PP2A and microtubules function in 5-aminolevulinic acid-mediated H2 O2 signaling in Arabidopsis guard cells

ALA induces stomatal opening through regulation among PTPA, PP2AC, and SnRK2.6

5-Aminolevulinic acid (ALA), as a new natural plant growth regulator, has been proved to regulate protein phosphatase 2A (PP2A) activity to promote stomatal opening in apple (Malus domestica) leaves. However, the molecular mechanisms underlying remain unclear. Here, we cloned and transformed MdPTPA, MdPP2AC, and MdSnRK2.6 of apple into tobaccos (Nicotiana tabacum) and found that over-expression (OE)-MdPTPA or OE-MdPP2AC promoted stomatal aperture while OE-MdSnRK2.6 induced stomatal closure under normal or drought condition. The Ca2+ and H2O2 levels in the guard cells of OE-MdPTPA and OE-MdPP2AC was decreased but flavonols increased, and the results in OE-SnRK2.6 was contrary. Exogenous ALA stimulated PP2A activity but depressed SnRK2.6 activity in transgenic tobaccos, leading to less Ca2+, H2O2 and more flavonols in guard cells, and consequently stomatal opening. OE-MdPTPA improved stomatal opening and plant growth but impaired drought tolerance, while OE-MdSnRK2.6 improved drought tolerance but depressed the leaf P n. Only OE-MdPP2AC improved stomatal opening, leaf P n, plant growth, as well as drought tolerance. These suggest that the three genes involved in ALA-regulating stomatal movement have their respective unique biological functions. Yeast two-hybrid (Y2H) assays showed that MdPP2AC interacted with MdPTPA or MdSnRK2.6, respectively, but no interaction of MdPTPA with MdSnRK2.6 was found. Yeast three-hybrid (Y3H) assay showed that MdPTPA promoted the interactions between MdPP2AC and MdSnRK2.6. Therefore, we propose a regulatory module of PTPA-PP2AC-SnRK2.6 that may be involved in mediating the ALA-inducing stomatal aperture in green plants.

Microtubule Reorganization During ABA-Induced Stomatal Closure in Arabidopsis

Plants must cope with diverse environmental stresses during growth and development, among which drought is one of the most concerning global threats. Recent studies have shown that the disassembly of the microtubule cytoskeleton plays an essential role during ABA-induced stomatal closure in response to drought stress. To facilitate studies on the mechanisms of ABA-induced microtubule rearrangement during stomatal closure, we describe procedures for observing guard cells treated with ABA, visualizing the microtubule cytoskeleton in guard cells, and their subsequent quantitative analysis. We include both representative images and the quantification results to illustrate these experiments.

ALA Upregulates MdPTPA Expression to Increase the PP2A Activity and Promote Stomatal Opening in Apple Leaves

5-Aminolevulinic acid (ALA) is a new natural plant growth regulator that inhibits abscisic acid (ABA)-induced stomatal closure. Studies have shown that protein phosphatase 2 A (PP2A) is involved in ALA-ABA antagonistically regulating stomatal movement; however, the molecular mechanisms underlying remain unclear. Here, we report that ALA promoted MdPP2A activity and the MdPP2AC expression in the epidermis of apple (Malus × domestica Borkh. cv. Fuji) leaves. Y2H (Yeast two hybrid), BiFC (Bimolecular fluorescence complement), and FLC (Firefly luciferase complementation imaging assay) analysis showed that MdPP2AC interacted with MdPTPA, a phosphortyrosyl phosphatase activator. Furthermore, the transient overexpression or interference-expression of MdPTPA transgenic apple leaves were developed. The results showed that overexpression of MdPTPA promoted stomatal opening by reducing Ca²⁺ and H₂O₂ but increasing flavonols in guard cells. Conversely, when the MdPTPA was silenced in transient transgenic apple leaves, the Ca²⁺, H₂O₂ and flavonols in guard cells and stomatal movement were completely conversed. In the transgenic apple leaves, exogenous ALA stimulated PP2A but repressed SnRK2.6 activity, while the responses are the same as that in the wild type. Therefore, we propose that MdPTPA, which increases the PP2A activity, mediates ALA signaling to promote stomatal opening in apple leaves.

