Abstract:

Border cells (BCs), originated from root-cap meristem, are a kind of active cells deposited in the periphery of roots to protect the root tips from extracellular biotic and abiotic stresses. Some investigators believe it is a constitutive expression in BC’s production, while others suggest that it is an induced process regulated by intro- and extro-signals. BCs in different plant species show completely different responses to temperature stress. In this paper, we investigated the factors associated with the production and development of BCs in peanut (Arachis hypogaea L.), which had never been reported before. Two peanut cultivars, Xinchang Xiaojingsheng and Jiangxi Yibazhua were employed and their seeds were aerobic cultured after imbibition. The BC number and activity (percentage of BC survival/total) as well as their responses to pectin methylesterase (PME) activity, BC liquor, temperature, and osmotic potential were measured. Under normal cultural condition, the number of BCs increased with the development and elongation of the root. The first BC formation almost synchronously occurred with root tip emergence. When the root length was 13–17 mm at 25℃, the number of BCs reached the maximum (about 10 000 BCs). When the BCs were removed from root tip, a new generation of BCs was induced within 72 h. During this period, the PME activity increased quickly to the maximum within 4 h and then decreased to pre-induction background levels subsequently, suggesting that the PME may play an important role in production and development of BC. The number and survival percentage of detached BCs were significantly different when the roots cultured in the media with three different BC liquor concentrations containing extracellular inhibitor of BC. The extracellular inhibitor of BC released could inhibit not only the production of BC but also its activity, and the inhibition was strong with the increment of the concentration. The extracellular inhibitor affected more greatly on BC production as compared with the BC activity. Temperature directly or indirectly affected the root growth, and regulated the production and development of BCs. Both high and low temperatures inhibited the root growth and affected the production rate, maximum number, and activity of BC, and low temperature had more severe harmness to the development of the peanut root. Temperature had greater impact on BC activity as compared with secretion rate of BC. The activities of detached BCs in different culture conditions show significant difference. The survival percentage of detached border cells was significantly higher under culture in the aerobic or mannitol than in MS medium or ddH2O, suggesting that maintenance of mucus layer osmolarity in root apex can maintained the survival percentage of BCs at a high level within a relative long time. The results supported the hypothesis of induced model of BC formation that regulated by the intrinsic and extrinsic signals.

Key words: Peanut, Root border cell, Development control, Root apical meristem, Extracellular inhibitor