Laboratory of Phytopathology, Wageningen University, Droevendaalsesteeg 1, 6708 PB, Wageningen, The Netherlands.
Mol Plant Pathol. 2013 Jan;14(1):19-29. doi: 10.1111/j.1364-3703.2012.00825.x. Epub 2012 Sep 2.
D-Galacturonic acid is the most abundant monosaccharide component of pectic polysaccharides that comprise a significant part of most plant cell walls. Therefore, it is potentially an important nutritional factor for Botrytis cinerea when it grows in and through plant cell walls. The d-galacturonic acid catabolic pathway in B. cinerea consists of three catalytic steps converting d-galacturonic acid to pyruvate and l-glyceraldehyde, involving two nonhomologous galacturonate reductase genes (Bcgar1 and Bcgar2), a galactonate dehydratase gene (Bclgd1) and a 2-keto-3-deoxy-l-galactonate aldolase gene (Bclga1). Knockout mutants in each step of the pathway (ΔBcgar1/ΔBcgar2, ΔBclgd1 and ΔBclga1) showed reduced virulence on Nicotiana benthamiana and Arabidopsis thaliana leaves, but not on Solanum lycopersicum leaves. The cell walls of N. benthamiana and A. thaliana leaves were shown to have a higher d-galacturonic acid content relative to those of S. lycopersicum. The observation that mutants displayed a reduction in virulence, especially on plants with a high d-galacturonic acid content in the cell walls, suggests that, in these hosts, d-galacturonic acid has an important role as a carbon nutrient for B. cinerea. However, additional in vitro growth assays with the knockout mutants revealed that B. cinerea growth is reduced when d-galacturonic acid catabolic intermediates cannot proceed through the entire pathway, even when fructose is present as the major, alternative carbon source. These data suggest that the reduced virulence of d-galacturonic acid catabolism-deficient mutants on N. benthamiana and A. thaliana is not only a result of the inability of the mutants to utilize an abundant carbon source as nutrient, but also a result of the growth inhibition by catabolic intermediates.
半乳糖醛酸是果胶多糖中最丰富的单糖成分，果胶多糖构成了大多数植物细胞壁的重要部分。因此，当灰葡萄孢在植物细胞壁中生长和穿透时，半乳糖醛酸可能是其重要的营养因素。灰葡萄孢的 D-半乳糖醛酸分解代谢途径包括三个催化步骤，将 D-半乳糖醛酸转化为丙酮酸和 L-甘油醛，涉及两个非同源半乳糖醛酸还原酶基因（Bcgar1 和 Bcgar2）、一个半乳糖酸脱水酶基因（Bclgd1）和一个 2-酮-3-脱氧-L-半乳糖醛酸醛缩酶基因（Bclga1）。该途径中每个步骤的敲除突变体（ΔBcgar1/ΔBcgar2、ΔBclgd1 和 ΔBclga1）在烟草原生质体和拟南芥叶片上的毒力降低，但在番茄叶片上没有降低。与番茄叶片相比，烟草原生质体和拟南芥叶片的细胞壁具有更高的 D-半乳糖醛酸含量。观察到突变体的毒力降低，特别是在细胞壁中 D-半乳糖醛酸含量较高的植物上，表明在这些宿主中，D-半乳糖醛酸作为 B. cinerea 的碳营养物质具有重要作用。然而，对敲除突变体的额外体外生长测定表明，即使存在果糖作为主要的替代碳源，当 D-半乳糖醛酸分解代谢中间体不能通过整个途径进行时，灰葡萄孢的生长受到抑制。这些数据表明，D-半乳糖醛酸分解代谢缺陷突变体在烟草原生质体和拟南芥上的毒力降低不仅是由于突变体无法将丰富的碳源作为营养物质利用的结果，也是由于分解代谢中间体的生长抑制所致。
