Involvement of reactive oxygen species in endosperm cap weakening and embryo elongation growth during lettuce seed germination.

Yu Zhang,Shanli Chen,Zhenjiang Xu,Jianxun Chen,Xiaofeng Wang,Xi Huang,Zhaowan Shi,Bingxian Chen

doi:10.1093/jxb/eru167

Abstract

Endosperm cap (CAP) weakening and embryo elongation growth are prerequisites for the completion of lettuce seed germination. Although it has been proposed that the cell wall loosening underlying these processes results from an enzymatic mechanism, it is still unclear which enzymes are involved. Here it is shown that reactive oxygen species (ROS), which are non-enzymatic factors, may be involved in the two processes. In Guasihong lettuce seeds imbibed in water, O2·(-) and H2O2 accumulated and peroxidase activity increased in the CAP, whereas its puncture force decreased. In addition, in the radicle, the increase in embryo growth potential was accompanied by accumulation of O2·(-) and an increase in peroxidase activity. Imbibing seeds in 0.3% sodium dichloroisocyanurate (SDIC) reduced endosperm viability and the levels of O2·(-), H2O2, and peroxidase activity in the CAP, whereas the decrease in its puncture force was inhibited. However, in the embryo, SDIC did not affect the accumulation of O2·(-), peroxidase activity, and the embryo growth potential. As a result, SDIC caused atypical germination, in which the endosperm ruptured at the boundary between the CAP and lateral endosperm. ROS scavengers and ROS generation inhibitors inhibited the CAP weakening and also decreased the embryo growth potential, thus decreasing the percentage of seed germination. Exogenous ROS and ROS generation inducers increased the percentage of CAP rupture to some extent, and the addition of H2O2 to 0.3% SDIC enabled some seeds to undergo typical germination.

Highlights

Seed germination begins with water uptake and ends with the emergence of the radicle (RAD) through the surrounding seed tissues (Bewley et al, 2013), and is a consequence of the competing interaction between the growth potential of the embryo and the limiting mechanical force of its surrounding tissues (Nambara et al, 2010)
In Guasihong lettuce seeds imbibed in water, O2·– and H2O2 accumulated and peroxidase activity increased in the CAP, whereas its puncture force decreased
The rupture of the endosperm during seed germination is a result of two processes: growth of the embryo, and weakening of the micropylar endosperm, by which its Abbreviations: CAP, endosperm cap; DAB, 3,3-diaminobenzidine; DPI, diphenylene iodonium chloride; NBT, nitroblue tetrazolium; RAD, radicle; ROS, reactive oxygen species; SDIC, sodium dichloroisocyanurate; TMB, 3,3′,5,5′-tetramethylbenzidine; TTC, triphenyltetrazolium chloride

Summary

Introduction

Seed germination begins with water uptake and ends with the emergence of the radicle (RAD) through the surrounding seed tissues (Bewley et al, 2013), and is a consequence of the competing interaction between the growth potential of the embryo and the limiting mechanical force of its surrounding tissues (Nambara et al, 2010). The rupture of the endosperm during seed germination is a result of two processes: growth of the embryo (normally the elongation of the RAD and/or hypocotyl), and weakening of the micropylar endosperm (or endosperm cap, CAP), by which its Abbreviations: CAP, endosperm cap; DAB, 3,3-diaminobenzidine; DPI, diphenylene iodonium chloride; NBT, nitroblue tetrazolium; RAD, radicle; ROS, reactive oxygen species; SDIC, sodium dichloroisocyanurate; TMB, 3,3′,5,5′-tetramethylbenzidine; TTC, triphenyltetrazolium chloride. Lettuce seed germination has been extensively studied, the mechanisms underlying the rupture of the CAP, especially its weakening process, are still largely unknown (Bewley, 1997; Dutta et al, 1994; Nonogaki and Morohashi, 1999; Wang et al, 2004)

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