No abstract

Salinity is among the environmental factors that affect plant growth and development and constrain agricultural productivity. Salinity stress triggers increases in cytosolic free Ca2+ concentration ([Ca2+]i) via Ca2+ influx across the plasma membrane. Salinity stress, as well as other stresses, induces the production of reactive oxygen species (ROS). It is well established that ROS also triggers increases in [Ca2+]i. However, the relationship and interaction between salinity stress-induced [Ca2+]i increases and ROS-induced [Ca2+]i increases remain poorly understood. Using an aequorin-based Ca2+ imaging assay we have analyzed [Ca2+]i changes in response to NaCl and H2O2 treatments in Arabidopsis thaliana. We found that NaCl and H2O2 together induced larger increases in [Ca2+]i in Arabidopsis seedlings than either NaCl or H2O2 alone, suggesting an additive effect on [Ca2+]i increases. Following a pre-treatment with either NaCl or H2O2, the subsequent elevation of [Ca2+]i in response to a second treatment with either NaCl or H2O2 was significantly reduced. Furthermore, the NaCl pre-treatment suppressed the elevation of [Ca2+]i seen with a second NaCl treatment more than that seen with a second treatment of H2O2. A similar response was seen when the initial treatment was with H2O2; subsequent addition of H2O2 led to less of an increase in [Ca2+]i than did addition of NaCl. These results imply that NaCl-gated Ca2+ channels and H2O2-gated Ca2+ channels may differ, and also suggest that NaCl- and H2O2-evoked [Ca2+]i may reduce the potency of both NaCl and H2O2 in triggering [Ca2+]i increases, highlighting a feedback mechanism. Alternatively, NaCl and H2O2 may activate the same Ca2+ permeable channel, which is expressed in different types of cells and/or activated via different signaling pathways.

Inflorescence regeneration in vitro provides a simplified approach for the study of inflorescence development. In this study, high frequency of regenerated inflorescences was established using Arabidopsis stage-10 pistil as the explants on the inducing medium containing the 2 mg/L zeatin and 0.01 mg/L indole-3-acetic acid. TERMINAL FLOWER 1 (TFL1) expression was detected in callus at 6 days after transferred to inducing medium, and LEAFY (LFY) expression was detectable subsequently, suggesting that both genes play important roles as they function on inflorescence development in the plant. To investigate the formation of the stem cell organizing center, we examined the WUSCHEL (WUS) and CLAVATA3 (CLV3) expression within callus during inflorescence regeneration. WUS signals start to accumulate on callus at 4 days after induction, and then, the CLV3 signals are induced on callus at 5 days on the inflorescence-inducing medium. The expression domain of WUS is below that of CLV3, indicating that the patterns of the organizing center and stem cell formation are similar to that in zygotic and somatic embryogenesis. However, more cells of the organizing center were observed within callus than pro-embryo, suggesting that inflorescence differentiation requires more cells of the organizing center. Furthermore, it was found that the WUS expression is controlled by the ratio of cytokinin with auxin. The results suggest that other factors besides WUS and CLV3 are required for inflorescence regeneration.

To survive, plants must respond rapidly and effectively to various stress factors, including biotic and abiotic stresses. Salinity stress triggers the increase of cytosolic free Ca2+ concentration ([Ca2+]i) via Ca2+ influx across the plasma membrane, as well as bacterial flg22 and plant endogenous peptide Pep1. However, the interaction between abiotic stress-induced [Ca2+]i increases and biotic stress-induced [Ca2+]i increases is still not clear. Employing an aequorin-based Ca2+ imaging assay, in this work, we investigated the [Ca2+]i changes in response to flg22, Pep1, and NaCl treatments in Arabidopsis thaliana. We observed an additive effect on the [Ca2+]i increase which induced by flg22, Pep1, and NaCl. Our results indicate that biotic and abiotic stresses may activate different Ca2+ permeable channels. Further, calcium signal induced by biotic and abiotic stresses was independent in terms of spatial and temporal patterning.