Exogenous 5-aminolevulinic acid alleviates low-temperature damage by modulating the xanthophyll cycle and nutrient uptake in tomato seedlings

5-Aminolevulinic acid (ALA), an antioxidant existing in plants, has been widely reported to participate in the process of coping with cold stress of plants. In this study, exogenous ALA promoted the growth of tomato plants and alleviated the appearance of purple tomato leaves under low-temperature stress. At the same time, exogenous ALA improved antioxidant enzyme activities, SlSOD gene expression, Fv/Fm, and proline contents and reduced H₂O₂ contents, SlRBOH gene expression, relative electrical conductivity, and malondialdehyde contents to alleviate the damage caused by low temperature to tomato seedlings. Compared with low-temperature stress, spraying exogenous ALA before low-temperature stress could restore the indicators of photochemical quenching, actual photochemical efficiency, electron transport rate, and nonphotochemical quenching to normal. Exogenous ALA could increase the total contents of the xanthophyll cycle pool, the positive de-epoxidation rate of the xanthophyll cycle and improved the expression levels of key genes in the xanthophyll cycle under low-temperature stress. In addition, we found that exogenous ALA significantly enhanced the absorption of mineral nutrients, promoted the transfer and distribution of mineral nutrients to the leaves, and improved the expression levels of mineral nutrient absorption-related genes, which were all conducive to the improved adaptation of tomato seedlings under low-temperature stress. In summary, the application of exogenous ALA can increase tomato seedlings' tolerance to low-temperature stress by improving the xanthophyll cycle and the ability of the absorption of mineral nutrients in tomato seedlings.

Effect of 5-Aminolevulinic Acid (5-ALA) on Leaf Chlorophyll Fast Fluorescence Characteristics and Mineral Element Content of Buxus megistophylla Grown along Urban Roadsides

It is well known that trees grown on roadsides suffer from stressful environments, including poor soils, bad weather, and harmful gases from automobile exhaust. Improving the adaptability of roadside trees to adverse environments is important for urban management. An experiment was carried out with six-year-old Buxus megistophylla Levl. hedgerows, where 20 mg/L 5-aminolevulinic acids (5-ALA) solution was sprayed on the blade surface at the end of April. Three months later, plant morphology, chlorophyll fast fluorescence characteristics, antioxidant enzyme activities and the mineral element content were investigated. The results showed that leaf size and thickness were significantly greater with 5-ALA treatment, and the leaf color was also greener than those of the control. 5-ALA treatment significantly promoted the electron transfer activity of the PSII reaction center on the donor side, the reaction center itself and the receptor side. It reduced energy dissipation through the heat with increased photochemical quantum yields. The activities of antioxidant enzymes, including superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) in leaves and roots, were stimulated by 5-ALA treatment. The content of soluble sugars and free proline in leaves was significantly increased by 5-ALA treatment, as were the absorption and accumulation of several kinds of mineral nutrient elements, such as nitrogen, phosphate, calcium, magnesium, iron, copper and boron. Additionally, 5-ALA application significantly increased the content of cadmium, mercury, chromium and lead in the roots but decreased them in the leaves. This implies that 5-ALA may induce a mechanism in B. megistophylla in which toxic elements were intercepted in roots to avoid accumulation in leaves, which ensured healthy growth of the aboveground tissues. 5-ALA may regulate the absorption and utilization of mineral nutrient elements in soil with the interception of toxic heavy metal elements in roots, promote leaf photosynthetic performance, induce the accumulation of soluble sugars and free proline, and improve the antioxidant enzyme systems for plants to adapt to the stressful environment of urban roads. These results provide a basis for 5-ALA applications alongside city roads.

Arabidopsis KASH Proteins SINE1 and SINE2 Are Involved in Microtubule Reorganization During ABA-Induced Stomatal Closure

Abscisic acid (ABA) induces stomatal closure by utilizing complex signaling mechanisms, allowing for sessile plants to respond rapidly to ever-changing environmental conditions. ABA regulates the activity of plasma membrane ion channels and calcium-dependent protein kinases, Ca2+ oscillations, and reactive oxygen species (ROS) concentrations. Throughout ABA-induced stomatal closure, the cytoskeleton undergoes dramatic changes that appear important for efficient closure. However, the precise role of this cytoskeletal reorganization in stomatal closure and the nature of its regulation are unknown. We have recently shown that the plant KASH proteins SINE1 and SINE2 are connected to actin organization during ABA-induced stomatal closure but their role in microtubule (MT) organization remains to be investigated. We show here that depolymerizing MTs using oryzalin can restore ABA-induced stomatal closure deficits in sine1-1 and sine2-1 mutants. GFP-MAP4-visualized MT organization is compromised in sine1-1 and sine2-1 mutants during ABA-induced stomatal closure. Loss of SINE1 or SINE2 results in loss of radially organized MT patterning in open guard cells, aberrant MT organization during stomatal closure, and an overall decrease in the number of MT filaments or bundles. Thus, SINE1 and SINE2 are necessary for establishing MT patterning and mediating changes in MT rearrangement, which is required for ABA-induced stomatal closure.