Kim T‐H , Böhmer M , Hu H , Nishimura N and Schroeder JI (2010) Guard cell signal transduction network: advances in understanding abscisic acid, CO2, and Ca2+ signaling. Annual Review of Plant Biology 61: 561–591.
Chloroplasts are a common feature of guard cells in the majority of species examined, yet the function of these organelles has been the subject of debate and remains to be confirmed. The number of chloroplasts found in guard cells is species-dependent, but typically ranges between 8-15, compared with 30-70 in palisade mesophyll cells (Willmer & Fricker, 1996) resulting in between 25-100 fold lower chlorophyll content per cell in guard cells than in mesophyll cells. However, guard cell volume is about 10 fold lower than mesophyll, which means that the chloroplasts could provide a significant energy source. A noticeable difference in guard cell chloroplasts is that starch accumulates in the dark and disappears in the light, the opposite to mesophyll (Willmer & Fricker, 1996). Although this may also be species-dependent as Arabidopsis has been shown to be practically free of starch in the morning and accumulates it during the night.
Guard cell photosynthesis and stomatal function - …
A compelling argument supporting a role for guard cell chloroplasts in stomatal physiology has been the fact that stomata remain functional in epidermal strips or protoplasts where any mesophyll influence has been removed. Experimental evidence exists for four primary ways in which guard cell chloroplasts could contribute to stomatal function: (1) ATP and/or reductants produced by guard cell electron transport are used in osmoregulation; (2) chloroplasts are used in the blue-light signalling and responses; (3) starch stored in guard cell chloroplasts is used to synthesize malate to counter-balance K+ for stomatal opening; (4) guard cell photosynthesis produces osmotically active sugars (see Figure 1).
What is the role of the guard cells in the process of ..
Transgenic plants with impairments in photosynthetic function have recently been used to address the role of guard cell chloroplasts and guard cell photosynthesis in stomatal function. Despite severe reductions in either electron transport or Calvin cycle processes, stomata in transgenic plants were still able to achieve similar stomatal conductances as wild type controls. Tobacco plants with reduced amounts of Rubisco (von Caemmerer et al. 2004) revealed similar reduction in guard cell photosynthetic efficiency to those found in mesophyll cells, although no differences in stomatal behaviour were observed. The fact that stomata opened in response to a step-change in light despite high internal CO2 concentration (Ci) implied that the stomata in these plants were insensitive to Ci. Major reductions in carboxylation capacity of photosynthesis and impaired rates of electron transport (via reductions in the b6f complex) also resulted in no phenotypic stomatal response despite decreases in sucrose content, suggesting that something other than sucrose concentration acts as the osmoregulator during opening (Baroli et al. 2008). A minor regulatory role for photosynthetic electron transport was suggested in studies conducted on antisense SBPase plants in which stomatal opening in response to red light was greater in transgenic plants compared with wild type controls, possibly due to increased ATP availability (Lawson et al. 2008). However, reduced ATP availability in tobacco plants with reductions in the cytochrome b6f complex showed no effect on red light induced stomatal opening (Baroli et al. 2008). Although reduction in guard cell photosynthesis and Calvin cycle activity have either shown no or only minor effects on stomatal function and behaviour, studies on transgenic antisense PEPc potato plants have supported a role for malate and PEPc activity in guard cells, with reduced PEPc activity showing delays in stomatal opening which was accelerated in over-expressing plants (Gehlen et al. 1996). This work is supported by recent findings that show reduced rates of stomatal opening and final conductance in Amaranthus edulis mutants deficient in PEPc (Cousins et al. 2007). Additionally, stomata in plants with 12% wild-type fructose-1,6-bishpatase (FBPase) activity showed significantly faster opening responses and higher final conductance with increasing irradiance, despite lower photosynthetic rates and elevated Ci concentrations (Muschak et al. 1999). Zea mays plants with increased amount of NADP-malic enzyme (ME) which converts malate to pyruvate have also been shown to have a reduced stomatal conductance (Laporte et al. 2002). Guard cell chloroplasts do not necessarily need to play a role in providing energy or osmotica for stomatal function, they could act as part of the sensory or signalling pathways. Such pathways may involve reactive oxygen species such as H2O2, which has been shown to be involved in ABA signalling. Alterations of stomatal conductance in plants with changes in the redox state of ascorbic acid, which is an important component of the antioxidant system, suggest a regulatory role of hydrogen peroxide production.
Plant chloroplasts are commonly found in guard cells ..
Plants growing in drier conditions tend to have small numbers of tiny stomata and only on their lower leaf surface, to save water loss. Most plants regulate the size of stomata with guard cells. Each stoma is surrounded by a pair of sausage-shaped guard cells. In low light the guard cells lose water and become flaccid, causing the stomata to close. They would normally only close in the dark when no carbon dioxide is needed for photosynthesis.