Royal Botanic Gardens, Kew, Richmond, Surrey, UK.
Royal Botanic Garden Edinburgh, Edinburgh, UK.
Ann Bot. 2018 Nov 3;122(5):767-776. doi: 10.1093/aob/mcx146.
Helicocytic stomata are characterized by an inward spiral of mesogenous cells surrounding a central stomatal pore. They represent a relatively rare feature that occurs in some drought-tolerant angiosperm species. In some Begonia species with thick leaves, the stomata are not only helicocytic but also clustered into groups that are spaced apart by at least one cell. This paper presents a detailed ontogenetic study of this characteristic non-contiguous stomatal patterning in a developmental and phylogenetic context.
Light microscopy and both scanning and transmission electron microscopy were used to examine stomatal development in several species of Begonia. Published reports of stomatal development in Begonia and other angiosperms were reviewed to provide a comprehensive discussion of the evolution of stomatal patterning.
Helicocytic stomata develop from meristemoids that undergo a series of oriented asymmetric divisions to produce a spiral of mesogene stomatal lineage ground cells (SLGCs) surrounding a stoma. A clear developmental similarity between anisocytic and helicocytic stomata is positively correlated with the number of iterations of amplifying divisions that result in SLGCs. Stomatal clusters develop from asymmetric divisions in neighbouring SLGCs. Within each cluster, non-contiguous spacing of meristemoids is maintained by asymmetric divisions oriented away from each developing meristemoid.
Formation of non-contiguous stomatal clusters in Begonia relies on two primary developmental factors in the epidermis: an inwardly spiralling series of amplifying divisions that result in helicocytic stomata, and the development of a variable number of meristemoids from neighbouring SLGCs within each cluster. Optimization of these features on an angiosperm phylogeny indicates that the occurrence of amplifying divisions could be pre-adaptive for these factors. Both factors have been thoroughly studied in terms of developmental genetics in Arabidopsis, suggesting gene orthologues that could be implicated in Begonia stomatal patterning.
具有内向旋卷的中柱细胞围绕中央气孔的气孔称为旋卷气孔，这是一些耐旱被子植物的一种相对罕见的特征。在一些叶片较厚的秋海棠属植物中，气孔不仅呈旋卷状，而且还聚集成群，彼此之间至少间隔一个细胞。本文从发育和系统发生的角度详细研究了这种非连续的气孔模式的特征。
利用光镜、扫描电镜和透射电镜观察了几种秋海棠属植物的气孔发育。回顾了秋海棠属和其他被子植物气孔发育的文献报道，以全面讨论气孔模式进化。
旋卷气孔由经历一系列定向不对称分裂的分生细胞原基发育而来，产生一个围绕气孔的中柱衍生的气孔谱系细胞（SLGCs）螺旋。拟卵形和旋卷气孔之间的明显发育相似性与导致 SLGCs 的扩增分裂的迭代次数呈正相关。气孔簇由相邻 SLGC 中的不对称分裂发育而来。在每个簇内，通过远离每个发育中的分生细胞原基的定向不对称分裂来维持分生细胞原基的非连续间隔。
秋海棠非连续气孔簇的形成依赖于表皮中的两个主要发育因素：导致旋卷气孔的向内螺旋式扩增分裂系列，以及每个簇中从相邻 SLGC 发育的可变数量的分生细胞原基。在被子植物系统发育树上对这些特征的优化表明，扩增分裂的发生可能是这些因素的预适应。在拟南芥中，这两个因素在发育遗传学方面都得到了深入研究，表明可能与秋海棠气孔模式有关的基因同源物。
