The stimulation of cell extension by ethylene and auxin in aquatic plants

Dual activities of ACC synthase: Novel clues regarding the molecular evolution of ACS genes

Ethylene plays profound roles in plant development. The rate-limiting enzyme of ethylene biosynthesis is 1-aminocyclopropane-1-carboxylate (ACC) synthase (ACS), which is generally believed to be a single-activity enzyme evolving from aspartate aminotransferases. Here, we demonstrate that, in addition to catalyzing the conversion of S-adenosyl-methionine to the ethylene precursor ACC, genuine ACSs widely have C_β-S lyase activity. Two N-terminal motifs, including a glutamine residue, are essential for conferring ACS activity to ACS-like proteins. Motif and activity analyses of ACS-like proteins from plants at different evolutionary stages suggest that the ACC-dependent pathway is uniquely developed in seed plants. A putative catalytic mechanism for the dual activities of ACSs is proposed on the basis of the crystal structure and biochemical data. These findings not only expand our current understanding of ACS functions but also provide novel insights into the evolutionary origin of ACS genes.

Keeping the shoot above water - submergence triggers antithetical growth responses in stems and petioles of watercress (Nasturtium officinale)

The molecular mechanisms controlling underwater elongation are based extensively on studies on internode elongation in the monocot rice (Oryza sativa) and petiole elongation in Rumex rosette species. Here, we characterize underwater growth in the dicot Nasturtium officinale (watercress), a wild species of the Brassicaceae family, in which submergence enhances stem elongation and suppresses petiole growth. We used a genome-wide transcriptome analysis to identify the molecular mechanisms underlying the observed antithetical growth responses. Though submergence caused a substantial reconfiguration of the petiole and stem transcriptome, only little qualitative differences were observed between both tissues. A core submergence response included hormonal regulation and metabolic readjustment for energy conservation, whereas tissue-specific responses were associated with defense, photosynthesis, and cell wall polysaccharides. Transcriptomic and physiological characterization suggested that the established ethylene, abscisic acid (ABA), and GA growth regulatory module for underwater elongation could not fully explain underwater growth in watercress. Petiole growth suppression is likely attributed to a cell cycle arrest. Underwater stem elongation is driven by an early decline in ABA and is not primarily mediated by ethylene or GA. An enhanced stem elongation observed in the night period was not linked to hypoxia and suggests an involvement of circadian regulation.

Transcriptome profiling of the floating-leaved aquatic plant Nymphoides peltata in response to flooding stress

BACKGROUND

Waterlogging or flooding is one of the most challenging abiotic stresses experienced by plants. Unlike many flooding-tolerant plants, floating-leaved aquatic plants respond actively to flooding stress by fast growth and elongation of its petioles to make leaves re-floating. However, the molecular mechanisms of this plant group responding to flood have not been investigated before. Here, we investigated the genetic basis of this adaptive response by characterizing the petiole transcriptomes of a floating-leaved species Nymphoides peltata under normal and flooding conditions.

RESULTS

Clean reads under normal and flooding conditions with pooled sampling strategy were assembled into 124,302 unigenes. A total of 8883 unigenes were revealed to be differentially expressed between normal and flooding conditions. Among them, top ranked differentially expressed genes were mainly involved in antioxidant process, photosynthesis process and carbohydrate metabolism, including the glycolysis and a modified tricarboxylic acid cycle - alanine metabolism. Eight selected unigenes with significantly differentiated expression changes between normal and flooding conditions were validated by qRT-PCR.

CONCLUSIONS

Among these processes, antioxidant process and glycolysis are commonly induced by waterlogging or flooding environment in plants, whereas photosynthesis and alanine metabolism are rarely occurred in other flooding-tolerant plants, suggesting the significant contributions of the two processes in the active response of N. peltata to flooding stress. Our results provide a valuable genomic resource for future studies on N. peltata and deepen our understanding of the genetic basis underlying the response to flooding stress in aquatic plants.

Functional investigation of two 1-aminocyclopropane-1-carboxylate (ACC) synthase-like genes in the moss Physcomitrella patens

Two ACC synthase-like (ACL) proteins in the moss Physcomitrella patens have no ACS activity, and PpACL1 functions as an L -cystine/ L -cysteine C-S lyase. The ethylene biosynthetic pathway has been well characterized in higher plants, and homologs of a key enzyme in this pathway, ACS, have been reported in several algae and mosses, including Physcomitrella patens. However, the function of the ACS homologs in P. patens has not been investigated. In this research, we cloned two putative ACS genes from the P. patens genome, namely PpACS-Like 1 and 2, and investigated whether their encoded proteins had in vitro and in vivo ACS activity. In vitro biochemical assays using purified PpACL1 and PpACL2 showed that neither protein had ACS activity. Subsequently, we generated transgenic Arabidopsis lines expressing 35S:PpACL1 and 35S:PpACL2, and found that the transgenic etiolated seedlings that overexpressed either of these proteins lacked the constitutive triple response phenotype and did not emit excess levels of ethylene, indicating that neither of the PpACS-Like proteins had in vivo ACS activity. Furthermore, we found that PpACL1 functions as a C-S lyase that uses L-cystine and L-cysteine as substrates, rather than as an aminotransferase. Together, these results indicated that PpACL1 and PpACL2 are not true ACS genes as those found in higher plants.

