Zygotic embryo cell wall responses to drying in three gymnosperm species differing in seed desiccation sensitivity

Abstract

Plant cell walls (CWs) are dynamic in that they can change conformation during ontogeny and in response to various stresses. Though seeds are the main propagatory units of higher plants, little is known of the conformational responses of zygotic embryo CWs to drying. This study employed cryo-scanning electron microscopy to compare the effects of desiccation on zygotic embryo CW morphology across three gymnosperm species that were shown here to differ in seed desiccation sensitivity: Podocarpus henkelii (highly desiccation-sensitive), Podocarpus falcatus (moderately desiccation-sensitive), and Pinus elliottii (desiccation-tolerant). Fresh/imbibed (i.e. fresh Podocarpus at shedding and imbibed Pi. elliottii) embryos showed polyhedral cells with regular walls, typical of turgid cells with an intact plasmalemma. Upon desiccation to c. 0.05 g g (dry mass basis), CWs assumed an undulating conformation, the severity of which appeared to depend on the amount and type of dry matter accumulated. After desiccation, intercellular spaces between cortical cells in all species were comparably enlarged relative to those of fresh/imbibed embryos. After rehydration, meristematic and cotyledonary CWs of P. henkelii and meristematic CWs of P. falcatus remained slightly undulated, suggestive of plasmalemma and/or CW damage, while those of Pi. elliottii returned to their original conformation. Cell areas in dried-rehydrated P. henkelii root meristem and cotyledon were also significantly lower than those from fresh embryos, suggesting incomplete recovery, even though embryo water contents were comparable between the two states. Electrolyte leakage measurements suggest that the two desiccation-sensitive species incurred significant plasmalemma damage relative to the tolerant species upon desiccation, in agreement with the CW abnormalities observed in these species after rehydration. Immunocytochemistry studies revealed that of the four CW epitopes common to embryos of all three species, an increase in arabinan (LM6) upon desiccation and rehydration in desiccation-tolerant Pi. elliottii was the only difference, although this was not statistically significant. Seed desiccation sensitivity in species like P. henkelii and P. falcatus may therefore be partly based on the inability of the plasmalemma and consequently CWs of dried embryos to regain their original conformation following rehydration.

Alleviating delayed field curing-induced deterioration in rice seeds by cathodic water invigoration: a comparison of seed and seedling responses in two upland rice species and their interspecific hybrid.

“…For example, while marked CWL folding in embryo root meristematic cells of non-invigorated cured O. glaberrima reversed upon imbibition, slight CWL folding was evident in cured O. sativa whereas no CWL folding was apparent in the hybrid seeds. However, CWL abnormalities such as separation of adjacent CWLs and plasmalemma separation were evident in O. sativa and hybrid probably due to differences in CWL anatomical and/or chemical properties (based on observation reported for other species by Webb and Arnott 1982;Oliver, 2007;Woodenberg et al, 2018). This is definitely a potential useful area of investigation for future studies.…”

“…Differences in CWL folding between O. glaberrima and O. sativa or the hybrid may also be a result of differences in their CWL properties (Oliver, 2007). Several factors including species, tissues within a seed, size of CWL, lipids, ratios of proteins, and starches in a seed, and proportion of celluloses, hemicelluloses, pectins in the CWL may influence the water retention capacity of the protoplast and determine the degree of shrinkage upon water loss (Webb and Arnott, 1982;Woodenberg et al, 2018). For example, accumulation of lipids during desiccation offers better intracellular mechanical stabilization because of their ability to occupy more subcellular space and minimize cell shrinkage (Farrant 2000;Woodernberg et al, 2018).…”

“…101 Cellular membranes are a major site of ultrastructural damage during desiccation (Leprince et al, 1993;Walters et al, 2002;Benamar et al, 2003;Woodenberg et al, 2018) and their ability to repair upon rehydration is key prerequisite of desiccation tolerance in plant cells and tissues (Webb and Arnott, 1982;Bewley, 1997;Walters and Koster, 2007). Mitochondrial membranes are the primary targets for ageing-and desiccation induced damage (Benamar et al, 2003;Wang et al, 2012a).…”

“…Delayed field curing in these tropical environments may disturb the metabolically and physically quiescent state of the seeds and subsequently compromise seed quality by allowing for deteriorative processes such as unbalanced metabolism at intermediate water contents, which could lead to oxidative stress (Walters and Koster, 2007;Kranner et al, 2010) and damage cellular ultrastructure and biological macromolecules (e.g., lipids, proteins, DNA and RNA) (Powell and Matthews, 2012). Cellular membranes may serve as a major site of damage to cell ultrastructure during desiccation (Leprince et al, 1993;Walters et al, 2002;Benamar et al, 2003;Woodenberg et al, 2018) as cells shrink and cause tensions to be created within cell membranes leading to the shearing of membrane connections to the CWL (reviewed in Walters et al, 2002;Woodenberg et al, 2018) and loss of membrane material through vesiculations (reviewed in Walters et al, 2002;Corbineau et al 2004). Therefore, the ability of a protoplasm to minimize damage from desiccation and/or rehydration cycles, maintain cellular and ultrastructure integrity in the dry state and mobilize repair mechanisms on rehydration are key to establishing desiccation tolerance in plants and plant tissues (Bewley, 1997;Walters and Koster, 2007) and seed viability (Webb and Arnott, 1982).…”