Disruption of abscisic acid signaling constitutively activates Arabidopsis resistance to the necrotrophic fungus Plectosphaerella cucumerina.

Plant resistance to necrotrophic fungi is regulated by a complex set of signaling pathways that includes those mediated by the hormones salicylic acid (SA), ethylene (ET), jasmonic acid (JA), and abscisic acid (ABA). The role of ABA in plant resistance remains controversial, as positive and negative regulatory functions have been described depending on the plant-pathogen interaction analyzed. Here, we show that ABA signaling negatively regulates Arabidopsis (Arabidopsis thaliana) resistance to the necrotrophic fungus Plectosphaerella cucumerina. Arabidopsis plants impaired in ABA biosynthesis, such as the aba1-6 mutant, or in ABA signaling, like the quadruple pyr/pyl mutant (pyr1pyl1pyl2pyl4), were more resistant to P. cucumerina than wild-type plants. In contrast, the hab1-1abi1-2abi2-2 mutant impaired in three phosphatases that negatively regulate ABA signaling displayed an enhanced susceptibility phenotype to this fungus. Comparative transcriptomic analyses of aba1-6 and wild-type plants revealed that the ABA pathway negatively regulates defense genes, many of which are controlled by the SA, JA, or ET pathway. In line with these data, we found that aba1-6 resistance to P. cucumerina was partially compromised when the SA, JA, or ET pathway was disrupted in this mutant. Additionally, in the aba1-6 plants, some genes encoding cell wall-related proteins were misregulated. Fourier transform infrared spectroscopy and biochemical analyses of cell walls from aba1-6 and wild-type plants revealed significant differences in their Fourier transform infrared spectratypes and uronic acid and cellulose contents. All these data suggest that ABA signaling has a complex function in Arabidopsis basal resistance, negatively regulating SA/JA/ET-mediated resistance to necrotrophic fungi.

Contraction speed of the actomyosin cytoskeleton in the absence of the cell membrane

The contraction of the actomyosin cytoskeleton, which is produced by the sliding of myosin II along actin filaments, drives important cellular activities such as cytokinesis and cell migration. To explain the contraction velocities observed in such physiological processes, we have studied the contraction of intact cytoskeletons of Dictyostelium discoideum cells after removing the plasma membrane using Triton X-100. The technique developed in this work allows for the quantitative measurement of contraction rates of individual cytoskeletons. The relationship of the contraction rates with forces was analyzed using three different myosins with different in vitro sliding velocities. The cytoskeletons containing these myosins were always contractile and the contraction rate was correlated with the sliding velocity of the myosins. However, the values of the contraction rate were two to three orders of magnitude slower than expected from the in vitro sliding velocities of the myosins, presumably due to internal and external resistive forces. The contraction process also depended on actin cross-linking proteins. The lack of α-actinin increased the contraction rate 2-fold and reduced the capacity of the cytoskeleton to retain internal materials, while the lack of filamin resulted in the ATP-dependent disruption of the cytoskeleton. Interestingly, the myosin-dependent contraction rate of intact contractile rings is also reportedly much slower than the in vitro sliding velocity of myosin, and is similar to the contraction rates of cytoskeletons (different by only 2–3 fold), suggesting that the contraction of intact cells and cytoskeletons is limited by common mechanisms.