Abstract
Pectin, a major component of the primary cell walls of dicot plants, is synthesized in Golgi, secreted into the wall as methylesters and subsequently de-esterified by pectin methylesterase (PME). Pectin remodelling by PMEs is known to be important in regulating cell expansion in plants, but has been poorly studied in cotton. In this study, genome-wide analysis showed that PMEs are a large multi-gene family (81 genes) in diploid cotton (Gossypium raimondii), an expansion over the 66 in Arabidopsis and suggests the evolution of new functions in cotton. Relatively few PME genes are expressed highly in fibres based on EST abundance and the five most abundant in fibres were cloned and sequenced from two cotton species. Their significant sequence differences and their stage-specific expression in fibres within a species suggest sub-specialisation during fibre development. We determined the transcript abundance of the five fibre PMEs, total PME enzyme activity, pectin content and extent of de-methylesterification of the pectin in fibre walls of the two cotton species over the first 25–30 days of fibre growth. There was a higher transcript abundance of fibre-PMEs and a higher total PME enzyme activity in G. barbadense (Gb) than in G. hirsutum (Gh) fibres, particularly during late fibre elongation. Total pectin was high, but de-esterified pectin was low during fibre elongation (5–12 dpa) in both Gh and Gb. De-esterified pectin levels rose thereafter when total PME activity increased and this occurred earlier in Gb fibres resulting in a lower degree of esterification in Gb fibres between 17 and 22 dpa. Gb fibres are finer and longer than those of Gh, so differences in pectin remodelling during the transition to wall thickening may be an important factor in influencing final fibre diameter and length, two key quality attributes of cotton fibres.
Highlights
The primary cell walls (PCWs) surrounding all dicot plant cells are composed of cellulose microfibrils complexed with interconnecting xyloglucan polymers that are all embedded in a pectin polymer matrix [1]
As pectin methylesterase (PME) are generally encoded by multigene families in most plants and only a partial fragment (Genbank Accession: ABH93358) of a single cotton PME (Carbohydrate Esterase Family 8) was present in the Carbohydrate Active enZyme (CAZy) database we sought to identify cotton PME orthologues, those expressed in fibres, from amongst the extensive collection of cotton ESTs in Public databases
102 gene models were identified in G. raimondii with a Blast hit to AtPME2 or AtPME31 and/or our 33 identified EST contigs, 21 of those were characterised as PME inhibitors (PMEI) by the NCBI Conserved Domain Database (CDD) Search tool and lacked a PME catalytic domain
Summary
The primary cell walls (PCWs) surrounding all dicot plant cells are composed of cellulose microfibrils complexed with interconnecting xyloglucan polymers that are all embedded in a pectin polymer matrix [1]. The pectic matrix provides an environment for the deposition, slippage and extension of the structural cellulosic-glycan network during cell growth, regulates cell wall porosity and is the major adhesive material between adjacent cells [2]. The porosity properties of the matrix influence hydration status and affect the movement and access of cell wall-modifying proteins that interact with the microfibrils and other polymers and proteins in the wall [2]. D-galacturonic acid containing 100–200 galacturonic acid (GalA) residues [5]. HGA is synthesized in the Golgi with 70–80% of the GalA residues methylesterified at the C-6 carboxyl position and deposited into the cell wall [6]. RG-II is not structurally related to RG-I, it has a backbone of around 9 GalA residues that are (1R4)-a-linked and is substituted by up to 4 heteropolymeric side chains [7]
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