The stimulating effects of ethylene and auxin on petiole elongation and on hyponastic curvature are independent processes in submerged Rumex palustris

The flooding-tolerant plant species Rumex palustris (Sm.) responds to complete submergence with stimulation of petiole elongation mediated by the gaseous hormone ethylene. We examined the involvement of auxin in petiole elongation. The manipulation of petiolar auxin levels by removing the leaf blade, or by addition of synthetic auxins or auxin transport inhibitors, led to the finding that auxin plays an important role in submergence-induced petiole elongation in R. palustris. A detailed kinetic analysis revealed a transient effect of removing the auxin source (leaf blade), explaining why earlier studies in which less frequent measurements were taken failed to identify any role for auxin in petiole elongation. We previously showed that the onset of stimulated petiole elongation depends on a more upright petiole angle being reached by means of hyponastic (upward) curvature, a differential growth process that is also regulated by ethylene and auxin. This raised the possibility that both ethylene and auxin stimulate elongation only indirectly by influencing hyponastic growth. We show here that the action of ethylene and auxin in promoting petiole elongation in submerged R. palustris is independent of the promoting effect that these hormones also exert on the hyponastic curvature of the same petiole.

Cell specificity in auxin- and ethylene-induced 'supergrowth' in Riella helicophylla

In gemmalings of Riella helicophylla, auxin and ethylene stimulate elongation growth, especially of pillar cells. When the two hormones are supplied simultaneously, the effects are additive, i.e. the result is 'supergrowth'. In the cells of the meristem, elongation is enhanced by auxin, but not by ethylene when given alone. However, these cells also respond with 'supergrowth' to a combined treatment with auxin and ethylene. The antiauxin p-chlorophenoxyisobutyric acid suppresses both the ethylene stimulation of cell growth and the additive 'supergrowth'. The results support the concept that auxin pre-conditions the cells to the ethylene-dependent growth event. We suggest that the response elicited by the specific cell types could be related to differences in their level of endogenous auxin.

Ethylene-induced growth and proton excretion in the aquatic plant Nymphoides peltata

In the presence of auxin, ethylene can promote growth in petioles of N. peltata (S.G. Gmel.) O. Kuntze. Acid buffer will also stimulate growth in the tissue and, in abraded petiole segments, ethylene-stimulated growth is accompanied by a marked acidification of the medium. Auxin stimulates growth in this tissue and, for various auxin and ethylene treatments, the amount of growth is closely correlated with the degree of medium acidification. The results are consistent with predictions of the 'acid-growth' theory, and provide the first evidence that ethylene acts by an 'acid-growth' mechanism.

Inhibition by silver ions of gas space (aerenchyma) formation in adventitious roots of Zea mays L. subjected to exogenous ethylene or to oxygen deficiency

We have studied the role of ethylene in accelerating the lytic formation of gas spaces (aerenchyma) in the cortex of adventitious roots of maize (Zea mays L.) growing in poorly aerated conditions. Such roots had previously been shown to contain increased concentrations of ethylene. Ten day-old maize plants bearing seminal roots and one whorl of emerging adventitious roots were grown in nutrient solution bubbled with air, ethylene in air (0.1 to 5.0 μl l(-1)), or allowed to become oxygen-deficient in nonaerated (but not completely anaerobic) solution. Additions of 0.1 μl l(-1) ethylene or more promoted the formation of aerenchyma, with lysis of up to 47% of the cortical cells. The effects of non-aeration were similar to those of exogenous ethylene. When silver ions, an ethylene antagonist, were present at low, non-toxic concentrations (circa 0.6 μM), aerenchyma formation was prevented in ethylene treated roots and in those exposed to oxygen deficiency. Silver ions also blocked the inhibiting effect of exogenous ethylene on root extension. By contrast, the suppression of aerenchyma formation by silver ions under oxygendeficient conditions was associated with a retardation of root extension, indicating the importance of aerenchyma for root growth in poorly aerated media. Rates of production of ethylene by excised roots were stimulated by a previous non-aeration treatment. The effectiveness of Ag(+) in inhibiting equally the action on cortical cells of exogenous ethylene and of non-aeration, supports the view that gas space (aerenchyma) formation in adventitious roots 'adpted' to oxygendeficient environments is mediated by increased concentrations of endogenous ethylene. The possibility that extra ethylene could arise from increased biosynthesis of a precursor in root tissues with a restricted oxygen supply is discussed.

Ethylene and auxin-induced cell growth in relation to auxin transport and metabolism and ethylene production in the semi-aquatic plant, Regnellidium diphyllum

Cell elongation in the rachis of the semiaquatic fern Regnellidium diphyllum is induced by the addition of ethylene or indoleacetic acid (IAA). Experiments with whole plants or rachis segments have shown that ethylene-induced growth requires the presence of auxin. Ethylene does not cause a modification in either endogenous auxin levels or in the extent of auxin metabolism but auxin transport is reduced. Rates of ethylene production in Regnellidium are not altered by either mechanical excitation or by the addition of auxin. A two-hormone control of cell expansion is proposed in which an initial, auxin-dependent growth event pre-conditions the cells to a further subsequent (or synchronous) ethylene-dependent growth event.

The effect of ethylene and auxin on cell wall extensibility of the Semi-aquatic fern, Regnellidium diphyllum

Ethylene and auxin both enhance cell elongation growth in the rachis of the frond of Regnellidium diphyllum. Measurements of the stress relaxation modulus of the walls of methanol-killed rachis segments show that both auxin and ethylene cause an increase in cell wall extensibility, that the effects are additive, and that they occur in the presence of hypertonic solutions of mannitol that preclude cell elongation. The results are taken as evidence for the operation of two separate mechanisms for cell wall loosening.