Maternal control of embryogenesis by MPK6 and its upstream MKK4/MKK5 in Arabidopsis

Corresponding author: Shuqun Zhang

In flowering plants, developing embryos (胚胎) reside in maternal sporophytes (孢子). It is known that maternal generation influences the development of next-generation embryos; however, little is known about the signaling components in the process. Previously, we demonstrated that Arabidopsis mitogen-activated protein kinase 6 (MPK6; 丝裂原活化蛋白激酶) and MPK3 play critical roles in plant reproduction. In addition, we noticed that a large fraction of seeds from mpk6 single-mutant plants showed a wrinkled (有皱纹的) seed coat (种皮) or a burst-out embryo phenotype. Here, we report that these seed phenotypes can be traced back to defective embryogenesis. The defective embryos have shorter suspensors (胚柄) and reduced growth along the longitudinal axis (纵轴). Furthermore, the cotyledons (子叶) fail to bend over (弯曲) to progress to the bent-cotyledon stage (手杖胚期). As a result of the uneven circumference along the axis, the seed coat wrinkles to develop raisin-like (类似于葡萄干的) morphology after dehydration (脱水). In more severe cases, the embryo can be pushed out from the micropylar end (珠孔端), resulting in the burst-out embryo seed phenotype. Genetic analyses demonstrated that the defective embryogenesis of the mpk6 mutant is a maternal effect. Heterozygous or homozygous mpk6 embryos have defects only in mpk6 homozygous maternal plants, but not in wild-type or heterozygous maternal plants. The loss of function of MKK4/MKK5 also results in the same phenotypes, suggesting that MKK4/MKK5 might act upstream of MPK6 in this pathway. The maternal-mediated embryo defects are associated with changes in auxin activity maxima (最大量) and PIN localization. In summary, this research demonstrates that the Arabidopsis MKK4/MKK5–MPK6 cascade is an important player in the maternal control of embryogenesis.

在有花植物中，胚胎是在母本孢子内发育的。母本会影响下一代胚胎的发育，然而该进程中的信号组分还未知。之前，作者已经证明了拟南芥的丝裂原活化蛋白激酶MPK6和MPK3在植物的生殖中扮演着重要的角色。另外，作者注意到在mpk6单突变体的植物中许多的种子种皮褶皱，或者是胚胎爆裂而出。本文的试验表明这些表型可以追溯到胚胎发生。缺陷胚胎的胚柄较短，纵轴的生长减少。此外，子叶不能弯曲以进入手杖胚期。在轴向上的不均衡发育导致了种皮的褶皱，在脱水后形成了类似葡萄干的形态。在一些更极端的表型中，胚胎被从珠孔端挤出，导致了一个胚胎外露的种子表型。仅只有母本纯合的mpk6突变体胚胎存在缺陷，而在野生型或者母本杂合mpk6植株中胚胎均表现正常。MKK4/MKK5的缺失突变体同样会导致胚胎缺陷的表型，表明MKK4/MKK5可能在这条通路中处于MPK6的上游。母本介导的胚胎缺陷与生长素最大活力的变化及PIN的定位相关。总的来说，本文研究揭示了拟南芥的MKK4/MKK5–MPK6级联对于母本控制胚胎发生是至关重要的。

个人简介：1981-1985年，南京大学，生物学士；1985-1988年，中科院水生生物研究所，博士生；1988-1990年，University of Texas，访问学者；1990-1994，University of Texas，植物科学博士；1995-1998，Rutgers University，博士后；1998-2003年，University of Missouri，助理教授；2003-2009年，University of Missouri，助理教授；2009-至今，University of Missouri，生物化学院教授；2011-至今，浙江大学，植物科学研究所，兼职教授。

doi: 10.1111/tpj.13737