ABO8Participates In ABA-mediated Mitochondria ROS Signaling To Regulate Root Growth

Abscisic acid (ABA) plays crucial roles in plant growth and development, as well as in plant responses to abiotic and biotic stress. ABA can stimulate the production of reactive oxygen species (ROS), which greatly influence plant growth and development under environmental stresses such as water deficiency and high salinity. Although hazardous to cells when exceeded, ROS at low concentrations are important signaling molecules that regulate stomatal movement, prevent pathogen invasion, promote programmed cell death, and redirect plant growth. Besides. ABA and ROS also regulate root growth.abo8(ABA overly sensitive8) mutant was isolated in a genetic screen, for its hypersensitivity to ABA in root growth and seed germination. ABO8, encoding a mitochondria-localized pentatricopeptide repeat (PPR) protein, is responsible for the splicing of nad4intron3. ABO8is highly expressed in primary and lateral root tips, as well as in lateral root primordia. Mitochondrial dysfunction in abo8leads to higher accumulation of ROS in the root tips of abo8than in those of wild type (WT). ABA also stimulates ROS production both in WT and abo8. Addition of the reducing agent GSH partially recovers the hypersensitivity of abo8to ABA in seed germination and root growth. These results indicate that the hypersensitivity to ABA of abo8in root growth and seed germination is caused by the high accumulation of ROS.The molecular mechanism of ROS in regulating plant growth remains obscure. As indicated by low expression level of proDR5::GUS and proIAA2::GUS, auxin accumulation/signaling is reduced in abo8, which is further downregulated by ABA treatment. Addition of GSH induces the expression of proDR5::GUS in WT, and partially recovers the low expression of proDR5::GUS in abo8. The active auxin indole-3-acetic acid (IAA) can be oxidized into the less active form of2-oxindole-3-acetic acid (OxIAA) in root tips, we thus infer that the highly accumulated ROS were responsible for the reduced auxin accumulation/signaling in the root tips of abo8.Further studies showed that the root meristem activity of abo8decreased, containing short meristem with bigger cells and more columella stem cell (CSC) layers. These phenotypes can be aggravated by ABA treatment while partially recovered by GSH treatment. Further more, ABA treatment and abo8mutation both inhibit the expression of PLETHORA1(PLT1) and PLT2, which is also partially recovered by GSH addition. Compared with WT, root growth in plt1and plt2mutants are more sensitive to ABA. abo8, plt1and abo8/plt1generate a similar number of meristem cells, while the CSC phenotype of abo8/plt1double mutant is similar as plt1. The phenotype of triple mutant abo8/plt1/plt2resembles plt1/plt2. Overexpression of PLT2greatly rescues the root meristem defeciency in abo8, with or without ABA treatment. These genetic data show that ABA and ROS both inhibit root meristem activity by down regulating the PLT expression. In summary, our results indicate that ABA-mediated ROS signaling may regulates root growth and development by controlling auxin accumulation/signaling and PLT expression-mediated modulation of root meristem activity in Arabidopsis.