Calcium: A Central Regulator of Plant Growth and Development

Abstract

Today no one questions the assertion that Ca2+ is a crucial regulator of growth and development in plants. The myriad processes in which this ion participates is large and growing and involves nearly all aspects of plant development (recent reviews in Harper et al., 2004; Hetherington and Brownlee, 2004; Hirschi, 2004; Reddy and Reddy, 2004; Bothwell and Ng, 2005). Despite this wealth of research, the concept of Ca2+ as an intracellular regulator is relatively recent and within the professional life span of many people who are still active and working on this topic today. The aim of this essay is to identify those lines of thought and research that led to the idea that Ca2+ is a second messenger in plant cell growth and development. This essay thus focuses primarily on work starting in the mid sixties and extending to the mid eighties. I do not provide an exhaustive review of the history of Ca2+ research, nor do I attempt to treat modern aspects of Ca2+ research. However, I do strive to identify the roots of modern Ca2+ research and to chart the origin of the current revolution. Ca2+ is an essential element; however, its role is elusive. When examining total Ca2+ in plants, the concentration is quite large (mM), but its requirement is that of a micronutrient (μM). Ca2+ is not usually limiting in field conditions, still there are several defects that can be associated with low levels of this ion, including poor root development, leaf necrosis and curling, blossom end rot, bitter pit, fruit cracking, poor fruit storage, and water soaking (Simon, 1978; White and Broadley, 2003). The underlying causes for these effects are not entirely clear; nevertheless, two areas within the cell have been recognized as being important targets. First is the cell wall, where Ca2+ plays a key role in cross-linking acidic pectin residues. The second is the cellular membrane system, where low [Ca2+]e increases the permeability of the plasma membrane. These are briefly discussed below. Since the 19th century, it has been appreciated that Ca2+ plays a crucial role in determining the structural rigidity of the cell wall (reviewed in Wyn Jones and Lunt, 1967; Burstrom, 1968). During cell wall formation, the acidic pectin residues (e.g., galacturonic acid) are secreted as methyl esters, and only later deesterified by pectin methylesterase, liberating carboxyl groups, which bind Ca2+. It follows that low [Ca2+]e should make the cell wall more pliable and easily ruptured, whereas high concentrations should rigidify the wall and make it less plastic. It had become apparent in the mid to late fifties that modifying the [Ca2+]e produced a pronounced effect on cell growth. Thus, elevating the [Ca2+]e led to an inhibition in shoot or coleoptile growth, whereas reducing its concentration promoted cell and tissue elongation (Bennet-Clark, 1956; Tagawa and Bonner, 1957). Strong support for the Ca2+/pectate interaction came from a quantitative examination of the cation exchange capacity of the coleoptile cell wall, which was shown to be due to the number of free pectic carboxyl groups (Jansen et al., 1960). Still further support came from studies using the cation chelator EDTA, which had been employed to macerate plant tissues without destroying the cell structure (Letham, 1958). The explanation centered around the idea that EDTA, by chelating Ca2+, led to a marked weakening or loss of pectates in the middle lamella, thus removing the agent that cemented cells together. The importance of the Ca2+/pectate interaction as a regulator of growth encouraged researchers to include a role for auxin in this scheme, particularly because it was becoming evident that Ca2+ and auxin had antagonistic actions. Thus, auxin promoted shoot growth and inhibited root growth, whereas Ca2+ inhibited shoot growth and promoted root growth. Working with oat coleoptiles, Bennet-Clark (1956) proposed that there might be a direct antagonism between indoleacetic acid (IAA) and Ca2+. Noting that Ca2+, and the lanthanide praseodymium, inhibited IAA-induced elongation, whereas EDTA reversed the inhibitory activity of Ca2+, and even promoted growth, Bennet-Clark (1956) suggested that IAA acts as a Ca2+/Mg2+ chelator. This model proposed that IAA removes Ca2+ and leads to a loss of Ca2+ pectates, which are replaced by pectate free acids or methyl esters. The latter, because they are not cross-linked, would render the wall plastic and able to elongate (Bennet-Clark, 1956). This idea was challenged by Cleland (1960), who demonstrated that IAA does not enhance the loss of Ca2+ from the cell wall, nor does it cause a redistribution of Ca2+ between pectin and proto-pectin. Somewhat later, Burling and Jackson (1965) used atomic absorption spectroscopy to show that Ca2+ accumulated in the cell walls of elongating coleoptiles and that this accumulation was unaffected by auxin. Further studies by Baker and Ray (1965) and Ray and Baker (1965) established the separation in action between C