Gravity‐dependent differentiation and root coils in Arabidopsis thaliana wild type and phospholipase‐A‐I knockdown mutant grown on the International Space Station

Abstract

Arabidopsis roots on 45° tilted agar in 1-g grow in wave-like figures. In addition to waves, formation of root coils is observed in several mutants compromised in gravitropism and/or auxin transport. The knockdown mutant ppla-I-1 of patatin-related phospholipase-A-I is delayed in root gravitropism and forms increased numbers of root coils. Three known factors contribute to waving: circumnutation, gravisensing and negative thigmotropism. In microgravity, deprivation of wild type (WT) and mutant roots of gravisensing and thigmotropism and circumnutation (known to slow down in microgravity, and could potentially lead to fewer waves or increased coiling in both WT and mutant). To resolve this, mutant ppla-I-1 and WT were grown in the BIOLAB facility in the International Space Station. In 1-g, roots of both types only showed waving. In the first experiment in microgravity, the mutant after 9 days formed far more coils than in 1-g but the WT also formed several coils. After 24 days in microgravity, in both types the coils were numerous with slightly more in the mutant. In the second experiment, after 9 days in microgravity only the mutant formed coils and the WT grew arcuated roots. Cell file rotation (CFR) on the mutant root surface in microgravity decreased in comparison to WT, and thus was not important for coiling. Several additional developmental responses (hypocotyl elongation, lateral root formation, cotyledon expansion) were found to be gravity-influenced. We tentatively discuss these in the context of disturbances in auxin transport, which are known to decrease through lack of gravity.

Physiological responses of a halophytic shrub to salt stress by Na2SO4 and NaCl: oxidative damage and the role of polyphenols in antioxidant protection

“…In agreement with our previous results (Reginato et al 2014), P. strombulifera showed a greater sensitivity to Na 2 SO 4 than to NaCl, as indicated by the important oxidative damage induced in tissues when the SO 4 2− anion is present in the medium. Malondialdehyde is considered a reliable indicator of oxidative stress resulting from degradation of membrane lipids under several abiotic constraints (Zhang and Kirkham 1996; Hernández et al 2001).…”

“…Stimulation of shoot growth at Ψ o values up to −1.9 MPa (500 mM) NaCl is an interesting halophytic response found in our studies (Reginato et al 2014). Findings in other Prosopis species indicate that the NaCl tolerance of P. strombulifera exceeds the limits described for most halophytic plants (Catalán et al 1994; Almeida Viégas et al 2004; Felker 2007).…”

“…These authors argued that truly salt-tolerant species possessing efficient mechanisms for Na + exclusion from the cytosol may not require a high level of antioxidant activity, as they simply do not allow excessive ROS production in the first instance. This would be the case of NaCl-treated plants of P. strombulifera , whose capacity for ion compartmentation and osmoregulation in their tissues and adequate hydric equilibrium has been demonstrated in our previous work (Llanes et al 2013; Reginato et al 2014). …”

“…However, P. strombulifera is much less tolerant to Na 2 SO 4 than to NaCl. Plants grown in the presence of Na 2 SO 4 showed immediate and significant reduction of shoot height and leaf number per plant, accompanied by senescence symptoms such as chlorosis, necrosis and leaf abscission (Reginato et al 2014). …”

Molecular mechanisms of gravity perception and signal transduction in plants

“…Genetic studies of the secretory phospholipase А 2 β in relation to the auxin sensitivity of its gene expression indicated that the enzyme is needed for stem and hypocotyl gravitropism and auxin action during gravity response (Lee et al 2003). A knockdown mutant of patatin-related phospholipase A-I (ppla-I-1) was shown to have delayed gravitropic responses in roots, hypocotyls, and flower stalks (Scherer and Pietrzyk 2014). A possible reason for this is that the enzyme mediates auxin-induced gene expression (Effendi et al 2014).…”