ZYGOTIC EMBRYO CELL WALL RESPONSES TO DRYING IN THREE GYMNOSPERM SPECIES DIFFERING IN SEED DESICCATION SENSITIVITY
Woodenberg, W.R., Sershen, Varghese, B. and Pammenter, N.
School of Life Sciences, University of KwaZulu-Natal, Westville Campus, Durban, RSA
Contact: Wynston Woodenberg, firstname.lastname@example.org
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 CW morphology of zygotic embryos across three gymnosperm species that were shown to differ in seed desiccation sensitivity: Podocarpus henkelii (highly desiccation-sensitive); Podocarpus falcatus (moderately desiccation-sensitive); and Pinus elliottii (desiccation-tolerant). Hydrated embryos of all three species showed polyhedral cells with regular walls, typical of turgid cells with an intact plasmalemma. Upon desiccation to c. 0.05 g g-1 (dry mass basis), CWs assumed an undulating conformation, the severity of which seemed to depend on the amount and kind of dry matter accumulated. After desiccation, intercellular spaces between cortical cells in all species were comparably enlarged relative to those of hydrated embryos. After rehydration, embryo CWs of P. henkelii and meristematic CWs of P. falcatus remained slightly undulated, suggestive of plasmalemma and/or CW damage, while those of P. elliottii returned to their original conformation. Cell areas in dried-rehydrated P. henkelii meristem and cotyledon were also significantly lower than hydrated cells from undried embryos, suggestive of incomplete recovery even though water contents of the two states were comparable. Electrolyte leakage measurements suggest that the two desiccation-sensitive species incurred significant plasmalemma damage relative to the tolerant species upon desiccation, which may have contributed to the CW abnormalities 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 P. elliottii was the only significant difference. 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. Moreover, embryo CWs of the desiccation-sensitive species may lack the ability to produce additional arabinan during dehydration and rehydration, which compromises CW flexibility and prevents its full conformational recovery following rehydration.