Promotion of Testa Rupture during Garden Cress Germination Involves Seed Compartment-Specific Expression and Activity of Pectin Methylesterases  

Abstract

Pectin methylesterase (PME) controls the methylesterification status of pectins and thereby determines the biophysical properties of plant cell walls, which are important for tissue growth and weakening processes. We demonstrate here that tissue-specific and spatiotemporal alterations in cell wall pectin methylesterification occur during the germination of garden cress (Lepidium sativum). These cell wall changes are associated with characteristic expression patterns of PME genes and resultant enzyme activities in the key seed compartments CAP (micropylar endosperm) and RAD (radicle plus lower hypocotyl). Transcriptome and quantitative real-time reverse transcription-polymerase chain reaction analysis as well as PME enzyme activity measurements of separated seed compartments, including CAP and RAD, revealed distinct phases during germination. These were associated with hormonal and compartment-specific regulation of PME group 1, PME group 2, and PME inhibitor transcript expression and total PME activity. The regulatory patterns indicated a role for PME activity in testa rupture (TR). Consistent with a role for cell wall pectin methylesterification in TR, treatment of seeds with PME resulted in enhanced testa permeability and promoted TR. Mathematical modeling of transcript expression changes in germinating garden cress and Arabidopsis (Arabidopsis thaliana) seeds suggested that group 2 PMEs make a major contribution to the overall PME activity rather than acting as PME inhibitors. It is concluded that regulated changes in the degree of pectin methylesterification through CAP-and RAD-specific PME and PME inhibitor expression play a crucial role during Brassicaceae seed germination.

Tuning of pectin methylesterification: consequences for cell wall biomechanics and development

“…Database analysis has shown that 69 genes encode putative PMEIs/invertase inhibitors in Arabidopsis (Scheler et al 2014). Monocots have fewer PMEIs, consistent with remarks above; 35 genes encoding putative PMEIs were found in rice (Wang et al 2013b).…”

“…High PME activity is thus associated with the ABA-induced delay in radicle elongation and endosperm weakening; two processes that take place prior to the completion of germination (Müller et al 2006). In addition to the strong temporal regulation of PME and PMEI expression in seeds, PMEs and PMEIs have also been found to be differentially expressed in a tissue-specific manner during seed germination of the close Arabidopsis relative Lepidium sativum (Scheler et al 2014). In support of a role for PME activity and the resulting changes in DM in the timing and process of testa rupture, changes in expression were temporally focused around testa rupture (Scheler et al 2014).…”

“…Interestingly, the PRO region is highly similar in structure to specific domains of PMEIs. Mathematical modeling predicted that the major contribution of the PRO-containing group 2 PME family proteins to overall PME activity in different Lepidium sativum seed compartments is in de-methylesterification rather than in PMEI-like activity, supporting the hypothesis that the PRO region is generally cleaved off during protein maturation (Scheler et al 2014).…”

“…In poplar (Populus trichocarpa), 89 ORFs have been annotated as putative full length PMEs compared to 41 ORFs in rice (Oryza sativa) (Coutinho et al 2003;Wang et al 2013b) Coutinho et al (2003 listed 66 ORFs in Arabidopsis annotated as PME corresponding to 6.81 % of all carbohydrate-active enzymes in the species (Coutinho et al 2003). Scheler et al (2014) identified 67 putative PMEs in Arabidopsis, with 22 falling into group 1, and 45 into group 2.…”

Genomics: A Hallmark to Monitor Molecular and Biochemical Processes Leading Toward a Better Perceptive of Seed Aging and ex-situ Conservation

“…Therefore, monitoring of seed aging needs speedy viability analysis, which should be innovative, low-cost, and equally reliable (Dona et al, 2013). Current progress in genomics and seed biology has unraveled new horizons for efficient handling and conservation of ex-situ seed germplasm under extensive storage period (Scheler et al, 2015). Therefore, genomic and biochemical markers could be used to monitor seed aging; however, comprehensive knowledge of the complicated network of molecular and biochemical processes that mitigate seed longevity is needed in order to select appropriate markers, which can then be used to gather useful information about the aging and viability of ex-situ seed stocks.…”