaf water potential (Psi(L)) of chilled seedlings decreased to -2.2 MPa resulting in a complete loss of turgor potential (psi(p)). Ineffective g(s) control early in chilling resulted from decreased root hydraulic conductance (L(r)) due to increased water viscosity and factor(s) intrinsic to the roots. After 24 h chilling, Psi(L) and psi(p) of chilled seedlings recovered to control levels due to stomatal control of transpiration and increased L(r). The impact of the temporal changes in g(s) and L(r) on maize seedling water relations during chilling was analysed using a simple, quantitative hydraulic model. It was determined that g(s) is critical to stabilizing Psi(L) at non-lethal levels in chilled seedlings at 2.5 h and 24 h chilling. However, there was also a significant contribution due to increased L(r) at 24 h chilling so that psi(p) increased to control levels. As a first step in determining the factor(s) responsible for the increase in L(r), cDNA microarrays were used to quantify the transcript levels of eight aquaporins obtained from mature root tissue at 24 h chilling. None of these were significantly up-regulated, suggesting that the increase in L(r) was not due to regulation of these aquaporins at the transcriptional level.

Gas Exchange and Co-regulation of Photochemical and Nonphotochemical Quenching in Bean during Chilling at Ambient and Elevated Carbon Dioxide.

The effects of elevated (700 micromol mol(-1)) and ambient (350 micromol mol(-1)) CO(2) on gas exchange parameters and chlorophyll fluorescence were measured on bean (Phaseolus vulgaris) during 24 h chilling treatments at 6.5 degrees C. Consistent with previous research on this cultivar, photosynthetic decline during chilling was not significantly affected by CO(2) while post-chilling recovery was more rapid at elevated compared to ambient CO(2). Our primary focus was whether there were also CO(2)-mediated differences in demand on nonphotochemical quenching (NPQ) processes during the chilling treatments. We found that photosystem II quantum yield and total NPQ were similar between the CO(2) treatments during chilling. In both CO(2) treatments, chilling caused a shift from total NPQ largely composed of q(E), the protective, rapidly responding component of NPQ, to total NPQ dominated by the more slowly relaxing q(I), related to both protective and damage processes. The switch from q(E) to q(I) during chilling was more pronounced in the elevated CO(2) plants. Using complementary plots of the quantum yields of photochemistry and NPQ we demonstrate that, despite CO(2) effects on the partitioning of NPQ into q(E) and q(I) during chilling, total NPQ was regulated at both CO(2) levels to maximize photochemical utilization of absorbed light energy and dissipate only that fraction of light energy that was in excess of the capacity of photosynthesis. Photodamage did occur during chilling but was repaired within 3 h recovery from chilling in both CO(2) treatments.