Cucurbit Genetics Cooperative Report 12:82-83 (article
Control of Chlorophyll During Plant Development: Hypothesis
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The term "control" in the title pertains to a series of steps that transforms proplastids into chloroplasts, the organelles of chlorophyll synthesis. These steps occur in competent cells that are exposed to light. According to present hypothesis there are two systems of control: one at the organelle level and another at the organismal level. The organelle system controls the steps that lead to normal chloroplasts under favorable intracellular conditions. But intracellular conditions potentially vary indifferent organs and at different stages during plant development. Furthermore, these conditions are affected by fluctuations in the external environment. The organismal system, acting as a buffer to each variations, tends to maintain favorable internal conditions for effective control by the organelle system. The focus here is on the organismal system.
The control at the organismal level is perceived as a homeostat of plastid transformation (HPT). The term "homeostat" is derived from the concept of homeostasis. The HPT consists of different nuclear genes that act in a selective manner, singly or in combination, as homeostatic regulators. Thus, the capacity of competent cells to transform proplastids into chloroplasts in different organs and at different developmental stages is sustained by these regulators. Some mutants of these regulators adversely affect or completely block the course of plastid transformation.
In a broader sense, HPT enables higher plants to carry on photosynthesis persistently and efficiently throughout life, assuming normal fluctuations in the external environment. HPT probably played a role in the evolution of higher organisms. This is because persistent production of photosynthates during plant development was advantageous not only to the producers, the autotrophs, but also to their animal predators, the heterotrophs.
The above hypothesis originated from studies of precocious depletion of chlorophyll in Cucurbita. The supporting evidence is based on the identification of two groups of genes that are unique to their specific effects.
The first group targets specific organs selectively. This group consists of gene B and its selective activators and selective suppressors. B is a major nuclear element that brings about precocious depletion of chlorophyll in fruits in all known genetic backgrounds. But B can be expressed or suppressed in other organs (e.g., leaf blades, stems) depending on the presence of selective activators such as Ses-B+ and Ac-B or selective suppressors, such as Ses-B and Ac-B+. These findings suggested that the action of B, B+, Ses-B+, Ac-B and Ac-B+ is organ-specific, and that B+, Ses-B, and Ac-B+ are effective homeostatic regulators. The information on the behavior of gene B has been published, but see also the two preceding articles in present issue of CGC Report.
The second group of genes targets leaf blades at a particular time during plant development. Usually the first five to seven sequential leaves on the main stem are not affected (Shifriss, unpublished). A similar manifestation is exhibited by certain cultivars of Amaranthus tricolor (e.g., 'Illumination') except that in these cultivars the entire shoot tip is affected sometime during development. As a result, the upper portion of an affected plant is completely devoid of chlorophyll. Separate progenies obtained from self-pollination of the upper and lower portions of such a plant behave developmentally in identical manner.
The time and extent of gene expression in both groups are highly affected by non-genetic fluctuations. This is particularly true for heterozygotes.
The hypothesis of homeostatic regulators can be tested. First, consider the future synthesis of two isogenic B+ inbreds; one carrying Ses-B and another, Ses-B+. These inbreds will appear indistinguishable phenotypically. However, when tested for photosynthetic activity in diverse environments the difference between them will become evident. Either the Ses-B inbred will be consistently superior over the Ses-B+ inbred or each will be superior in a different ecological niche. Second molecular analysis will demonstrate that the DNA sequences of some of the homeostatic regulators in Cucurbita are shared by many distantly related species of higher plants, and that these sequences influence the potential of crop yield.
While light triggers the process of plastid transformation, the evidence in Cucurbita and other taxa suggests the existence of an hierarchy of regulators that sustains this process during development. any alternative to the HPT hypothesis should offer a more convincing interpretation for the kinds of specificity as well as for the widespread distribution of such genes as Ses-B and Ses-B+ among the B+ cultivars of Cucurbita.