Hey I Can Chan has given a pretty good answer regarding the options available in Pathfinder, but it’s fairly slim pickings. D&D 3.5 doesn’t have much better, sadly, but here’s what there is.
Initiate of Nature, Player’s Guide to Faerûn
Requires cleric or druid level 5th, as well as a nature god as a patron deity (the feat actually lists the nature gods of Faerûn in its requirement, but for playing in not-Faerûn, the feat is not specific to any one of them). This feat allows you to rebuke plants as an evil cleric rebukes undead, allowing you to to command a plant-creature army if you can find enough of them. It also lets you rebuke animals, and adds several plant-based spells to your spell list.
If for whatever reason this is unavailable to you, you can take both Plant Defiance and Plant Control to allow you to both turn and rebuke plant creatures. No animals, no extra spells, and it takes two feats, so it’s horribly overpriced, but as SRD material it is accessible.
Leadership can attract creatures as cohorts and followers, so you can get a significant plant creature companion as well as a number of low-level plant creature followers if you desire.
Note that Draconomicon has a feat Dragon Cohort which allows you to get a cohort without Leadership, and that cohort is a dragon-type creature 3 levels higher than you could usually have as a cohort. Swap Dragon for Plant and this could be a good deal. No followers this way though.
Plant Devotion, Complete Champion
Doesn’t have anything to do with plant allies, just makes your more plant-like. Still, a natural armor bonus and fortification isn’t bad for a feat, and it is fitting. Note that Complete Champion allows clerics to trade in a domain for the corresponding devotion feat: this is not required. A cleric with the plant domain can still take plant devotion as a feat.
I was honestly shocked that this was all I could find out of all of 3.5’s prestige classes.
Holt Warden, Complete Champion
Holt wardens are clerics with the plant domain who join the Guardians of the Green. It’s a prestige class that advances divine spellcasting at every level, and its prerequisites consist of skill ranks you probably want anyway plus being able to cast spells of the Plant Domain, making it pretty painless to take.
As Brian mentioned, this grants the ability to rebuke plants. Initiate of Nature is massively cheaper, and because of its other benefits, is probably worth taking even if you do take levels of holt warden.
Aside from that, you get the ability to spontaneously swap out spells for Plant Domain spells (but this replaces your existing ability to do that with cure or inflict spells), and you get the druid’s Nature Sense, Woodland Stride, Trackless Step, and Timeless Body features. You also get a 1/day, 10-minute-casting-time heal spell, quite a bit earlier than the real thing becomes available, which is nice enough. Immunity to venom, various divination-through-the-forest rituals, and the ability to give all living creatures who hear you +2 Wisdom and restore some spells to divine spellcasters.
Ultimately, it’s OK, but not great. Becomes much better if Initiate of Nature is unavailable.
Upon Which to Base Homebrew
These are things you cannot reasonably get as a cleric, even in 3.5, but might be viable for porting to Pathfinder as feats or an archetype.
Channel Plants, Races of the Wild
This class feature, found on the arcane hierophant prestige class, allows you to cast spells from any non-magical, non-creature plant. Considering that the previous feature, Channel Animals, allowed you to do the same from any animal creature, extending this to plant creatures would be reasonable enough (in fact, considering their usual rarity, casting from plant creatures is inferior to casting from any non-creature plant).
Greenbound Summoning, Lost Empires of Faerûn
I mention this feat only because it is fantastically powerful. It is extremely difficult for cleric to qualify for it and it is inappropriate in most games. It does, however, allow you to have powerful plant summons. As part of an archetype that adds summon nature’s ally to your cleric list, perhaps, it might work? I’d either delay it until ~10th level or greatly modify the greenbound template, though.