Wounded leaves of Arabidopsis thaliana show transient immunity to Botrytis cinerea, the causal agent of grey mould. Using a fluorescent probe, histological staining and a luminol assay, we now show that reactive oxygen species (ROS), including H2O2 and O2 −, are produced within minutes after wounding. ROS are formed in the absence of the enzymes Atrboh D and F and can be prevented by diphenylene iodonium (DPI) or catalase. H2O2 was shown to protect plants upon exogenous application. ROS accumulation and resistance to B. cinerea were abolished when wounded leaves were incubated under dry conditions, an effect that was found to depend on abscisic acid (ABA). Accordingly, ABA biosynthesis mutants (aba2 and aba3) were still fully resistant under dry conditions even without wounding. Under dry conditions, wounded plants contained higher ABA levels and displayed enhanced expression of ABA-dependent and ABA-reporter genes. Mutants impaired in cutin synthesis such as bdg and lacs2.3 are already known to display a high level of resistance to B. cinerea and were found to produce ROS even when leaves were not wounded. An increased permeability of the cuticle and enhanced ROS production were detected in aba2 and aba3 mutants as described for bdg and lacs2.3. Moreover, leaf surfaces treated with cutinase produced ROS and became more protected to B. cinerea. Thus, increased permeability of the cuticle is strongly linked with ROS formation and resistance to B. cinerea. The amount of oxalic acid, an inhibitor of ROS secreted by B. cinerea could be reduced using plants over expressing a fungal oxalate decarboxylase of Trametes versicolor. Infection of such plants resulted in a faster ROS accumulation and resistance to B. cinerea than that observed in untransformed controls, demonstrating the importance of fungal suppression of ROS formation by oxalic acid. Thus, changes in the diffusive properties of the cuticle are linked with the induction ROS and attending innate defenses.

The extensive support for abscisic acid (ABA) involvement in the complex regulatory networks controlling stress responses and development in plants contrasts with the relatively recent role assigned to nitric oxide (NO). Because treatment with exogenous ABA leads to enhanced production of NO, it has been widely considered that NO participates downstream of ABA in controlling processes such as stomata movement, seed dormancy, and germination. However, data on leaf senescence and responses to stress suggest that the functional interaction between ABA and NO is more complex than previously thought, including not only cooperation but also antagonism. The functional relationship is probably determined by several factors including the time- and place-dependent pattern of accumulation of both molecules, the threshold levels, and the regulatory factors important for perception. These factors will determine the actions exerted by each regulator. Here, several examples of well-documented functional interactions between NO and ABA are analysed in light of the most recent reported data on seed dormancy and germination, stomata movements, leaf senescence, and responses to abiotic and biotic stresses.

ORCID IDs: 0000-0003-1506-4683 (R.M.); 0000-0003-4881-6343 (V.H.M.); 0000-0002-4354-6295 (C.G.R.). TANGLED1 (TAN1) and AUXIN-INDUCED-IN-ROOTS9(AIR9) are microtubule-binding proteins that localize to the division site in plants. Their function in Arabidopsis (Arabidopsis thaliana) remained unclear because neither tan1 nor air9 single mutants have a strong phenotype. We show that tan1 air9 double mutants have a synthetic phenotype consisting of short, twisted roots with disordered cortical microtubule arrays that are hypersensitive to a microtubule-depolymerizing drug. The tan1 air9 double mutants have significant defects in division plane orientation due to failures in placing the new cell wall at the correct division site. Full-length TAN1 fused to yellow fluorescent protein, TAN1-YFP, and several deletion constructs were transformed into the double mutant to assess which regions of TAN1 are required for its function in root growth, root twisting, and division plane orientation. TAN1-YFP expressed in tan1 air9 significantly rescued the double mutant phenotype in all three respects. Interestingly, TAN1 missing the first 126 amino acids, TAN1-DI-YFP, failed to rescue the double mutant phenotype, while TAN1 missing a conserved middle region, TAN1-DII-YFP, significantly rescued the mutant phenotype in terms of root growth and division plane orientation but not root twisting. We use the tan1 air9 double mutant to discover new functions for TAN1 and AIR9 during phragmoplast guidance and root morphogenesis.Plant cells are typically constrained by cell walls (Cosgrove, 2005) that are also connected via plasmodesmata (Brunkard and Zambryski, 2017), and they do not migrate relative to each other. In the absence of significant cell migration, the entire plant body must be built through elegant coordination between the division, expansion, and differentiation of cells. Therefore, division plane orientation, or the spatial control of cytokinesis, has important roles in plant development and growth (Pickett-Heaps et al., 1999).The key steps of plant division plane orientation occur during interphase, G2, and mitosis. During interphase, the cortical microtubule array typically aligns perpendicular to the cell expansion axis (Baskin, 2001), and during G2 (Gunning and Wick, 1985), this promotes the formation of a land plant-specific microtubule and microfilament array called the preprophase band (PPB;Pickett-Heaps and Northcote, 1966). Although the location of the PPB accurately predicts the future division site (Gunning et al., 1978;Van Damme et al., 2007;Rasmussen et al., 2013;Lipka et al., 2014;Martinez et al., 2017), its role in division plane establishment has recently been called into question by mutants that do not form obvious PPBs but manage to produce relatively orderly roots. These plants have mutations in the tonneau1a locus (Zhang et al., 2016) and mutations in three related tonneau1 recruiting motif (trm) loci (Schaefer et al., 2017). Whether the PPB establishes the future division site or is the signpost of an ear...

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