Expansins: roles in plant growth and potential applications in crop improvement

Key message

Results from various expansin related studies have demonstrated that expansins present an opportunity to improve various crops in many different aspects ranging from yield and fruit ripening to improved stress tolerance.

Abstract

The recent advances in expansin studies were reviewed. Besides producing the strength that is needed by the plants, cell walls define cell shape, cell size and cell function. Expansins are cell wall proteins which consist of four sub families; α-expansin, β-expansin, expansin-like A and expansin-like B. These proteins mediate cell wall loosening and they are present in all plants and in some microbial organisms and other organisms like snails. Decades after their initial discovery in cucumber, it is now clear that these small proteins have diverse biological roles in plants. Through their ability to enable the local sliding of wall polymers by reducing adhesion between adjacent wall polysaccharides and the part they play in cell wall remodeling after cytokinesis, it is now clear that expansins are required in almost all plant physiological development aspects from germination to fruiting. This is shown by the various reports from different studies using various molecular biology approaches such as gene achieve these many roles through their non-enzymatic wall loosening ability. This paper reviews and summarizes some of the reported functions of expansins and outlines the potential uses of expansins in crop improvement programs.

     

Expansins and cell growth

Expansins are now generally accepted to be key regulators of wall extension during growth. Several alternative roles for expansins have emerged in which the emphasis of their action is on wall breakdown or softening in processes such as fruit ripening, pollination, germination and abscission. Expansins are commonly encoded by substantial gene families and have classically been divided into two subfamilies, referred to as alpha- and beta-expansins. Two further subfamilies have now been identified: gamma-expansins, which were first described in Arabidopsis, and delta-expansins, which were identified in rice and are absent from Arabidopsis. Both are truncated versions of alpha- and beta-expansins, with gamma-expansins representing the amino-terminal half of a mature expansin and delta-expansins the carboxy-terminal half of a beta-expansin. Functional roles for gamma- and delta-expansins have yet to be defined, although recent data indicate a signalling role for gamma-expansins.     

Plant growth modelling and applications: the increasing importance of plant architecture in growth models

Background

Modelling plant growth allows us to test hypotheses and carry out virtual experiments concerning plant growth processes that could otherwise take years in field conditions. The visualization of growth simulations allows us to see directly and vividly the outcome of a given model and provides us with an instructive tool useful for agronomists and foresters, as well as for teaching. Functional–structural (FS) plant growth models are nowadays particularly important for integrating biological processes with environmental conditions in 3-D virtual plants, and provide the basis for more advanced research in plant sciences.

Scope

In this viewpoint paper, we ask the following questions. Are we modelling the correct processes that drive plant growth, and is growth driven mostly by sink or source activity? In current models, is the importance of soil resources (nutrients, water, temperature and their interaction with meristematic activity) considered adequately? Do classic models account for architectural adjustment as well as integrating the fundamental principles of development? Whilst answering these questions with the available data in the literature, we put forward the opinion that plant architecture and sink activity must be pushed to the centre of plant growth models. In natural conditions, sinks will more often drive growth than source activity, because sink activity is often controlled by finite soil resources or developmental constraints.

PMA06

This viewpoint paper also serves as an introduction to this Special Issue devoted to plant growth modelling, which includes new research covering areas stretching from cell growth to biomechanics. All papers were presented at the Second International Symposium on Plant Growth Modeling, Simulation, Visualization and Applications (PMA06), held in Beijing, China, from 13–17 November, 2006. Although a large number of papers are devoted to FS models of agricultural and forest crop species, physiological and genetic processes have recently been included and point the way to a new direction in plant modelling research.Key words: Biomechanics, carbon allocation, functional–structural plant models, meristem, nitrogen, phenotypic plasticity, root architecture, simulation, sink, source, PMA06     

Modeling plant growth and development

Computational plant models or 'virtual plants' are increasingly seen as a useful tool for comprehending complex relationships between gene function, plant physiology, plant development, and the resulting plant form. The theory of L-systems, which was introduced by Lindemayer in 1968, has led to a well-established methodology for simulating the branching architecture of plants. Many current architectural models provide insights into the mechanisms of plant development by incorporating physiological processes, such as the transport and allocation of carbon. Other models aim at elucidating the geometry of plant organs, including flower petals and apical meristems, and are beginning to address the relationship between patterns of gene expression and the resulting plant form.     

