The Plant Invertase/Pectin Methylesterase Inhibitor Superfamily

Abstract

Invertases (INVs) and pectin methylesterases (PMEs) are essential enzymes coordinating carbohydrate metabolism, stress responses, and sugar signaling. INVs catalyzes the cleavage of sucrose into glucose and fructose, exerting a pivotal role in sucrose metabolism, cellulose biosynthesis, nitrogen uptake, reactive oxygen species scavenging as well as osmotic stress adaptation. PMEs exert a dynamic control of pectin methylesterification to manage cell adhesion, cell wall porosity, and elasticity, as well as perception and signaling of stresses. INV and PME activities can be regulated by specific proteinaceous inhibitors, named INV inhibitors (INVIs) and PME Inhibitors (PMEIs). Despite targeting different enzymes, INVIs and PMEIs belong to the same large protein family named "Plant Invertase/Pectin Methylesterase Inhibitor Superfamily." INVIs and PMEIs, while showing a low aa sequence identity, they share several structural properties. The two inhibitors showed mainly alpha-helices in their secondary structure and both form a non-covalent 1:1 complex with their enzymatic counterpart. Some PMEI members are organized in a gene cluster with specific PMEs. Although the most important physiological information was obtained in Arabidopsis thaliana, there are now several characterized INVI/PMEIs in different plant species. This review provides an integrated and updated overview of this fascinating superfamily, from the specific activity of characterized isoforms to their specific functions in plant physiology. We also highlight INVI/PMEIs as biotechnological tools to control different aspects of plant growth and defense. Some isoforms are discussed in view of their potential applications to improve industrial processes. A review of the nomenclature of some isoforms is carried out to eliminate confusion about the identity and the names of some INVI/PMEI member. Open questions, shortcoming, and opportunities for future research are also presented.

Keywords: CW integrity; biotechnological applications; degree of methylesterification; invertase inhibitors; pectin methylesterase inhibitors; plant growth and defence; sucrose metabolism.

Figure 1

(A) Invertase inhibitor (INVI) and pectin methylesterase inhibitor (PMEI) structural organizations. The INVI/PMEIs domain are preceded by a signal peptide (SP) or a transmembrane domain (TM) for the targeting to the endomembrane system leading to the different subcellular localization. PMEI/INVIs can possess one, both, and neither of these motifs. Some PMEIs (from 1 to 3 isoforms; also named PRO region) can be clustered with a C-terminal PME. (B) Sucrose hydrolysis by invertase activity yielding glucose and fructose. (C) De-methylesterification of homogalacturonan (HG) by pectin methylesterases, with consequent production of negatively charged carboxyl groups, methanol, and protons.

Figure 2

Crystal structure of different INVI/PMEI isoforms alone or in complex with their enzymatic counterpart. (A) NtCIF (PDB ID; 1RJ1) and (B) NtCIF in complex with AtCWINV-1 (PDB ID; 2XQR). (C) AtPMEI1 dimer (PDB ID; IX8Z) and (D) AcPMEI in complex with SlPME-1 (PDB ID; 1Xg2). (E) Sequence Comparison of AtPMEI1 and NtCIF. The most conserved motifs are highlighted in light blue for PMEI and in red for INVI. The four cysteine conserved in both inhibitors are highlighted in green. The α symbols followed by numbers indicates the different alpha-helices.

Figure 3

Subcellular localization and activities of INVs and INVIs. Sucrose can be cleaved in the apoplast by a CW-INV in glucose and fructose, which are transported into the cytoplasm by a hexose transporter. Sucrose can also be directly loaded by specific sucrose transporters into the cytosol or in the vacuole where it is cleaved by neutral invertase (N-INV; or sucrose synthase) or V-INV, respectively. The hexoses generated by INV activity can serve as substrate for growth as well as can regulate gene expression during growth and defense. Modulations of INVs by INVIs are dependent from different environmental stimuli and can influence different physiological processes. FRU, fructose; glc, glucose; SUC, Sucrose. ST, Sucrose Transporter; HT, Hexose Transporter.

Figure 4

Subcellular localization and activities of PMEI-PMEs and PMEIs. HG is methylesterified in the golgi apparatus, where PMEIs can avoid a premature pectin de-methylesterification by PME, which could cause a pectin jellification. Pectin is secreted in the apoplast in a high methylesterified form. In this compartment, a fine-tuning of PME activity is exerted by independent and clustered PMEIs to regulate the degree and pattern of pectin methylesterification in various plant physiology processes. Subtilisin-like proteases (Subtilases) can degrade the processing motif of PMEI-PME catalyzing the separation of the inhibitor from the PME domain.

Figure 5

Overview of INVIs and PMEIs functions in plant growth and development. INVIs control seed germination, sugar transport in roots; senescence and sugar fruit content. PMEIs play multiple roles in several physiological processes, such as pollen tube elongation, seed mucilage extrusion, and modification, flowering transition, silique development, mechanical strength of stem, phyllotaxis, rhyzotaxis, pavement cells morphogenesis, hypocotyl growth in the dark, and root growth, and they are also involved in fruit development and ripening.

Figure 6

Schematic representation of the involvement of INVI/PMEI members in plant-environment interactions. Plants face different pathogens and pests, such as bacteria, nematodes, fungi, viruses, and insects as well as multiple abiotic stresses. Different INVI/PMEI members have a pivotal role plant-microbe associations beneficial to the host plant like also during multiple plant-pathogen interactions. PMEIs and INVIs are also involved in plant responsiveness to different abiotic stresses and in heavy metal tolerance.