Expansins: ever-expanding numbers and functions

Expansins were first identified as cell-wall-loosening proteins that, at least in part, mediate pH-dependent extension of the plant cell wall and growth of the cell. More recently, it has been realized that expansins belong to two protein families, the alpha-and beta-expansins, and that they appear to be involved in regulating, besides cell expansion, a variety of plant processes, including morphogenesis, softening of fruits, and growth of the pollen tube of grasses through the stigma and the style. The Arabidopsis genome contains 26 alpha-expansin genes and the rice genome at least 26. There are more beta-expansin genes in monocots than in dicots, at least 14 in rice and five in Arabidopsis. Expansin genes are differentially regulated by environmental and hormonal signals, and hormonal regulatory elements have been found in their promoter regions. An analysis of exon/intron structure led to the hypothesis that alpha-and beta-expansins evolved from a common ancestral gene.     

Cobalt and plant development: interactions with ethylene in hypocotyl growth

Co²⁺ promoted elongation of hypocotyl segments of light-grown cucumber (Cucumis sativus) seedlings. Time course and dose response data are presented and interactions with IAA, gibberellin, cyclohexanol, and cotyledons described. Segments without cotyledons responded to Co²⁺ only if grown in gas-tight vessels with IAA added. When bases of cotyledons were ringed with an inhibitor of auxin transport, Co²⁺ caused no growth promotion in the hypocotyl. Co²⁺ prevented lateral swelling of hypocotyls treated with supraoptimal IAA. Removal of ethylene from the atmosphere reduced the Co²⁺ response, but Co²⁺ did not counteract the inhibitory effect of increased ethylene levels. These results are consistent with the hypothesis that Co²⁺ promotes hypocotyl elongation by inhibiting ethylene production. The hypothesis was confirmed by a direct demonstration that Co²⁺, at growth-promoting concentrations, powerfully inhibited ethylene production in the cucumber hypocotyl.     

Sonication and ultrasound: impact on plant growth and development

Plant biotechnology, and plant tissue culture in particular, could benefit from new means to stimulate plant growth and development. Although the number of studies is still limited, there is evidence that sonication using low frequencies of sound (as little as a few dozen Hz) to as high as ultrasound (several dozen kHz) may increase organogenesis. In this brief review, we look at those examples in detail and explore how sound influences growth and development. Where available, we try to offer a mechanism by which sound affects or influences plant growth.     

Cadaverine: A lysine catabolite involved in plant growth and development

The cadaverine (Cad) a diamine, imino compound produced as a lysine catabolite is also implicated in growth and development of plants depending on environmental condition. This lysine catabolism is catalyzed by lysine decarboxylase, which is developmentally regulated. However, the limited role of Cad in plants is reported, this review is tempted to focus the metabolism and its regulation, transport and responses, interaction and cross talks in higher plants. The Cad varied presence in plant parts/products suggests it as a potential candidate for taxonomic marker as well as for commercial exploitation along with growth and development.     

Characterization of the VIER F-BOX PROTEINE genes from Arabidopsis reveals their importance for plant growth and development

E3 ubiquitin ligases (E3s) target proteins for degradation by the 26S proteasome. In SKP1/CDC53/F-box protein-type E3s, substrate specificity is conferred by the interchangeable F-box protein subunit. The vast majority of the 694 F-box proteins encoded by the Arabidopsis thaliana genome remain to be understood. We characterize the VIER F-BOX PROTEINE (VFB; German for FOUR F-BOX PROTEINS) genes from Arabidopsis that belong to subfamily C of the Arabidopsis F-box protein superfamily. This subfamily also includes the F-box proteins TRANSPORT INHIBITOR RESPONSE1 (TIR1)/AUXIN SIGNALING F-BOX (AFB) proteins and EIN3 BINDING F-BOX proteins, which regulate auxin and ethylene responses, respectively. We show that loss of VFB function causes delayed plant growth and reduced lateral root formation. We find that the expression of a number of auxin-responsive genes and the activity of DR5:beta-glucuronidase, a reporter for auxin response, are reduced in the vfb mutants. This finding correlates with an increase in the abundance of an AUXIN/INDOLE-3-ACETIC ACID repressor. However, we also find that auxin responses are not affected in the vfb mutants and that a representative VFB family member, VFB2, cannot functionally complement the tir1-1 mutant. We therefore exclude the possibility that VFBs are functional orthologs of TIR1/AFB proteins.     

Steroid hormones and plant growth and development

The occurrence of steroid hormones in plants is briefly reviewed. Their effects on plant growth, development and flowering are also considered.     

Expansins in growing tomato leaves

An expansin-like protein from growing tomato leaves was identified by its ability to restore the 'acid-growth' response to heat-inactivated tomato walls and by its similarity to expansins from cucumber hypocotyls. Native walls from growing tomato leaves exhibit an endogenous acid-induced extension (creep) that resembles in various biochemical characteristics the acid-growth activity of cucumber hypocotyls. For example, the acid-growth activity is lost when the walls of tomato leaves are briefly heated and is largely restored by addition of a crude protein extract from the walls of growing leaves. Wall proteins from growing leaves enhance the stress relaxation spectrum of tomato walls in a fashion characteristic of cucumber expansins. HPLC fractionation of the crude wall protein from tomato leaves yielded an active fraction containing a major 27 kDa protein that cross-reacts with an antibody raised against cucumber expansin. The results show that tomato leafwalls possess at least one expansin that is responsible for the acid-growth property of leaves and indicate that cell wall extension in leaves shares an underlying protein mechanism common to cell wall expansion in stems.     

Reverse genetic characterization of two paralogous acetoacetyl CoA thiolase genes in Arabidopsis reveals their importance in plant growth and development

Acetoacetyl CoA thiolase (AACT, EC 2.3.1.9) catalyzes the condensation of two acetyl CoA molecules to form acetoacetyl CoA. Two AACT-encoding genes, At5g47720 (AACT1) and At5g48230 (AACT2), were functionally identified in the Arabidopsis genome by direct enzymological assays and functional expression in yeast. Promoter::GUS fusion experiments indicated that AACT1 is primarily expressed in the vascular system and AACT2 is highly expressed in root tips, young leaves, top stems and anthers. Characterization of T-DNA insertion mutant alleles at each AACT locus established that AACT2 function is required for embryogenesis and for normal male gamete transmission. In contrast, plants lacking AACT1 function are completely viable and show no apparent growth phenotypes, indicating that AACT1 is functionally redundant with respect to AACT2 function. RNAi lines that express reduced levels of AACT2 show pleiotropic phenotypes, including reduced apical dominance, elongated life span and flowering duration, sterility, dwarfing, reduced seed yield and shorter root length. Microscopic analysis reveals that the reduced stature is caused by a reduction in cell size and fewer cells, and male sterility is caused by loss of the pollen coat and premature degeneration of the tapetal cells. Biochemical analyses established that the roots of AACT2 RNAi plants show quantitative and qualitative alterations in phytosterol profiles. These phenotypes and biochemical alterations are reversed when AACT2 RNAi plants are grown in the presence of mevalonate, which is consistent with the role of AACT2 in generating the bulk of the acetoacetyl CoA precursor required for the cytosol-localized, mevalonate-derived isoprenoid biosynthetic pathway.     

Salicylic acid beyond defence: its role in plant growth and development

In recent years salicylic acid (SA) has been the focus of intensive research due to its function as an endogenous signal mediating local and systemic plant defence responses against pathogens. It has also been found that SA plays a role during the plant response to abiotic stresses such as drought, chilling, heavy metal toxicity, heat, and osmotic stress. In this sense, SA appears to be, just like in mammals, an 'effective therapeutic agent' for plants. Besides this function during biotic and abiotic stress, SA plays a crucial role in the regulation of physiological and biochemical processes during the entire lifespan of the plant. The discovery of its targets and the understanding of its molecular modes of action in physiological processes could help in the dissection of the complex SA signalling network, confirming its important role in both plant health and disease. Here, the evidence that supports the role of SA during plant growth and development is reviewed by comparing experiments performed by exogenous application of SA with analysis of genotypes affected by SA levels and/or perception.